Chromosome-level genome assembly of bunching onion illuminates genome evolution and flavor formation in Allium crops.

The Allium genus is cultivated globally as vegetables, condiments, or medicinal plants and is characterized by large genomes and strong pungency. However, the genome evolution and genomic basis underlying their unique flavor formation remain poorly understood. Herein, we report an 11.27-Gb chromosome-scale genome assembly for bunching onion (A. fistulosum). The uneven bursts of long-terminal repeats contribute to diversity in genome constituents, and dispersed duplication events largely account for gene expansion in Allium genomes. The extensive duplication and differentiation of alliinase and lachrymatory factor synthase manifest as important evolutionary events during flavor formation in Allium crops. Furthermore, differential selective preference for flavor-related genes likely lead to the variations in isoalliin content in bunching onions. Moreover, we reveal that China is the origin and domestication center for bunching onions. Our findings provide insights into Allium genome evolution, flavor formation and domestication history and enable future genome-assisted breeding of important traits in these crops.

S-Nitrosoglutathione Reductase Contributes to Thermotolerance by Modulating High Temperature-Induced Apoplastic H2O2 in Solanum lycopersicum.

S-nitrosoglutathione reductase (GSNOR) is considered as a critical regulator of plant stress tolerance for its impacts on protein S-nitrosylation through regulation of the S-nitrosothiol (SNO) level. However, the mechanism of GSNOR-mediated stress tolerance is still obscure. Here, we found that GSNOR activity was induced by high temperature in tomato (Solanum lycopersicum) plants, whereas mRNA level of SlGSNOR1 exhibited little response. Suppressing SlGSNOR1 expression by virus-induced gene silencing (VIGS) increased accumulation of SNO and nitrites under high temperature and reduced thermotolerance. The compromised thermotolerance was associated with less accumulation of abscisic acid (ABA) and salicylic acid (SA), attenuated activation of mitogen-activated protein kinase (MAPK) and reduced expression of heat shock protein. Intriguingly, SlGSNOR1 silencing impaired upregulation of RESPIRATORY BURST OXIDASE HOMOLOG1 (SlRBOH1) and apoplastic H2O2 accumulation in response to high temperature, whereas SlRBOH1 silencing abolished activation of GSNOR and led to a similar decline in thermotolerance as in SlGSNOR1-silenced plants. Importantly, H2O2 treatment recovered the thermotolerance and improved antioxidant capacity in SlGSNOR1-silenced plants. Our results suggest that GSNOR plays a role in regulating the SlRBOH1-dependent apoplastic H2O2 production in response to high temperature, while a balanced interaction between SNO and H2O2 is critical for maintaining the cellular redox homeostasis and thermotolerance.

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.

Melatonin promotes metabolism of bisphenol A by enhancing glutathione-dependent detoxification in Solanum lycopersicum L.

Bisphenol A (BPA), a widely distributed organic compound, is toxic to animals and plants. Here we show the mechanism of BPA detoxification by melatonin (MEL) in tomato, which is otherwise poorly understood in plants. BPA treatment decreased the quantum yield of photosystem II (Fv/Fm) and increased the membrane lipid peroxidation and reactive oxygen species (ROS) accumulation dose-dependently, whereas exogenous MEL alleviated the BPA effects on Fv/Fm, lipid peroxidation, ROS accumulation and BPA uptake. Furthermore, BPA elevated the glutathione (GSH) content, activities of glutathione S-transferase (GST), and glutathione reductase (GR), and the transcript levels of GSH1, GR1, GST1 and MEL biosynthesis genes (COMT, T5H, and SNAT), whereas BPA + MEL showed even a more profound induction. Silencing GSH1, GR1 and GST1 genes compromised the BPA detoxification potential of tomato plants as revealed by an increased level of ROS, lipid peroxidation and BPA uptake, and a decreased Fv/Fm and GST activity; these changes were alleviated by MEL application. Under in vitro conditions, BPA was glutathionylated by GSH, which was further catalyzed by GST to cysteine and N-acetylcysteine conjugates. These findings suggest a crucial role for MEL in BPA detoxification via GSH and GST, and can be useful to reduce BPA residue for food safety.

