Chunmei Ren

The Jasmonate-ZIM-Domain Proteins Interact with the WD-Repeat/bHLH/MYB Complexes to Regulate Jasmonate-Mediated Anthocyanin Accumulation and Trichome Initiation in Arabidopsis thaliana

This work examines the molecular mechanism of jasmonate regulation of anthocyanin biosynthesis and trichome initiation. It identifies three bHLH transcription factors and two MYB transcription factors as new targets of JAZ proteins, showing that JAZ proteins attenuate the transcriptional function of WD-repeat/bHLH/MYB complexes to regulate anthocyanin accumulation and trichome. Jasmonates (JAs) mediate plant responses to insect attack, wounding, pathogen infection, stress, and UV damage and regulate plant fertility, anthocyanin accumulation, trichome formation, and many other plant developmental processes. Arabidopsis thaliana Jasmonate ZIM-domain (JAZ) proteins, substrates of the CORONATINE INSENSITIVE1 (COI1)–based SCFCOI1 complex, negatively regulate these plant responses. Little is known about the molecular mechanism for JA regulation of anthocyanin accumulation and trichome initiation. In this study, we revealed that JAZ proteins interact with bHLH (Transparent Testa8, Glabra3 [GL3], and Enhancer of Glabra3 [EGL3]) and R2R3 MYB transcription factors (MYB75 and Glabra1), essential components of WD-repeat/bHLH/MYB transcriptional complexes, to repress JA-regulated anthocyanin accumulation and trichome initiation. Genetic and physiological evidence showed that JA regulates WD-repeat/bHLH/MYB complex-mediated anthocyanin accumulation and trichome initiation in a COI1-dependent manner. Overexpression of the MYB transcription factor MYB75 and bHLH factors (GL3 and EGL3) restored anthocyanin accumulation and trichome initiation in the coi1 mutant, respectively. We speculate that the JA-induced degradation of JAZ proteins abolishes the interactions of JAZ proteins with bHLH and MYB factors, allowing the transcriptional function of WD-repeat/bHLH/MYB complexes, which subsequently activate respective downstream signal cascades to modulate anthocyanin accumulation and trichome initiation.

A Leaky Mutation in DWARF4 Reveals an Antagonistic Role of Brassinosteroid in the Inhibition of Root Growth by Jasmonate in Arabidopsis

The F-box protein CORONATINE INSENSITIVE1 (COI1) plays a central role in jasmonate (JA) signaling and is required for all JA responses in Arabidopsis (Arabidopsis thaliana). To dissect JA signal transduction, we isolated the partially suppressing coi1 (psc1) mutant, which partially suppressed coi1 insensitivity to JA inhibition of root growth. The psc1 mutant partially restored JA sensitivity in coi1-2 background and displayed JA hypersensitivity in wild-type COI1 background. Genetic mapping, sequence analysis, and complementation tests revealed that psc1 is a leaky mutation of DWARF4 (DWF4) that encodes a key enzyme in brassinosteroid (BR) biosynthesis. Physiological analysis showed that an application of exogenous BR eliminated the partial restoration of JA sensitivity by psc1 in coi1-2 background and the JA hypersensitivity of psc1 in wild-type COI1 background. Exogenous BR also attenuated JA inhibition of root growth in the wild type. In addition, the expression of DWF4 was inhibited by JA, and this inhibition was dependent on COI1. These results indicate that (1) BR is involved in JA signaling and negatively regulates JA inhibition of root growth, and (2) the DWF4 is down-regulated by JA and is located downstream of COI1 in the JA-signaling pathway.

