HY5 functions as a systemic signal by integrating BRC1-dependent hormone signaling in tomato bud outgrowth.

Light plays an important role in determining plant architecture, which greatly influences crop yield. However, the precise mechanisms by which light signaling regulates bud outgrowth remain to be identified. Here, we show that light regulates bud outgrowth via both HY5 and brassinosteroid (BR)-dependent pathways in tomato. Inactivation of the red-light photoreceptor PHYB, or deficiencies in PHYB or the blue-light photoreceptor CRY1a, inhibits bud outgrowth and leads to decreased accumulation of HY5 protein and increased transcript level of BRANCHED1 (BRC1), a central integrator of branching signals. HY5, functioning as a mobile systemic signal from leaves, promotes bud outgrowth by directly suppressing BRC1 transcript and activating the transcript of BR biosynthesis genes within the lateral buds in tomato. Furthermore, BRC1 prevents the accumulation of cytokinin (CK) and gibberellin (GA) by directly inhibiting the transcript of CK synthesis gene LOG4, while increasing the transcript levels of CK and GA degradation genes (CKX7, GA2ox4, and GA2ox5), leading to an arrest of bud outgrowth. Moreover, bud outgrowth occurs predominantly in the day due to the suppression of BRC1 transcript by HY5. These findings demonstrate that light-inducible HY5 acts as a systemic signaling factor in fine-tuning the bud outgrowth of tomato.

The RALF2-FERONIA-MYB63 module orchestrates growth and defense in tomato roots.

Plant secreted peptides RAPID ALKALINISATION FACTORs (RALFs), which act through the receptor FERONIA (FER), play important roles in plant growth. However, it remains unclear whether and how RALF-FER contributes to the trade-off of plant growth-defense. Here, we used a variety of techniques such as CRISPR/Cas9, protein-protein interaction and transcriptional regulation methods to investigate the role of RALF2 and its receptor FER in regulating lignin deposition, root growth, and defense against Fusarium oxysporum f. sp. lycopersici (Fol) in tomato (Solanum lycopersicum). The ralf2 and fer mutants show reduced primary root length, elevated lignin accumulation, and enhanced resistance against Fol than the wild-type. FER interacts with and phosphorylates MYB63 to promote its degradation. MYB63 serves as an activator of lignin deposition by regulating the transcription of dirigent protein gene DIR19. Mutation of DIR19 suppresses lignin accumulation, and reverses the short root phenotype and Fol resistance in ralf2 or fer mutant. Collectively, our results demonstrate that the RALF2-FER-MYB63 module fine-tunes root growth and resistance against Fol through regulating the deposition of lignin in tomato roots. The study sheds new light on how plants maintain the growth-defense balance via RALF-FER.

Far-red light inhibits lateral bud growth mainly through enhancing apical dominance independently of strigolactone synthesis in tomato.

The ratio of red light to far-red light (R:FR) is perceived by light receptors and consequently regulates plant architecture. Regulation of shoot branching by R:FR ratio involves plant hormones. However, the roles of strigolactone (SL), the key shoot branching hormone and the interplay of different hormones in the light regulation of shoot branching in tomato (Solanum lycopersicum) are elusive. Here, we found that defects in SL synthesis genes CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7) and CCD8 in tomato resulted in more lateral bud growth but failed to reverse the FR inhibition of lateral bud growth, which was associated with increased auxin synthesis and decreased synthesis of cytokinin (CK) and brassinosteroid (BR). Treatment of auxin also inhibited shoot branching in ccd mutants. However, CK released the FR inhibition of lateral bud growth in ccd mutants, concomitant with the upregulation of BR synthesis genes. Furthermore, plants that overexpressed BR synthesis gene showed more lateral bud growth and the shoot branching was less sensitive to the low R:FR ratio. The results indicate that SL synthesis is dispensable for light regulation of shoot branching in tomato. Auxin mediates the response to R:FR ratio to regulate shoot branching by suppressing CK and BR synthesis.

Characterization and quality evaluation of QiXueShuFu Decoction based on fingerprint and ultra-performance liquid chromatography-quadrupole-orbitrap mass spectrometry.