COP9 Signalosome CSN4 and CSN5 Subunits Are Involved in Jasmonate-Dependent Defense Against Root-Knot Nematode in Tomato.

COP9 signalosome (CSN) is an evolutionarily conserved regulatory component of the ubiquitin/proteasome system that plays crucial roles in plant growth and stress tolerance; however, the mechanism of COP9-mediated resistance to root-knot nematodes (RKNs, e.g. Meloidogyne incognita) is not fully understood in plants. In the present study, we found that RKN infection in the roots rapidly increases the transcript levels of CSN subunits 4 and 5 (CSN4 and CSN5) and their protein accumulation in tomato (Solanum lycopersicum) plants. Suppression of CSN4 or CSN5 expression resulted in significantly increased number of egg masses and aggravated RKN-induced lipid peroxidation of cellular membrane but inhibited RKN-induced accumulation of CSN4 or CSN5 protein in tomato roots. Importantly, the RKN-induced accumulation of jasmonic acid (JA) and JA-isoleucine (JA-Ile), as well as the transcript levels of JA-related biosynthetic and signaling genes were compromised by CSN4 or CSN5 gene silencing. Moreover, protein-protein interaction assays demonstrated that CSN4 and CSN5B interact with the jasmonate ZIM domain 2 (JAZ2), which is the signaling component of the JA pathway. Silencing of CSN4 or CSN5 also compromises RKN-induced JAZ2 expression. Together, our findings indicate that CSN4 and CSN5 play critical roles in JA-dependent basal defense against RKN.

Histone acetylation recruits the SWR1 complex to regulate active DNA demethylation in Arabidopsis.

Active DNA demethylation is critical for controlling the DNA methylomes in plants and mammals. However, little is known about how DNA demethylases are recruited to target loci, and the involvement of chromatin marks in this process. Here, we identify 2 components of the SWR1 chromatin-remodeling complex, PIE1 and ARP6, as required for ROS1-mediated DNA demethylation, and discover 2 SWR1-associated bromodomain-containing proteins, AtMBD9 and nuclear protein X1 (NPX1). AtMBD9 and NPX1 recognize histone acetylation marks established by increased DNA methylation 1 (IDM1), a known regulator of DNA demethylation, redundantly facilitating H2A.Z deposition at IDM1 target loci. We show that at some genomic regions, H2A.Z and DNA methylation marks coexist, and H2A.Z physically interacts with ROS1 to regulate DNA demethylation and antisilencing. Our results unveil a mechanism through which DNA demethylases can be recruited to specific target loci exhibiting particular histone marks, providing a conceptual framework to understand how chromatin marks regulate DNA demethylation.

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).

The bZip transcription factor HY5 mediates CRY1a-induced anthocyanin biosynthesis in tomato.

The production of anthocyanin is regulated by light and corresponding photoreceptors. In this study, we found that exposure to blue light and overexpression of CRY1a are associated with increased accumulation of anthocyanin in tomato (Solanum lycopersicum L.). These responses are the result of changes in mRNA and the protein levels of SlHY5, which is a transcription factor. In vitro and in vivo experiments using electrophoretic mobility shift assay and ChIP-qPCR assays revealed that SlHY5 could directly recognize and bind to the G-box and ACGT-containing element in the promoters of anthocyanin biosynthesis genes, such as chalcone synthase 1, chalcone synthase 2, and dihydroflavonol 4-reductase. Silencing of SlHY5 in OE-CRY1a lines decreased the accumulation of anthocyanin. The findings presented here not only deepened our understanding of how light controls anthocyanin biosynthesis and associated photoprotection in tomato leaves, but also allowed us to explore potential targets for improving pigment production.

