Published Papers - Seo Mitsunori

The karrikin receptor KAI2 promotes drought resistance in Arabidopsis thaliana

Weiqiang Li, Kien Huu Nguyen, Ha Duc Chu, Chien Van Ha, Yasuko Watanabe, Yuriko Osakabe, Marco Antonio Leyva-González, Mayuko Sato, Kiminori Toyooka, Laura Voges, Maho Tanaka, Mohammad Golam Mostofa, Motoaki Seki, Mitsunori Seo, Shinjiro Yamaguchi, David C. Nelson, Chunjie Tian, Luis Herrera-Estrella, Lam Son Phan Tran

PLoS Genetics   13 ( 11 )   2017.11 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2017 Li et al. Drought causes substantial reductions in crop yields worldwide. Therefore, we set out to identify new chemical and genetic factors that regulate drought resistance in Arabidopsis thaliana. Karrikins (KARs) are a class of butenolide compounds found in smoke that promote seed germination, and have been reported to improve seedling vigor under stressful growth conditions. Here, we discovered that mutations in KARRIKIN INSENSITIVE2 (KAI2), encoding the proposed karrikin receptor, result in hypersensitivity to water deprivation. We performed transcriptomic, physiological and biochemical analyses of kai2 plants to understand the basis for KAI2-regulated drought resistance. We found that kai2 mutants have increased rates of water loss and drought-induced cell membrane damage, enlarged stomatal apertures, and higher cuticular permeability. In addition, kai2 plants have reduced anthocyanin biosynthesis during drought, and are hyposensitive to abscisic acid (ABA) in stomatal closure and cotyledon opening assays. We identified genes that are likely associated with the observed physiological and biochemical changes through a genome-wide transcriptome analysis of kai2 under both well-watered and dehydration conditions. These data provide evidence for crosstalk between ABA- and KAI2-dependent signaling pathways in regulating plant responses to drought. A comparison of the strigolactone receptor mutant d14 (DWARF14) to kai2 indicated that strigolactones also contributes to plant drought adaptation, although not by affecting cuticle development. Our findings suggest that chemical or genetic manipulation of KAI2 and D14 signaling may provide novel ways to improve drought resistance.

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Wounding triggers callus formation via dynamic hormonal and transcriptional changes

Momoko Ikeuchi, Akira Iwase, Bart Rymen, Alice Lambolez, Mikiko Kojima, Yumiko Takebayashi, Jefri Heyman, Shunsuke Watanabe, Mitsunori Seo, Lieven De Veylder, Hitoshi Sakakibara, Keiko Sugimoto

Plant Physiology   175 ( 3 ) 1158 - 1174   2017.11 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2017 American Society of Plant Biologists. All rights reserved. Wounding is a primary trigger of organ regeneration, but how wound stress reactivates cell proliferation and promotes cellular reprogramming remains elusive. In this study, we combined transcriptome analysis with quantitative hormonal analysis to investigate how wounding induces callus formation in Arabidopsis (Arabidopsis thaliana). Our time course RNA-seq analysis revealed that wounding induces dynamic transcriptional changes, starting from rapid stress responses followed by the activation of metabolic processes and protein synthesis and subsequent activation of cell cycle regulators. Gene ontology analyses further uncovered that wounding modifies the expression of hormone biosynthesis and response genes, and quantitative analysis of endogenous plant hormones revealed accumulation of cytokinin prior to callus formation. Mutants defective in cytokinin synthesis and signaling display reduced efficiency in callus formation, indicating that de novo synthesis of cytokinin is critical for wound-induced callus formation. We further demonstrate that type-B ARABIDOPSIS RESPONSE REGULATOR-mediated cytokinin signaling regulates the expression of CYCLIN D3;1 (CYCD3;1) and that mutations in CYCD3;1 and its homologs CYCD3;2 and 3 cause defects in callus formation. In addition to these hormone-mediated changes, our transcriptome data uncovered that wounding activates multiple developmental regulators, and we found novel roles of ETHYLENE RESPONSE FACTOR 115 and PLETHORA3 (PLT3), PLT5, and PLT7 in callus generation. All together, these results provide novel mechanistic insights into how wounding reactivates cell proliferation during callus formation.