QiXueShuFu Decoction (QXSFD) modified from the Bazhen Decoction which was originally from the classic Ming Dynasty is a traditional folk formula that boosts the body's immune system. However, its ambiguous chemical components limited its quality control evaluation. In this study, ultra-performance liquid chromatography (UPLC) fingerprint combined with multivariate analysis was used to evaluate the quality of 15 batches of QXSFD, and UPLC quadrupole-orbitrap mass spectrometry was used to further examine the chemical components in QXSFD, after which representative compounds from each disassembled prescription were selected for comparison. Fifteen batches of samples had 33 common peaks in which 11 differential components could be used as a reference for subsequent quality control. One hundred forty-three components were identified from QXSFD. Saponins were mainly derived from the monarch, terpenes from the minister, and polysaccharides and glycosides from the assistant. In addition, quantitative assay revealed that the content of ferulic acid, chlorogenic acid, 2,3,5,4'-tetrahydroxystilbene-2-O-ß-D-glucoside and 3,6'-disinapoyl sucrose in the whole prescription were higher than the contents of each disassembled prescription. This is the first comprehensive quality report on the chemical components of QXSFD, which is important for pharmacodynamic material basis and quality control.

Brassinosteroid signaling integrates multiple pathways to release apical dominance in tomato.

The control of apical dominance involves auxin, strigolactones (SLs), cytokinins (CKs), and sugars, but the mechanistic controls of this regulatory network are not fully understood. Here, we show that brassinosteroid (BR) promotes bud outgrowth in tomato through the direct transcriptional regulation of BRANCHED1 (BRC1) by the BR signaling component BRASSINAZOLE-RESISTANT1 (BZR1). Attenuated responses to the removal of the apical bud, the inhibition of auxin, SLs or gibberellin synthesis, or treatment with CK and sucrose, were observed in bud outgrowth and the levels of BRC1 transcripts in the BR-deficient or bzr1 mutants. Furthermore, the accumulation of BR and the dephosphorylated form of BZR1 were increased by apical bud removal, inhibition of auxin, and SLs synthesis or treatment with CK and sucrose. These responses were decreased in the DELLA-deficient mutant. In addition, CK accumulation was inhibited by auxin and SLs, and decreased in the DELLA-deficient mutant, but it was increased in response to sucrose treatment. CK promoted BR synthesis in axillary buds through the action of the type-B response regulator, RR10. Our results demonstrate that BR signaling integrates multiple pathways that control shoot branching. Local BR signaling in axillary buds is therefore a potential target for shaping plant architecture.

The APETALA2a/DWARF/BRASSINAZOLE-RESISTANT 1 module contributes to carotenoid synthesis in tomato fruits.

Ethylene (ET) signaling plays a critical role in the ripening of climacteric fruits such as tomato. Brassinosteroids (BRs) were found to promote the ripening of both climacteric and non-climacteric fruits. However, the mechanism of interaction between ET and BRs during fruit ripening is unclear. Here, we found that BR synthesis and signaling increased after the onset of fruit ripening. Overexpression of the BR synthesis gene DWARF (DWF) promotedfruit softening, lycopene synthesis and ET production, whereas defect of DWF inhibited them. BRASSINAZOLE RESISTANT 1 (BZR1) as a key component of BR signaling, enhanced fruit lycopene content by directly activating the transcription of PSY1 gene. Interestingly, the increases in BR synthesis and BZR1 protein levels were dependent on ET signaling. Knocking out the ET-induced APETALA2a (AP2a) suppressed the expression of DWF and BR accumulation. Molecular assays demonstrated that AP2a was a positive regulator of DWF expression. Furthermore, 28-homobrassinolide, a bioactive BR, partially compensated the defects of lycopene accumulation and expression of PSY1 in ap2a mutant fruits. The results demonstrated that AP2a mediated ET signaling to regulate BR synthesis and signaling. BRs played critical roles in lycopene synthesis after onset of fruit ripening.

SlMPK1- and SlMPK2-mediated SlBBX17 phosphorylation positively regulates CBF-dependent cold tolerance in tomato.