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.

Brassinosteroids act as a positive regulator for resistance against root-knot nematode involving RESPIRATORY BURST OXIDASE HOMOLOG-dependent activation of MAPKs in tomato.

Interplay of hormones with reactive oxygen species (ROS) fine-tunes the response of plants to stress; however, the crosstalk between brassinosteroids (BRs) and ROS in nematode resistance is unclear. In this study, we found that low BR biosynthesis or lack of BR receptor increased, whilst exogenous BR decreased the susceptibility of tomato plants to Meloidogyne incognita. Hormone quantification coupled with hormone mutant complementation experiments revealed that BR did not induce the defence response by triggering salicylic acid (SA), jasmonic acid/ethylene (JA/ET) or abscisic acid (ABA) signalling pathway. Notably, roots of BR-deficient plants had decreased apoplastic ROS accumulation, transcript of RESPIRATORY BURST OXIDASE HOMOLOG1 (RBOH1) and WHITEFLY INDUCED1 (WFI1), and reduced activation of mitogen-activated protein kinase 1/2 (MPK1/2) and MPK3. Silencing of RBOH1, WFI1, MPK1, MPK2 and MPK3 all increased the root susceptibility to nematode and attenuated BR-induced resistance against the nematode. Significantly, suppressed transcript of RBOH1 compromised BR-induced activation of MPK1/2 and MPK3. These results strongly suggest that RBOH-dependent MPK activation is involved in the BR-induced systemic resistance against the nematode.

Brassinosteroid-mediated apoplastic H2 O2 -glutaredoxin 12/14 cascade regulates antioxidant capacity in response to chilling in tomato.

Brassinosteroids (BRs) regulate plant development and stress response. Although much has been learned about their roles in plant development, the mechanisms by which BRs regulate plant stress tolerance remain unclear. Chilling is a major stress that adversely affects plant growth. Here, we report that BR positively regulates chilling tolerance in tomato. BR partial deficiency aggravated chilling-induced oxidized protein accumulation, membrane lipid peroxidation, and decrease of maximum quantum efficiency of photosystem II (Fv/Fm). By contrast, overexpression of BR biosynthetic gene Dwarf or treatment with 24-epibrassinolide (EBR) attenuated chilling-induced oxidative damages and resulted in an increase of Fv/Fm. BR increased transcripts of RESPIRATORY BURST OXIDASE HOMOLOG1 (RBOH1) and GLUTAREDOXIN (GRX) genes, and BR-induced chilling tolerance was associated with an increase in the ratio of reduced/oxidized 2-cysteine peroxiredoxin (2-Cys Prx) and activation of antioxidant enzymes. However, RBOH1-RNAi plants failed to respond to EBR as regards to the induction of GRX genes, activation of antioxidant capacity, and attenuation of chilling-induced oxidative damages. Furthermore, silencing of GRXS12 and S14 compromised EBR-induced increases in the ratio of reduced/oxidized 2-Cys Prx and activities of antioxidant enzymes. Our study suggests that BR enhances chilling tolerance through a signalling cascade involving RBOH1, GRXs, and 2-Cys Prx in tomato.

Tomato CRY1a plays a critical role in the regulation of phytohormone homeostasis, plant development, and carotenoid metabolism in fruits.