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GA as a regulatory link between the showy floral traits color and scent

Jasmin Ravid, Ben Spitzer-Rimon, Yumiko Takebayashi, Mitsunori Seo, Alon Cna'ani, Javiera Aravena-Calvo, Tania Masci, Moran Farhi, Alexander Vainstein

New Phytologist   215 ( 1 ) 411 - 422   2017.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust Emission of volatiles at advanced stages of flower development is a strategy used by plants to lure pollinators to the flower. We reveal that GA negatively regulates floral scent production in petunia. We used Agrobacterium-mediated transient expression of GA-20ox in petunia flowers and a virus-induced gene silencing approach to knock down DELLA expression, measured volatile emission, internal pool sizes and GA levels by GC-MS or LC–MS/MS, and analyzed transcript levels of scent-related phenylpropanoid-pathway genes. We show that GA has a negative effect on the concentrations of accumulated and emitted phenylpropanoid volatiles in petunia flowers; this effect is exerted through transcriptional/post-transcriptional downregulation of regulatory and biosynthetic scent-related genes. Both overexpression of GA20-ox, a GA-biosynthesis gene, and suppression of DELLA, a repressor of GA-signal transduction, corroborated GA's negative regulation of floral scent. We present a model in which GA-dependent timing of the sequential activation of different branches of the phenylpropanoid pathway during flower development may represent a link between the showy traits controlling pollinator attraction, namely color and scent.

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Primitive auxin response without TIR1 and Aux/IAA in the charophyte alga Klebsormidium nitens

Kinuka Ohtaka, Koichi Hori, Yuri Kanno, Mitsunori Seo, Hiroyuki Ohta

Plant Physiology   174 ( 3 ) 1621 - 1632   2017.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2017 American Society of Plant Biologists. All rights reserved. The phytohormone auxin regulates many aspects of growth and development in land plants, but the origin and evolution of auxin signaling and response mechanisms remain largely unknown. Indeed, it remains to be investigated whether auxin-related pathways diverged before the emergence of land plants. To address this knowledge deficit, we analyzed auxin responses in the charophyte alga Klebsormidium nitens NIES-2285, whose ancestor diverged from a green algal ancestor during the evolution of land plants. This strain is the same as Klebsormidium flaccidum NIES-2285, for which the draft genome was sequenced in 2014, and was taxonomically reclassified as K. nitens. This genome sequence revealed genes involved in auxin responses. Furthermore, the auxin indole-3-acetic acid (IAA) was detected in cultures of K. nitens, but K. nitens lacks the central regulators of the canonical auxin-signaling pathway found in land plants. Exogenous IAA inhibited cell division and cell elongation in K. nitens. Inhibitors of auxin biosynthesis and of polar auxin transport also inhibited cell division and elongation. Moreover, exogenous IAA rapidly induced expression of a LATERAL ORGAN BOUNDARIES-DOMAIN transcription factor. These results suggest that K. nitens has acquired the part of the auxin system that regulates transcription and cell growth without the requirement for the central players that govern auxin signaling in land plants.

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Overexpression of oligouridylate binding protein 1b results in ABA hypersensitivity

Cam Chau Nguyen, Kentaro Nakaminami, Akihiro Matsui, Shunsuke Watanabe, Yuri Kanno, Mitsunori Seo, Motoaki Seki