B-box (BBX) proteins are an important class of zinc finger transcription factors that play a critical role in plant growth and stress response. However, the mechanisms of how BBX proteins participate in the cold response in tomato remain unclear. Here, using approaches of reverse genetics, biochemical and molecular biology we characterized a BBX transcription factor, SlBBX17, which positively regulates cold tolerance in tomato (Solanum lycopersicum). Overexpressing SlBBX17 enhanced C-repeat binding factor (CBF)-dependent cold tolerance in tomato plants, whereas silencing SlBBX17 increased plant susceptibility to cold stress. Crucially, the positive role of SlBBX17 in CBF-dependent cold tolerance was dependent on ELONGATED HYPOCOTYL5 (HY5). SlBBX17 physically interacted with SlHY5 to directly promote the protein stability of SlHY5 and subsequently increased the transcriptional activity of SlHY5 on SlCBF genes under cold stress. Further experiments showed that cold-activated mitogen-activated protein kinases, SlMPK1 and SlMPK2, also physically interact with and phosphorylate SlBBX17 to enhance the interaction between SlBBX17 and SlHY5, leading to enhanced CBF-dependent cold tolerance. Collectively, the study unveiled a mechanistic framework by which SlMPK1/2-SlBBX17-SlHY5 regulated transcription of SlCBFs to enhance cold tolerance, thereby shedding light on the molecular mechanisms of how plants respond to cold stress via multiple transcription factors.

The transcription factor SPL13 mediates strigolactone suppression of shoot branching by inhibiting cytokinin synthesis in Solanum lycopersicum.

Plant architecture imposes a large impact on crop yield. IDEAL PLANT ARCHITECTURE 1 (IPA1), which encodes a SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor, is a target of molecular design for improving grain yield. However, the roles of SPL transcription factors in regulating tomato (Solanum lycopersicum) plant architecture are unclear. Here, we show that the expression of SPL13 is down-regulated in the lateral buds of strigolactone (SL)-deficient ccd mutants and is induced by GR24 (a synthetic analog of SL). Knockout of SPL13 by CRISPR/Cas9 resulted in higher levels of cytokinins (CKs) and transcripts of the CK synthesis gene ISOPENTENYL TRANSFERASES 1 (IPT1) in the stem nodes, and more growth of lateral buds. GR24 suppresses CK synthesis and lateral bud growth in ccd mutants, but is not effective in spl13 mutants. On the other hand, silencing of the IPT1 gene inhibited bud growth of spl13 mutants. Interestingly, SL levels in root extracts and exudates are significantly increased in spl13 mutants. Molecular studies indicated that SPL13 directly represses the transcription of IPT1 and the SL synthesis genes CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7) and MORE AXILLARY GROWTH 1 (MAX1). The results demonstrate that SPL13 acts downstream of SL to suppress lateral bud growth by inhibiting CK synthesis in tomato. Tuning the expression of SPL13 is a potential approach for decreasing the number of lateral shoots in tomato.

The phyB-dependent induction of HY5 promotes iron uptake by systemically activating FER expression.

Iron (Fe) deficiency affects global crop productivity and human health. However, the role of light signaling in plant Fe uptake remains uncharacterized. Here, we find that light-induced Fe uptake in tomato (Solanum lycopersicum L.) is largely dependent on phytochrome B (phyB). Light induces the phyB-dependent accumulation of ELONGATED HYPOCOTYL 5 (HY5) protein both in the leaves and roots. HY5 movement from shoots to roots activates the expression of FER transcription factor, leading to the accumulation of transcripts involved in Fe uptake. Mutation in FER abolishes the light quality-induced changes in Fe uptake. The low Fe uptake observed in phyB, hy5, and fer mutants is accompanied by lower photosynthetic electron transport rates. Exposure to red light at night increases Fe accumulation in wild-type fruit but has little effects on fruit of phyB mutants. Taken together, these results demonstrate that Fe uptake is systemically regulated by light in a phyB-HY5-FER-dependent manner. These findings provide new insights how the manipulation of light quality could be used to improve Fe uptake and hence the nutritional quality of crops.