Blue light photoreceptors, cryptochromes (CRYs), regulate multiple aspects of plant growth and development. However, our knowledge of CRYs is predominantly based on model plant Arabidopsis at early growth stage. In this study, we elucidated functions of CRY1a gene in mature tomato (Solanum lycopersicum) plants by using cry1a mutants and CRY1a-overexpressing lines (OE-CRY1a-1 and OE-CRY1a-2). In comparison with wild-type plants, cry1a mutants are relatively tall, accumulate low biomass, and bear more fruits, whereas OE-CRY1a plants are short stature, and they not only flower lately but also bear less fruits. RNA-seq, qRT-PCR, and LC-MS/MS analysis revealed that biosynthesis of gibberellin, cytokinin, and jasmonic acid was down-regulated by CRY1a. Furthermore, DNA replication was drastically inhibited in leaves of OE-CRY1a lines, but promoted in cry1a mutants with concomitant changes in the expression of cell cycle genes. However, CRY1a positively regulated levels of soluble sugars, phytofluene, phytoene, lycopene, and ß-carotene in the fruits. The results indicate the important role of CRY1a in plant growth and have implications for molecular interventions of CRY1a aimed at improving agronomic traits.

24-Epibrassinolide alleviates organic pollutants-retarded root elongation by promoting redox homeostasis and secondary metabolism in Cucumis sativus L.

Environmental pollution by organic pollutants (OPs) has become a global concern due to its detrimental effects on the environment and human health. As plants are used to remediate contaminated sites, understanding the responses of plants to various OPs and fortification of plant tolerance are of great significance. In this work, we studied the biochemical and molecular responses of cucumber plants to three well-known OPs, 2,4,6-trichlorophenol, chlorpyrifos and oxytetracycline in the absence or presence of 24-epibrassinolide (EBR), a potent regulator of plant growth and stress tolerance. The results showed that the selected three OPs retarded root elongation; however, the phytotoxic effects of OPs were attenuated by exogenous EBR. OPs induced accumulations of both hydrogen peroxide (H2O2) and nitric oxide (NO) in root tips and resulted in an increased malondialdehyde (MDA) content, an indicator of membrane lipid peroxidation. Exogenous EBR reduced accumulations of H2O2, NO and MDA in the roots by increasing the expression of antioxidant and detoxification genes and the activities of the corresponding enzymes. Intriguingly, EBR not only promoted the activities of glutathione S-transferase and glutathione reductase, but also increased the content of reduced glutathione without altering the content of oxidized glutathione, which resulted in a reduced redox state under OPs stress. Furthermore, EBR increased the free radical scavenging capacity, flavonoid content and the activity and transcription of secondary metabolism related enzymes. Our results suggest that EBR treatment may fortify secondary metabolism to enhance antioxidant capacity in response to OPs treatment, which might have potential implication in phytoremediation of OPs.

Nitric oxide is involved in the oxytetracycline-induced suppression of root growth through inhibiting hydrogen peroxide accumulation in the root meristem.

Use of antibiotic-contaminated manure in crop production poses a severe threat to soil and plant health. However, few studies have studied the mechanism by which plant development is affected by antibiotics. Here, we used microscopy, flow cytometry, gene expression analysis and fluorescent dyes to study the effects of oxytetracycline (OTC), a widely used antibiotic in agriculture, on root meristem activity and the accumulation of hydrogen peroxide (H2O2) and nitric oxide (NO) in the root tips of tomato seedlings. We found that OTC caused cell cycle arrest, decreased the size of root meristem and inhibited root growth. Interestingly, the inhibition of root growth by OTC was associated with a decline in H2O2 levels but an increase in NO levels in the root tips. Diphenyliodonium (DPI), an inhibitor of H2O2 production, showed similar effects on root growth as those of OTC. However, exogenous H2O2 partially reversed the effects on the cell cycle, meristem size and root growth. Importantly, cPTIO (the NO scavenger) and tungstate (an inhibitor of nitrate reductase) significantly increased H2O2 levels in the root tips and reversed the inhibition of root growth by OTC. Out results suggest that OTC-induced NO production inhibits H2O2 accumulation in the root tips, thus leading to cell cycle arrest and suppression of root growth.

A novel ethylene responsive factor CitERF13 plays a role in photosynthesis regulation.