Plant Signaling and Behavior   12 ( 2 )   2017.02 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2017 Taylor & Francis Group, LLC. Oligouridylate binding protein 1b (UBP1b), a marker protein of plant stress granules (SGs), plays a role in heat stress tolerance in plants. A previous microarray analysis revealed that the expression of several ABA signaling-related genes is higher in UBP1b-overexpressing Arabidopsis plants (UBP1b-ox) subjected to both non-stressed and heat stress conditions. Root elongation and seed germination assays demonstrated that UBP1b-ox exhibited hypersensitivity to ABA. RT-qPCR analysis confirmed that mitogen-activated protein kinase (MAPK) cascade genes, such as MPK3, MKK4, and MKK9 were upregulated in UBP1b-ox plants. ABA receptor genes, including PYL5 and PYL6, were also upregulated in UBP1b-ox plants. mRNA of WRKY33–a downstream gene of MPK3 and an upstream gene of ethylene biosynthesis, exhibited high levels of accumulation, although the level of endogenous ABA was not significantly different between UBP1b-ox and control plants. In addition, RNA decay analysis revealed that WRKY33 was more stable in UBP1b-ox plants, indicating that the mRNA of WRKY33 was protected within UBP1b SGs. Collectively, these data demonstrate that UBP1b plays an important role in plant response to ABA.

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Jasmonates are induced by the PAMP flg22 but not the cell death-inducing elicitor Harpin in Vitis rupestris

Xiaoli Chang, Mitsunori Seo, Yumiko Takebayashi, Yuji Kamiya, Michael Riemann, Peter Nick

Protoplasma   254 ( 1 ) 271 - 283   2017.01 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2016, Springer-Verlag Wien. Plants employ two layers of defence that differ with respect to cell death: pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). In our previous work, we have comparatively mapped the molecular events in a cell system derived from the wild American grape Vitis rupestris, where cell death-independent defence can be triggered by PAMP flg22, whereas the elicitor Harpin activates a cell death-related ETI-like response. Both defence responses overlapped with respect to early events, such as calcium influx, apoplastic alkalinisation, oxidative burst, mitogen-activated protein kinase (MAPK) signalling, activation of defence-related genes and accumulation of phytoalexins. However, timing and amplitude of early signals differed. In the current study, we address the role of jasmonates (JAs) as key signalling compounds in hypersensitive cell death. We find, in V. rupestris, that jasmonic acid and its bioactive conjugate jasmonoyl-isoleucine (JA-Ile) rapidly accumulate in response to flg22 but not in response to Harpin. However, Harpin can induce programmed cell death, whereas exogenous methyl jasmonate (MeJA) fails to do so, although both signals induce a similar response of defence genes. Also in a second cell line from V. vinifera cv. `Pinot Noir', where Harpin cannot activate cell death and where flg22 fails to induce JA and JA-Ile, defence genes are activated in a similar manner. These findings indicate that the signal pathway culminating in cell death must act independently from the events culminating in the accumulation of toxic stilbenes.

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GTR1 is a jasmonic acid and jasmonoyl-L-isoleucine transporter in Arabidopsis thaliana

Yasuhiro Ishimaru, Takaya Oikawa, Takeshi Suzuki, Syohei Takeishi, Hideyuki Matsuura, Kosaku Takahashi, Shin Hamamoto, Nobuyuki Uozumi, Takafumi Shimizu, Mitsunori Seo, Hiroyuki Ohta, Minoru Ueda

Bioscience, Biotechnology and Biochemistry   81 ( 2 ) 249 - 255   2017 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2016 Japan Society for Bioscience, Biotechnology, and Agrochemistry. Jasmonates are major plant hormones involved in wounding responses. Systemic wounding responses are induced by an electrical signal derived from damaged leaves. After the signaling, jasmonic acid (JA) and jasmonoyl-L-isoleucine (JA-Ile) are translocated from wounded to undamaged leaves, but the molecular mechanism of the transport remains unclear. Here, we found that a JA-Ile transporter, GTR1, contributed to these translocations in Arabidopsis thaliana. GTR1 was expressed in and surrounding the leaf veins both of wounded and undamaged leaves. Less accumulations and translocation of JA and JA-Ile were observed in undamaged leaves of gtr1 at 30 min after wounding. Expressions of some genes related to wound responses were induced systemically in undamaged leaves of gtr1. These results suggested that GTR1 would be involved in the translocation of JA and JA-Ile in plant and may be contributed to correct positioning of JA and JA-Ile to attenuate an excessive wound response in undamaged leaves.