Brassinosteroid signaling positively regulates abscisic acid biosynthesis in response to chilling stress in tomato.

Brassinosteroids (BRs) and abscisic acid (ABA) are essential regulators of plant growth and stress tolerance. Although the antagonistic interaction of BRs and ABA is proposed to ensure the balance between growth and defense in model plants, the crosstalk between BRs and ABA in response to chilling in tomato (Solanum lycopersicum), a warm-climate horticultural crop, is unclear. Here, we determined that overexpression of the BR biosynthesis gene DWARF (DWF) or the key BR signaling gene BRASSINAZOLE-RESISTANT1 (BZR1) increases ABA levels in response to chilling stress via positively regulating the expression of the ABA biosynthesis gene 9-CIS-EPOXYCAROTENOID DIOXYGENASE1 (NCED1). BR-induced chilling tolerance was mostly dependent on ABA biosynthesis. Chilling stress or high BR levels decreased the abundance of BRASSINOSTEROID-INSENSITIVE2 (BIN2), a negative regulator of BR signaling. Moreover, we observed that chilling stress increases BR levels and results in the accumulation of BZR1. BIN2 negatively regulated both the accumulation of BZR1 protein and chilling tolerance by suppressing ABA biosynthesis. Our results demonstrate that BR signaling positively regulates chilling tolerance via ABA biosynthesis in tomato. The study has implications in production of warm-climate crops in horticulture.

Light-dependent activation of HY5 promotes mycorrhizal symbiosis in tomato by systemically regulating strigolactone biosynthesis.

Light quality affects mutualisms between plant roots and arbuscular mycorrhizal fungi (AMFs), which modify nutrient acquisition in plants. However, the mechanisms by which light systemically modulates root colonization by AMFs and phosphate uptake in roots remain unclear. We used a range of approaches, including grafting techniques, protein immunoblot analysis, electrophoretic mobility shift assay, chromatin immunoprecipitation, and dual-luciferase assays, to unveil the molecular basis of light signal transmission from shoot to root that mediates arbuscule development and phosphate uptake in tomato. The results show that shoot phytochrome B (phyB) triggers shoot-derived mobile ELONGATED HYPOCOTYL5 (HY5) protein accumulation in roots, and HY5 further positively regulates transcription of strigolactone (SL) synthetic genes, thus forming a shoot phyB-dependent systemic signaling pathway that regulates the synthesis and accumulation of SLs in roots. Further experiments with carotenoid cleavage dioxygenase 7 mutants and supplementary red light confirm that SLs are indispensable in the red-light-regulated mycorrhizal symbiosis in roots. Our results reveal a phyB-HY5-SLs systemic signaling cascade that facilitates mycorrhizal symbiosis and phosphate utilization in plants. The findings provide new prospects for the potential application of AMFs and light manipulation to effectively improve nutrient utilization and minimize the use of chemical fertilizers and associated pollution.

Brassinosteroid-mediated reactive oxygen species are essential for tapetum degradation and pollen fertility in tomato.

Phytohormone brassinosteroids (BRs) are essential for plant growth and development, but the mechanisms of BR-mediated pollen development remain largely unknown. In this study, we show that pollen viability, pollen germination and seed number decreased in the BR-deficient mutant d^im , which has a lesion in the BR biosynthetic gene DWARF (DWF), and in the bzr1 mutant, which is deficient in BR signaling regulator BRASSINAZOLE RESISTANT 1 (BZR1), compared with those in wild-type plants, whereas plants overexpressing DWF or BZR1 exhibited the opposite effects. Loss or gain of function in the DWF or BZR1 genes altered the timing of reactive oxygen species (ROS) production and programmed cell death (PCD) in tapetal cells, resulting in delayed or premature tapetal degeneration, respectively. Further analysis revealed that BZR1 could directly bind to the promoter of RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1), and that RBOH1-mediated ROS promote pollen and seed development by triggering PCD and tapetal cell degradation. In contrast, the suppression of RBOH1 compromised BR signaling-mediated ROS production and pollen development. These findings provide strong evidence that BZR1-dependent ROS production plays a critical role in the BR-mediated regulation of tapetal cell degeneration and pollen development in Solanum lycopersicum (tomato) plants.