Ethylene responsive factors (ERFs) act as critical downstream components of the ethylene signalling pathway in regulating plant development and stress responses. However little is known about its role in regulation of photosynthesis. Here, we identified an ethylene-inducible ERF gene in citrus, CitERF13. Transient over-expression of CitERF13 in N. tabacum leaves, resulted in a significant decrease in net photosynthetic rate. Closer examination of photosynthetic activity of PSII and PSI indicated that CitERF13 overexpression led to declines of Fv/Fm, Y(II) and Y(I). However, change in NPQ was less pronounced. CitERF13 overexpression also significantly reduced Vc,max, Jmax and AQY, indicating inhibition of the Calvin cycle. The expression of photosynthesis-related genes was suppressed to a variable extent in leaf blades transiently over-expressing CitERF13. CitERF13 transient overexpression in tobacco or citrus both resulted in a decline of Chlorophyll content and CitERF13 overexpressing tobacco leaf disc was more susceptible to chlorosis in response to MV-mediated oxidative stress. The results suggest that CitERF13 is potentially involved in suppressing photosynthesis through multiple pathways, for instance, inhibiting photochemical activity of photosynthesis, CO2 carboxylation capacity and chlorophyll metabolism.

HsfA1a upregulates melatonin biosynthesis to confer cadmium tolerance in tomato plants.

Melatonin regulates broad aspects of plant responses to various biotic and abiotic stresses, but the upstream regulation of melatonin biosynthesis by these stresses remains largely unknown. Herein, we demonstrate that transcription factor heat-shock factor A1a (HsfA1a) conferred cadmium (Cd) tolerance to tomato plants, in part through its positive role in inducing melatonin biosynthesis under Cd stress. Analysis of leaf phenotype, chlorophyll content, and photosynthetic efficiency revealed that silencing of the HsfA1a gene decreased Cd tolerance, whereas its overexpression enhanced plant tolerance to Cd. HsfA1a-silenced plants exhibited reduced melatonin levels, and HsfA1a overexpression stimulated melatonin accumulation and the expression of the melatonin biosynthetic gene caffeic acid O-methyltransferase 1 (COMT1) under Cd stress. Both an in vitro electrophoretic mobility shift assay and in vivo chromatin immunoprecipitation coupled with qPCR analysis revealed that HsfA1a binds to the COMT1 gene promoter. Meanwhile, Cd stress induced the expression of heat-shock proteins (HSPs), which was compromised in HsfA1a-silenced plants and more robustly induced in HsfA1a-overexpressing plants under Cd stress. COMT1 silencing reduced HsfA1a-induced Cd tolerance and melatonin accumulation in HsfA1a-overexpressing plants. Additionally, the HsfA1a-induced expression of HSPs was partially compromised in COMT1-silenced wild-type or HsfA1a-overexpressing plants under Cd stress. These results demonstrate that HsfA1a confers Cd tolerance by activating transcription of the COMT1 gene and inducing accumulation of melatonin that partially upregulates expression of HSPs.

Melatonin enhances thermotolerance by promoting cellular protein protection in tomato plants.

Melatonin is a pleiotropic signaling molecule that provides physiological protection against diverse environmental stresses in plants. Nonetheless, the mechanisms for melatonin-mediated thermotolerance remain largely unknown. Here, we report that endogenous melatonin levels increased with a rise in ambient temperature and that peaked at 40°C. Foliar pretreatment with an optimal dose of melatonin (10 µmol/L) or the overexpression of N-acetylserotonin methyltransferase (ASMT) gene effectively ameliorated heat-induced photoinhibition and electrolyte leakage in tomato plants. Both exogenous melatonin treatment and endogenous melatonin manipulation by overexpression of ASMT decreased the levels of insoluble and ubiquitinated proteins, but enhanced the expression of heat-shock proteins (HSPs) to refold denatured and unfolded proteins under heat stress. Meanwhile, melatonin also induced expression of several ATG genes and formation of autophagosomes to degrade aggregated proteins under the same stress. Proteomic profile analyses revealed that protein aggregates for a large number of biological processes accumulated in wild-type plants. However, exogenous melatonin treatment or overexpression of ASMT reduced the accumulation of aggregated proteins. Aggregation responsive proteins such as HSP70 and Rubisco activase were preferentially accumulated and ubiquitinated in wild-type plants under heat stress, while melatonin mitigated heat stress-induced accumulation and ubiquitination of aggregated proteins. These results suggest that melatonin promotes cellular protein protection through induction of HSPs and autophagy to refold or degrade denatured proteins under heat stress in tomato plants.