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N availability modulates the role of NPF3.1, a gibberellin transporter, in GA-mediated phenotypes in Arabidopsis

Laure C. David, Patrick Berquin, Yuri Kanno, Mitsunori Seo, Françoise Daniel-Vedele, Sylvie Ferrario-Méry

Planta   244 ( 6 ) 1315 - 1328   2016.12 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2016, Springer-Verlag Berlin Heidelberg. Main conclusion: AtNPF3.1gene expression is promoted by limiting nitrogen nutrition.Atnpf3.1mutants are affected in hypocotyl elongation and seed germination under conditions of low-nitrate availability. The NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER (NPF) family encodes nitrate or peptides transporters, some of which are also able to transport hormones. AtNPF3.1 has been described as a nitrate/nitrite/gibberellin transporter. Until now only its gibberellins (GAs) transport capacity have been proven in planta. We further analyzed its substrate specificity towards different GA species using a yeast heterologous system which revealed that (1) NPF3.1 transported not only bioactive GAs but also their precursors and metabolites and (2) the GAs' import activity of NPF3.1 was not affected by the presence of exogenous nitrate. Gene expression analysis along with germination assays and hypocotyl length measurements of loss of function mutants was used to understand the in planta role of NPF3.1. GUS staining revealed that this gene is expressed mainly in the endodermis of roots and hypocotyls, in shoots, stamens, and dry seeds. Germination assays in the presence of paclobutrazol, a GA biosynthesis inhibitor, revealed that the germination rate of npf3.1 mutants was lower compared to wild type when GA was added at the same time. Likewise, hypocotyl length measurements showed that the npf3.1 mutants were less sensitive to exogenous GA addition in the presence of paclobutrazol, compared to wild type. Moreover, this phenotype was observed only when plants were grown on low-nitrate supply. In addition, NPF3.1 gene expression was upregulated by low exogenous nitrate concentrations and the npf3.1 mutants exhibited a not yet described GA-related phenotype under these conditions. All together, these results indicated that NPF3.1 is indeed involved in GAs transport in planta under low-nitrate conditions.

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Abscinazole-E3M, a practical inhibitor of abscisic acid 8′-hydroxylase for improving drought tolerance

Jun Takeuchi, Masanori Okamoto, Ryosuke Mega, Yuri Kanno, Toshiyuki Ohnishi, Mitsunori Seo, Yasushi Todoroki

Scientific Reports   6   2016.11 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© The Author(s) 2016. Abscisic acid (ABA) is an essential phytohormone that regulates plant water use and drought tolerance. However, agricultural applications of ABA have been limited because of its rapid inactivation in plants, which involves hydroxylation of ABA by ABA 8′-hydroxylase (CYP707A). We previously developed a selective inhibitor of CYP707A, (-)-Abz-E2B, by structurally modifying S-uniconazole, which functions as an inhibitor of CYP707A and as a gibberellin biosynthetic enzyme. However, its synthetic yield is too low for practical applications. Therefore, we designed novel CYP707A inhibitors, Abz-T compounds, that have simpler structures in which the 1,2,3-triazolyl ring of (-)-Abz-E2B has been replaced with a triple bond. They were successfully synthesised in shorter steps, resulting in greater yields than that of (-)-Abz-E2B. In the enzymatic assays, one of the Abz-T compounds, (-)-Abz-E3M, acted as a strong and selective inhibitor of CYP707A, similar to (-)-Abz-E2B. Analysis of the biological effects in Arabidopsis revealed that (-)-Abz-E3M enhanced ABA's effects more than (-)-Abz-E2B in seed germination and in the expression of ABA-responsive genes. Treatment with (-)-Abz-E3M induced stomatal closure and improved drought tolerance in Arabidopsis. Furthermore, (-)-Abz-E3M also increased the ABA response in rice and maize. Thus, (-)-Abz-E3M is a more practical and effective inhibitor of CYP707A than (-)-Abz-E2B.