Transcription Factor WRKY70 Displays Important but No Indispensable Roles in Jasmonate and Salicylic Acid Signaling

The transcription factor WRKY70 was previously reported to be a common component in salicylic acid (SA) and jasmonate (JA) mediated signal pathways in Arabidopsis. Here, we present that the inactivation of the WRKY70 gene in wrky70-1 mutant does not alter the responses of both JA and SA, and that wrky70 mutation is unable to restore the coi1 mutant in JA responses. However, overexpression of WRKY70 reduces JA responses such as expression of JA-induced genes and JA-inhibitory root growth, and activates expression of SA-inducible PR1. These data indicate that the WRKY70 is important but not indispensable for JA and SA signaling, and that other regulators may display the redundant role with WRKY70 in modulation of JA and SA responses in Arabidopsis. Furthermore, we showed that JA inhibits expression of WRKY70 and PR1 by both COI1-dependent and COI1-independent pathways.

Brassinosteroid negatively regulates jasmonate inhibition of root growth in Arabidopsis

Jasmonate (JA) inhibits root growth of Arabidopsis thaliana seedlings. The mutation in COI1 that plays a central role in JA signaling displays insensitivity to JA inhibition of root growth. To dissect JA signaling pathway, we recently isolated one mutant named psc1, which partially suppresses coi1 insensitivity to JA inhibition of root growth. As we identified the PSC1 gene as an allele of DWF4 that encodes a key enzyme in brassinosteroid (BR) biosynthesis, we hypothesized and demonstrated that BR is involved in JA signaling and negatively regulates JA inhibition of root growth. In our Plant Physiology paper, we analyzed effects of psc1 or exogenous BR on the inhibition of root growth by JA. Here we show that treatment with brassinazole (Brz), a BR biosynthesis inhibitor, increased JA sensitivity in both coi1-2 and wild type, which further confirms that BR negatively regulates JA inhibition of root growth. Since effects of psc1, Brz, and exogenous BR on JA inhibition of root growth were mild, we suggests that BR negatively finely regulates JA inhibition of root growth in Arabidopsis.

Disruption of Fumarylacetoacetate Hydrolase Causes Spontaneous Cell Death under Short-Day Conditions in Arabidopsis

Disrupting fumarylacetoacetate hydrolase leads to cell death in Arabidopsis, indicating that the Tyr degradation pathway is essential for plant survival under short-day conditions. Fumarylacetoacetate hydrolase (FAH) hydrolyzes fumarylacetoacetate to fumarate and acetoacetate, the final step in the tyrosine (Tyr) degradation pathway that is essential to animals. Deficiency of FAH in animals results in an inborn lethal disorder. However, the role for the Tyr degradation pathway in plants remains to be elucidated. In this study, we isolated an Arabidopsis (Arabidopsis thaliana) short-day sensitive cell death1 (sscd1) mutant that displays a spontaneous cell death phenotype under short-day conditions. The SSCD1 gene was cloned via a map-based cloning approach and found to encode an Arabidopsis putative FAH. The spontaneous cell death phenotype of the sscd1 mutant was completely eliminated by further knockout of the gene encoding the putative homogentisate dioxygenase, which catalyzes homogentisate into maleylacetoacetate (the antepenultimate step) in the Tyr degradation pathway. Furthermore, treatment of Arabidopsis wild-type seedlings with succinylacetone, an abnormal metabolite caused by loss of FAH in the Tyr degradation pathway, mimicked the sscd1 cell death phenotype. These results demonstrate that disruption of FAH leads to cell death in Arabidopsis and suggest that the Tyr degradation pathway is essential for plant survival under short-day conditions.

Brassinosteroid enhances cytokinin-induced anthocyanin biosynthesis in Arabidopsis seedlings