A Plant Phytosulfokine Peptide Initiates Auxin-Dependent Immunity through Cytosolic Ca2+ Signaling in Tomato.

Phytosulfokine (PSK) is a disulfated pentapeptide that is an important signaling molecule. Although it has recently been implicated in plant defenses to pathogen infection, the mechanisms involved remain poorly understood. Using surface plasmon resonance and gene silencing approaches, we showed that the tomato (Solanum lycopersicum) PSK receptor PSKR1, rather than PSKR2, functioned as the major PSK receptor in immune responses. Silencing of PSK signaling genes rendered tomato more susceptible to infection by the economically important necrotrophic pathogen Botrytis cinerea Analysis of tomato mutants defective in either defense hormone biosynthesis or signaling demonstrated that PSK-induced immunity required auxin biosynthesis and associated defense pathways. Here, using aequorin-expressing tomato plants, we provide evidence that PSK perception by tomato PSKR1 elevated cytosolic [Ca2+], leading to auxin-dependent immune responses via enhanced binding activity between calmodulins and the auxin biosynthetic YUCs. Thus, our data demonstrate that PSK acts as a damage-associated molecular pattern and is perceived mainly by PSKR1, which increases cytosolic [Ca2+] and activates auxin-mediated pathways that enhance immunity of tomato plants to B. cinerea.

BZR1 Mediates Brassinosteroid-Induced Autophagy and Nitrogen Starvation in Tomato.

Autophagy, an innate cellular destructive mechanism, plays crucial roles in plant development and responses to stress. Autophagy is known to be stimulated or suppressed by multiple molecular processes, but the role of phytohormone signaling in autophagy is unclear. Here, we demonstrate that the transcripts of autophagy-related genes (ATGs) and the formation of autophagosomes are triggered by enhanced levels of brassinosteroid (BR). Furthermore, the BR-activated transcription factor brassinazole-resistant1 (BZR1), a positive regulator of the BR signaling pathway, is involved in BR-induced autophagy. Treatment with BR enhanced the formation of autophagosomes and the transcripts of ATGs in BZR1-overexpressing plants, while the effects of BR were compromised in BZR1-silenced plants. Yeast one-hybrid analysis and chromatin immunoprecipitation coupled with quantitative polymerase chain reaction revealed that BZR1 bound to the promoters of ATG2 and ATG6 The BR-induced formation of autophagosomes decreased in ATG2- and ATG6-silenced plants. Moreover, exogenous application of BR enhanced chlorophyll content and autophagosome formation and decreased the accumulation of ubiquitinated proteins under nitrogen starvation. Leaf chlorosis and chlorophyll degradation were exacerbated in BZR1-silenced plants and the BR biosynthetic mutant d^im but were alleviated in BZR1- and BZR1-1D-overexpressing plants under nitrogen starvation. Meanwhile, nitrogen starvation-induced expression of ATGs and autophagosome formation were compromised in both BZR1-silenced and d^im plants but were increased in BZR1- and BZR1-1D-overexpressing plants. Taken together, our results suggest that BZR1-dependent BR signaling up-regulates the expression of ATGs and autophagosome formation, which plays a critical role in the plant response to nitrogen starvation in tomato (Solanum lycopersicum).

Brassinosteroids Act as a Positive Regulator of Photoprotection in Response to Chilling Stress.