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The wheat ABA hypersensitive ERA8 mutant is associated with increased preharvest sprouting tolerance and altered hormone accumulation

Shantel A. Martinez, Keiko M. Tuttle, Yumiko Takebayashi, Mitsunori Seo, Kimberly Garland Campbell, Camille M. Steber

Euphytica   212 ( 2 ) 229 - 245   2016.11 [ Peer Review Accepted ]

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© 2016, Springer Science+Business Media Dordrecht (outside the USA). Wheat preharvest sprouting (PHS) is the germination of mature grain on the mother plant when rain occurs before harvest. Higher abscisic acid (ABA) hormone levels and sensitivity are associated with higher seed dormancy and PHS tolerance. Consistent with this, the ABA hypersensitive ENHANCED RESPONSE TO ABA8 (ERA8) mutant resulted in increased dormancy and PHS tolerance in soft white spring wheat `Zak'. ERA8 seeds were initially less responsive to germination-rescue by the hormone gibberellin (GA). ERA8 gained GA and lost ABA sensitivity more slowly than wild-type during dormancy loss through after-ripening and cold imbibition. This study examined if increased ABA sensitivity in ERA8 likely resulted from increased ABA signaling or increased ABA hormone levels. Zak ERA8 had higher initial grain dormancy although endogenous embryo ABA levels were similar in Zak ERA8 and wild-type, suggesting that increased dormancy was due to increased ABA signaling rather than increased ABA accumulation. ABA levels declined with Zak ERA8 after-ripening, suggesting that ABA turnover is not defective. Elevated ERA8 dormancy was also associated with increased embryonic jasmonic acid-Ile and aleurone indole-3-acetic acid (IAA) levels. The possible implication that other plant hormones may influence wheat seed dormancy and germination are discussed.

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AtSWEET13 and AtSWEET14 regulate gibberellin-mediated physiological processes

Yuri Kanno, Takaya Oikawa, Yasutaka Chiba, Yasuhiro Ishimaru, Takafumi Shimizu, Naoto Sano, Tomokazu Koshiba, Yuji Kamiya, Minoru Ueda, Mitsunori Seo

Nature Communications   7   2016.10 [ Peer Review Accepted ]

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© 2016 The Author(s). Transmembrane transport of plant hormones is required for plant growth and development. Despite reports of a number of proteins that can transport the plant hormone gibberellin (GA), the mechanistic basis for GA transport and the identities of the transporters involved remain incomplete. Here, we provide evidence that Arabidopsis SWEET proteins, AtSWEET13 and AtSWEET14, which are members of a family that had previously been linked to sugar transport, are able to mediate cellular GA uptake when expressed in yeast and oocytes. A double sweet13 sweet14 mutant has a defect in anther dehiscence and this phenotype can be reversed by exogenous GA treatment. In addition, sweet13 sweet14 exhibits altered long distant transport of exogenously applied GA and altered responses to GA during germination and seedling stages. These results suggest that AtSWEET13 and AtSWEET14 may be involved in modulating GA response in Arabidopsis.

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Dissection of Arabidopsis NCED9 promoter regulatory regions reveals a role for ABA synthesized in embryos in the regulation of GA-dependent seed germination

Mitsunori Seo, Yuri Kanno, Anne Frey, Helen M. North, Annie Marion-Poll

Plant Science   246   91 - 97   2016.05 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2016 Elsevier Ireland Ltd. Nine-cis-epoxycarotenoid dioxygenase (NCED) catalyzes the key step of abscisic acid (ABA) biosynthesis. There are five genes encoding NCED in Arabidopsis, which differentially regulate ABA biosynthesis in a spatiotemporal manner in response to endogenous and environmental stimuli. Previous studies have shown that NCED9 is expressed in testa and embryos during seed development. In the present study, we have identified promoter regions required for the expression of NCED9 in testa and embryos, respectively. Electrophoretic mobility shift assays (EMSA) and yeast one-hybrid (Y1H) assays showed that several homeodomain-leucine zipper (HD-Zip) proteins, namely ATHBs, bound to the sequence required for expression of NCED9 in testa, suggesting that they redundantly regulate NCED9 expression. By expressing the NCED9 gene under the control of a deleted NCED9 promoter in an nced9 mutant expression was limited to embryos. Transformants were complemented for the paclobutrazol resistant germination phenotype of the mutant, suggesting that the ABA synthesis mediated by NCED9 in embryos plays an important role in the regulation of gibberellin (GA)-dependent seed germination.