Glutathione-mediated regulation of nitric oxide, S-nitrosothiol and redox homeostasis confers cadmium tolerance by inducing transcription factors and stress response genes in tomato.

Glutathione (GSH) plays a critical role in plant growth, development and responses to stress. However, the mechanism by which GSH regulates tolerance to cadmium (Cd) stress still remains unclear. Here we show that inhibition of GSH biosynthesis by buthionine sulfoximine (BSO) aggravated Cd toxicity by increasing accumulation of reactive oxygen species (ROS) and reducing contents of nitric oxide (NO) and S-nitrosothiol (SNO) in tomato roots. In contrast, exogenous GSH alleviated Cd toxicity by substantially minimizing ROS accumulation and increasing contents of NO and SNO, and activities of antioxidant enzymes that eventually reduced oxidative stress. GSH-induced enhancement in Cd tolerance was closely associated with the upregulation of transcripts of several transcription factors such as ETHYLENE RESPONSIVE TRANSCRIPTION FACTOR 1 (ERF1), ERF2, MYB1 TRANSCRIPTION FACTOR- AIM1 and R2R3-MYB TRANSCRIPTION FACTOR- AN2, and some stress response genes. In addition, GSH modulated the cellular redox balance through maintaining increased GSH: GSSG and AsA: DHA ratios, and also increased phytochelatins contents. Nonetheless, GSH-induced alleviation of Cd phytotoxicity was also associated with increased sequestration of Cd into cell walls and vacuoles but not with Cd accumulation. Under Cd stress, while treatment with BSO slightly decreased vacuolar fraction of Cd, combined treatment with BSO and GSH noticeably increased that fraction. Our results suggest that GSH increases tomato tolerance to Cd stress not only by promoting the chelation and sequestration of Cd but also by stimulating NO, SNO and the antioxidant system through a redox-dependent mechanism.

Apoplastic H2 O2 plays a critical role in axillary bud outgrowth by altering auxin and cytokinin homeostasis in tomato plants.

Although phytohormones such as indole-3-acetic acid (IAA), cytokinin (CK) and strigolactone are important modulators of plant architecture, it remains unclear whether reactive oxygen species are involved in the regulation of phytohormone-dependent axillary bud outgrowth in plants. We used diverse techniques, including transcriptional suppression, HPLC-MS, biochemical methodologies and gene transcript analysis to investigate the signaling pathway for apoplastic hydrogen peroxide (H2 O2 )-induced axillary bud outgrowth. Silencing of tomato RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1) and WHITEFLY INDUCED 1 (WFI1), two important genes involved in H2 O2 production in the apoplast, enhanced bud outgrowth, decreased transcript of FZY - a rate-limiting gene in IAA biosynthesis and IAA accumulation in the apex - and increased the transcript of IPT2 involved in CK biosynthesis and CK accumulation in the stem node. These effects were fully abolished by the application of exogenous H2 O2 . Both decapitation and the silencing of FZY promoted bud outgrowth, and downregulated and upregulated the transcripts for IAA3 and IAA15, and IPT2, respectively. However, these effects were not blocked by treatment with exogenous H2 O2 but by napthaleneacetic acid (NAA) treatment. These results suggest that RBOHs-dependent apoplastic H2 O2 promotes IAA biosynthesis in the apex, which, in turn, inhibits CK biosynthesis and subsequent bud outgrowth in tomato plants.