Photoprotection is an important strategy adopted by plants to avoid photoinhibition under stress conditions. However, the way in which photoprotection is regulated is not fully understood. Here, we demonstrate that tomato (Solanum lycopersicum) mutants of brassinosteroid (BR) biosynthesis (dwf) and related signaling through BRASSINAZOLE-RESISTANT1 (bzr1) are more sensitive to (PSII and PSI photoinhibition, with decreased cyclic electron flow around PSI and lower nonphotochemical quenching, accumulation of PSII subunit S (PsbS), violaxanthin deepoxidase (VDE) activity, and D1 protein abundance. Chilling induced the accumulation of active BRs and activated BZR1, which directly activates the transcription of RESPIRATORY BURST OXIDASE HOMOLOG1 (RBOH1) and hydrogen peroxide production in the apoplast. While apoplastic hydrogen peroxide is essential for the induction of PROTON GRADIENT REGULATION5 (PGR5)-dependent cyclic electron flow, PGR5 participates in the regulation of chilling- and BR-dependent induction of nonphotochemical quenching, accumulation of D1, VDE, and PsbS proteins, transcription of genes involved in redox signaling, hormone signaling, and activity of several antioxidant enzymes. Mutations in BZR1 and PGR5 or suppressed transcription of RBOH1 compromised chilling- and BR-induced photoprotection, resulting in increased sensitivity to photoinhibition. These results demonstrate that BRs act as a positive regulator of photoprotection in a redox-PGR5-dependent manner in response to chilling stress in tomato.

SlHY5 Integrates Temperature, Light, and Hormone Signaling to Balance Plant Growth and Cold Tolerance.

During the transition from warm to cool seasons, plants experience decreased temperatures, shortened days, and decreased red/far-red (R/FR) ratios of light. The mechanism by which plants integrate these environmental cues to maintain plant growth and adaptation remains poorly understood. Here, we report that low temperature induced the transcription of PHYTOCHROME A and accumulation of LONG HYPOCOTYL5 (SlHY5, a basic Leu zipper transcription factor) in tomato (Solanum lycopersicum) plants, especially under short day conditions with low R/FR light ratios. Reverse genetic approaches and physiological analyses revealed that silencing of SlHY5 increased cold susceptibility in tomato plants, whereas overexpression of SlHY5 enhanced cold tolerance. SlHY5 directly bound to and activated the transcription of genes encoding a gibberellin-inactivation enzyme, namely GIBBERELLIN2-OXIDASE4, and an abscisic acid biosynthetic enzyme, namely 9-CIS-EPOXYCAROTENOID DIOXYGENASE6 (SlNCED6). Thus, phytochrome A-dependent SlHY5 accumulation resulted in an increased abscisic acid/gibberellin ratio, which was accompanied by growth cessation and induction of cold response. Furthermore, silencing of SlNCED6 compromises short day- and low R/FR-induced tomato resistance to cold stress. These findings provide insight into the molecular genetic mechanisms by which plants integrate environmental stimuli with hormones to coordinate their growth with impending cold temperatures. Moreover, this work reveals a molecular mechanism that plants have evolved for growth and survival in response to seasonal changes.

Brassinosteroids act as a positive regulator of NBR1-dependent selective autophagy in response to chilling stress in tomato.

Autophagy is a highly conserved and regulated catabolic process involved in the degradation of protein aggregates, which plays critical roles in eukaryotes. In plants, multiple molecular processes can induce or suppress autophagy but the mechanism of its regulation by phytohormones is poorly understood. Brassinosteroids (BRs) are steroid phytohormones that play crucial roles in plant response to stresses. Here, we investigate the role of BRs in NBR1-dependent selective autophagy in response to chilling stress in tomato. BRs and their signaling element BZR1 can induce autophagy and accumulation of the selective autophagy receptor NBR1 in tomato under chilling stress. Cold increased the stability of BZR1, which was promoted by BRs. Cold- and BR-induced increased BZR1 stability activated the transcription of several autophagy-related genes (ATGs) and NBR1 genes by directly binding to their promoters, which resulted in selective autophagy. Furthermore, silencing of these ATGs or NBR1 genes resulted in a decreased accumulation of several functional proteins and an increased accumulation of ubiquitinated proteins, subsequently compromising BR-induced cold tolerance. These results strongly suggest that BRs regulate NBR1-dependent selective autophagy in a BZR1-dependent manner in response to chilling stress in tomato.

A novel CO2 -responsive systemic signaling pathway controlling plant mycorrhizal symbiosis.