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GOLDEN 2-LIKE transcription factors for chloroplast development affect ozone tolerance through the regulation of stomatal movement

Yukari Nagatoshi, Nobutaka Mitsuda, Maki Hayashi, Shin Ichiro Inoue, Eiji Okuma, Akihiro Kubo, Yoshiyuki Murata, Mitsunori Seo, Hikaru Saji, Toshinori Kinoshita, Masaru Ohme-Takagi

Proceedings of the National Academy of Sciences of the United States of America   113 ( 15 ) 4218 - 4223   2016.04 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Stomatal movements regulate gas exchange, thus directly affecting the efficiency of photosynthesis and the sensitivity of plants to air pollutants such as ozone. The GARP family transcription factors GOLDEN 2-LIKE1 (GLK1) and GLK2 have known functions in chloroplast development. Here, we show that Arabidopsis thaliana (A. thaliana) plants expressing the chimeric repressors for GLK1 and-2 (GLK1/2-SRDX) exhibited a closed-stomata phenotype and strong tolerance to ozone. By contrast, plants that overexpress GLK1/2 exhibited an open-stomata phenotype and higher sensitivity to ozone. The plants expressing GLK1-SRDX had reduced expression of the genes for inwardly rectifying K+ (K+in) channels and reduced K+in channel activity. Abscisic acid treatment did not affect the stomatal phenotype of 35S:GLK1/2-SRDX plants or the transcriptional activity for K+in channel gene, indicating that GLK1/2 act independently of abscisic acid signaling. Our results indicate that GLK1/2 positively regulate the expression of genes for K+in channels and promote stomatal opening. Because the chimeric GLK1-SRDX repressor driven by a guard cell-specific promoter induced a closed-stomata phenotype without affecting chloroplast development in mesophyll cells, modulating GLK1/2 activity may provide an effective tool to control stomatal movements and thus to confer resistance to air pollutants.

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Highly sprouting-tolerant wheat grain exhibits extreme dormancy and cold imbibition-resistant accumulation of abscisic acid

Yu Ichi Kashiwakura, Daisuke Kobayashi, Yusuke Jikumaru, Yumiko Takebayashi, Eiji Nambara, Mitsunori Seo, Yuji Kamiya, Tetsuo Kushiro, Naoto Kawakami

Plant and Cell Physiology   57 ( 4 ) 715 - 732   2016.04 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2016 The Author 2016. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. Pre-harvest sprouting (PHS) of wheat (Triticum aestivum L.) grains induces hydrolyzing enzymes such as α-amylase, which considerably decreases wheat product quality. PHS occurs when cool and wet weather conditions before harvest break dormancy and induce grain germination. In this study, we used PHS-tolerant varieties, Gifu-komugi (Gifu) and OS38, to characterize the mechanisms of both dormancy breakage and dormancy maintenance at low temperatures. Physiologically mature Gifu grains exhibited dormancy after imbibition at 20°C, but germinated at 15°C. In contrast, OS38 grains remained dormant even at temperatures as low as 5°C. Embryo half-grains cut out from the dormant Gifu grains germinated by imbibition at 20°C, similar to conventional varieties worldwide. However, OS38 embryo half-grains were still dormant. Hormonome and pharmacological analyses suggested that ABA and gibberellin metabolism are important for temperature-dependent dormancy maintenance and breakage. Imbibition at 15°C decreased ABA levels but increased gibberellin levels in embryos of freshly harvested Gifu grains. Additionally, low temperatures induced expression of the ABA catabolism genes, TaABA8′OH1 and TaABA8′OH2, and the gibberellin biosynthesis gene, TaGA3ox2, in the embryos. However, in embryos of freshly harvested OS38 grains, ABA levels were increased while gibberellin levels were suppressed at 15°C. In these dormant embryos, low temperatures induced the TaNCED ABA biosynthesis genes, but suppressed TaABA8′OH2 and TaGA3ox2. These results show that the regulatory mechanism influencing the expression of ABA and gibberellin metabolism genes may be critical for dormancy maintenance and breakage at low temperatures. Our findings should help improve PHS-resistant wheat breeding programs.

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Staying Alive: Molecular Aspects of Seed Longevity

Sano N, Rajjou L, North HM, Debeaujon I, Marion-Poll A, Seo M

Plant and Cell Physiology   57   660 - 674   2016 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

Impact of the plastidial stringent response in plant growth and stress responses

Mikika Maekawa, Rina Honoki, Yuta Ihara, Ryoichi Sato, Akira Oikawa, Yuri Kanno, Hiroyuki Ohta, Mitsunori Seo, Kazuki Saito, Shinji Masuda

Nature Plants   1   2015.11 [ Peer Review Accepted ]

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© 2015 Nature Publishing Group. The regulatory nucleotide guanosine 5′-diphosphate 3′-diphosphate (ppGpp), which was originally identified in Escherichia coli, controls the expression of a large gene set and many enzyme activities. The ppGpp-dependent control of cell activities is referred to as the stringent response. Recently, genes responsible for the synthesis and degradation of ppGpp have been identified not only in bacteria, but also in eukaryotes, including plants and animals, indicating that the stringent response is, unexpectedly, widely conserved. However, the exact function of the eukaryotic stringent response remains elusive. Here, we isolated an Arabidopsis mutant that overproduces ppGpp in chloroplasts. This mutant shows metabolite reduction, dwarf chloroplasts and significantly suppressed plastidial transcription and translation. Under nutrient-deficient conditions, the mutant shows more robust growth than the wild type. These results indicate that the ppGpp-dependent control of the organelle function is crucial for the systematic growth of host organisms.

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Sustained low abscisic acid levels increase seedling vigor under cold stress in rice (Oryza sativa L.)

Ryosuke Mega, Ayano Meguro-Maoka, Akira Endo, Etsuo Shimosaka, Seiji Murayama, Eiji Nambara, Mitsunori Seo, Yuri Kanno, Suzanne R. Abrams, Yutaka Sato

Scientific Reports   5   2015.09 [ Peer Review Accepted ]

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Stress-induced abscisic acid (ABA) is mainly catabolized by ABA 8′-hydroxylase (ABA8ox), which also strictly regulates endogenous ABA levels. Although three members of the ABA8ox gene family are conserved in rice, it is not clear which stressors induce expression of these genes. Here, we found that OsABA8ox1 was induced by cold stress within 24h and that OsABA8ox2 and OsABA8ox3 were not. In contrast, OsABA8ox2 and OsABA8ox3 were ABA-inducible, but OsABA8ox1 was not. OsABA8ox1, OsABA8ox2, and OsABA8ox3 restored germination of a cyp707a1/a2/a3 triple mutant of Arabidopsis to rates comparable to those of the wild type, indicating that OsABA8ox1, OsABA8ox2, and OsABA8ox3 function as ABA-catabolic genes in vivo. Transgenic rice lines overexpressing OsABA8ox1 showed decreased levels of ABA and increased seedling vigor at 15°C. These results indicate that sustained low levels of ABA lead to increased seedling vigor during cold stress. On the other hand, excessively low endogenous ABA levels caused reduced drought and cold tolerance, although some of the transgenic rice lines expressing OsABA8ox1 at moderate levels did not show these harmful effects. Adequate regulation of endogenous ABA levels is thought to be crucial for maintaining seedling vigor under cold stress and for cold and drought tolerance in rice.