Elevated atmospheric carbon dioxide (eCO2 ) concentrations promote symbiosis between roots and arbuscular mycorrhizal fungi (AMF), modifying plant nutrient acquisition and cycling of carbon, nitrogen and phosphate. However, the biological mechanisms by which plants transmit aerial eCO2 cues to roots, to alter the symbiotic associations remain unknown. We used a range of interdisciplinary approaches, including gene silencing, grafting, transmission electron microscopy, liquid chromatography tandem mass spectrometry (LC-MS/MS), biochemical methodologies and gene transcript analysis to explore the complexities of environmental signal transmission from the point of perception in the leaves at the apex to the roots. Here we show that eCO2 triggers apoplastic hydrogen peroxide (H2 O2 )-dependent auxin production in tomato shoots followed by systemic signaling that results in strigolactone biosynthesis in the roots. This redox-auxin-strigolactone systemic signaling cascade facilitates eCO2 -induced AMF symbiosis and phosphate utilization. Our results challenge the current paradigm of eCO2 effects on AMF and provide new insights into potential targets for manipulation of AMF symbiosis for high nutrient utilization under future climate change scenarios.

Heat Shock Factor HsfA1a Is Essential for R Gene-Mediated Nematode Resistance and Triggers H2O2 Production1.

Plants generate reactive oxygen species (ROS) in the apoplast in response to pathogen attack, especially following resistance (R) gene-mediated pathogen recognition; however, the mechanisms activating ROS generation remain unknown. Here, we demonstrate that RKN (Meloidogyne incognita) infection rapidly induces ROS accumulation in the roots of tomato (Solanum lycopersicum) plants that contain the R gene Mi-1.2 but rarely induces ROS accumulation in the susceptible or Mi-1.2-silenced resistant genotypes. RNK also induces the hypersensitive response, a form of programmed cell death, in Mi-1.2 plants. RKN induces the expression of numerous class-A heat shock factor (HsfA) genes in resistant tomato plants. Silencing HsfA1a compromises Mi-1.2-mediated resistance, apoplastic H2O2 accumulation, and the transcription of whitefly induced 1 (Wfi1), which encodes a respiratory burst oxidase homolog. HsfA1a regulates Wfi1 transcription by binding to the Wfi1 promoter, and silencing of Wfi1 compromises Mi-1.2-mediated resistance. HsfA1a and Wfi1 are involved in Mi-1.2-triggered Hsp90 accumulation and basal defense in susceptible tomato. Thus, HsfA-1aWfi1-dependent ROS signaling functions as a crucial regulator of plant defense responses.

Light Signaling-Dependent Regulation of Photoinhibition and Photoprotection in Tomato.

Photoreceptor-mediated light signaling plays a critical role in plant growth, development, and stress responses but its contribution to the spatial regulation of photoinhibition and photoprotection within the canopy remains unclear. Here, we show that low-red/far-red (L-R/FR) ratio light conditions significantly alleviate PSII and PSI photoinhibition in the shade leaves of tomato (Solanum lycopersicum) plants. This protection is accompanied by a phytochrome A-dependent induction of LONG HYPOCOTYL5 (HY5). HY5 binds to the promoter of ABA INSENSITIVE5 (ABI5), triggering RESPIRATORY BURST OXIDASE HOMOLOG1 (RBOH1)-dependent H2O2 production in the apoplast. Decreased levels of HY5, ABI5, and RBOH1 transcripts increased cold-induced photoinhibition and abolished L-R/FR-induced alleviation of photoinhibition. L-R/FR illumination induced nonphotochemical quenching (NPQ) of chlorophyll a fluorescence and increased the activities of Foyer-Halliwell-Asada cycle enzymes and cyclic electron flux (CEF) around PSI. In contrast, decreased HY5, ABI5, and RBOH1 transcript levels abolished the positive effect of L-R/FR on photoprotection. Loss of PROTON GRADIENT REGULATION5-dependent CEF led to increased photoinhibition and attenuated L-R/FR-dependent NPQ. These data demonstrate that HY5 is an important hub in the cross talk between light and cold response pathways, integrating ABA and reactive oxygen species signaling, leading to the attenuation of photoinhibition by enhanced induction of photoprotection in shade leaves.