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A balanced JA/ABA status may correlate with adaptation to osmotic stress in Vitis cells

Ahmed Ismail, Mitsunori Seo, Yumiko Takebayashi, Yuji Kamiya, Peter Nick

Journal of Plant Physiology   185   57 - 64   2015.08 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2015 Elsevier GmbH. Water-related stress is considered a major type of plant stress. Osmotic stress, in particular, represents the common part of all water-related stresses. Therefore, plants have evolved different adaptive mechanisms to cope with osmotic-related disturbances. In the current work, two grapevine cell lines that differ in their osmotic adaptability, Vitis rupestris and Vitis riparia, were investigated under mannitol-induced osmotic stress. To dissect signals that lead to adaptability from those related to sensitivity, osmotic-triggered responses with respect to jasmonic acid (JA) and its active form JA-Ile, abscisic acid (ABA), and stilbene compounds, as well as the expression of their related genes were observed. In addition, the transcript levels of the cellular homeostasis gene NHX1 were examined. The data are discussed with a hypothesis suggesting that a balance of JA and ABA status might correlate with cellular responses, either guiding cells to sensitivity or to progress toward adaptation.

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Identification of Arabidopsis thaliana NRT1/PTR FAMILY (NPF) proteins capable of transporting plant hormones

Yasutaka Chiba, Takafumi Shimizu, Shinya Miyakawa, Yuri Kanno, Tomokazu Koshiba, Yuji Kamiya, Mitsunori Seo

Journal of Plant Research   128 ( 4 ) 679 - 686   2015.07 [ Peer Review Accepted ]

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© 2015, The Botanical Society of Japan and Springer Japan. NRT1/PTR FAMILY (NPF) proteins were originally identified as nitrate or di/tri-peptide transporters. Recent studies revealed that this transporter family also transports the plant hormones auxin (indole-3-acetic acid), abscisic acid (ABA), and gibberellin (GA), as well as secondary metabolites (glucosinolates). We developed modified yeast two-hybrid systems with receptor complexes for GA and jasmonoyl-isoleucine (JA-Ile), to detect GA and JA-Ile transport activities of proteins expressed in the yeast cells. Using these GA and JA-Ile systems as well as the ABA system that we had introduced previously, we determined the capacities of Arabidopsis NPFs to transport these hormones. Several NPFs induced the formation of receptor complexes under relatively low hormone concentrations. Hormone transport activities were confirmed for some NPFs by direct analysis of hormone uptake of yeast cells by liquid chromatography–tandem mass spectrometry. Our results suggest that at least some NPFs could function as hormone transporters.

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The WRKY45-dependent signaling pathway is required for resistance against Striga hermonthica parasitism

J. Musembi Mutuku, Satoko Yoshida, Takafumi Shimizu, Yasunori Ichihashi, Takanori Wakatake, Akira Takahashi, Mitsunori Seo, Ken Shirasu

Plant Physiology   168 ( 3 ) 1152 - 1163   2015.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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© 2015 American Society of Plant Biologists. All rights reserved. The root hemiparasite witchweed (Striga spp.) is a devastating agricultural pest that causes losses of up to $1 billion US annually in sub-Saharan Africa. Development of resistant crops is one of the cost-effective ways to address this problem. However, the molecular mechanisms underlying resistance are not well understood. To understand molecular events upon Striga spp. infection, we conducted genome-scale RNA sequencing expression analysis using Striga hermonthica-infected rice (Oryza sativa) roots. We found that transcripts grouped under the Gene Ontology term defense response were significantly enriched in up-regulated differentially expressed genes. In particular, we found that both jasmonic acid (JA) and salicylic acid (SA) pathways were induced, but the induction of the JA pathway preceded that of the SA pathway. Foliar application of JA resulted in higher resistance. The hebiba mutant plants, which lack the JA biosynthesis gene ALLENE OXIDE CYCLASE, exhibited severe S. hermonthica susceptibility. The resistant phenotype was recovered by application of JA. By contrast, the SA-deficient NahG rice plants were resistant against S. hermonthica, indicating that endogenous SA is not required for resistance. However, knocking down WRKY45, a regulator of the SA/benzothiadiazole pathway, resulted in enhanced susceptibility. Interestingly, NahG plants induced the JA pathway, which was down-regulated in WRKY45-knockdown plants, linking the resistant and susceptible phenotypes to the JA pathway. Consistently, the susceptibility phenotype in the WRKY45-knockdown plants was recovered by foliar JA application. These results point to a model in which WRKY45 modulates a cross talk in resistance against S. hermonthica by positively regulating both SA/benzothiadiazole and JA pathways.

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