To investigate whether brassinosteroids (BR) affects cytokinin (CK)-induced anthocyanin biosynthesis, seedlings of the Arabidopsis dwarf4 (dwf4) mutants including partially suppressing coi1 (psc1) and dwf4-102, which are defective in the BR biosynthesis, and the brassinosteroid-insensitive 1–4 (bri1-4) mutant defective in BR signalling were used for the analysis of CK-induced anthocyanin accumulation and the expression of anthocyanin biosynthetic genes and WD-repeat/Myb/bHLH transcription factors. The results show that the CK-induced anthocyanin accumulation was remarkably reduced in dwf4 and bri1-4 mutants, but distinctly increased in the wild type (WT) treated with BR. Moreover, the CK-induced expressions of the late anthocyanin biosynthetic genes including dihydroflavonol reductase, leucoanthocyanidin dioxygenase, and UDP-glucose: flavonoid-3-O-glucosyl transferase were significantly reduced in bri1-4 and dwf4-102 mutants compared to WT. In addition, the expressions of transcription factors production of anthocyanin pigment 1 (PAP1), glabra 3 (GL3), and enhancer of glabra 3 (EGL3) were induced by CK in WT but not in the bri1-4 and dwf4-102 mutants. These results indicate that BR enhanced the CK-induced anthocyanin biosynthesis by up-regulating the late anthocyanin biosynthetic genes and this regulation might be mediated by the transcription factors PAP1, GL3, and EGL3.

GDP-D-mannose epimerase regulates male gametophyte development, plant growth and leaf senescence in Arabidopsis

Plant GDP-D-mannose epimerase (GME) converts GDP-D-mannose to GDP-L-galactose, a precursor of both L-ascorbate (vitamin C) and cell wall polysaccharides. However, the genetic functions of GME in Arabidopsis are unclear. In this study, we found that mutations in Arabidopsis GME affect pollen germination, pollen tube elongation, and transmission and development of the male gametophyte through analysis of the heterozygous GME/gme plants and the homozygous gme plants. Arabidopsis gme mutants also exhibit severe growth defects and early leaf senescence. Surprisingly, the defects in male gametophyte in the gme plants are not restored by L-ascorbate, boric acid or GDP-L-galactose, though boric acid rescues the growth defects of the mutants, indicating that GME may regulate male gametophyte development independent of L-ascorbate and GDP-L-galactose. These results reveal key roles for Arabidopsis GME in reproductive development, vegetative growth and leaf senescence, and suggest that GME regulates plant growth and controls male gametophyte development in different manners.

Regulation of the WD-repeat/bHLH/MYB complex by gibberellin and jasmonate.

ABSTRACT The phytohormones gibberellin (GA) and jasmonate (JA) regulate various aspects of plant development, growth and defense. Previous studies showed that both DELLA repressors in GA pathway and JA-ZIM domain (JAZ) proteins in JA pathway interact with and repress the WD-repeat/bHLH/MYB transcriptional complex to inhibit trichome initiation, and GA and JA respectively induce DELLAs and JAZs degradation to synergistically enhance trichome formation. In this study, we showed that the DELLA protein RGA and JAZ1 competitively bind to ENHANCER OF GLABRA3 (EGL3), a bHLH component of the WD-repeat/bHLH/MYB complex. GA and JA differently affect the expression and protein stability of the components of the WD-repeat/bHLH/MYB complex, and EGL3 and GL3 repress the expression of JAZ genes as a feedback. The novel findings help to understand the mechanism of the WD-repeat/bHLH/MYB complex in GA/JA-regulated trichome formation.

Fumarylacetoacetate hydrolase is involved in salt stress response in Arabidopsis

Main conclusionFumarylacetoacetate hydrolase participates in positive regulation of salt stress in Arabidopsis.Fumarylacetoacetate hydrolase (FAH) catalyzes the hydrolysis of fumarylacetoacetate into fumarate and acetoacetate, the final step in the Tyr degradation pathway that is essential to animals. However, the Tyr degradation pathway is not well understood in plants. Previously, we found that mutation of the SHORT-DAY SENSITIVE CELL DEATH 1 (SSCD1) gene encoding FAH in Arabidopsis causes spontaneous cell death under short day, which first indicated that the Tyr degradation pathway also plays an important role in plants. In this study, we found that the SSCD1 gene was up-regulated by salt stress, and the sscd1 mutant was hypersensitive to salt stress. However, the double mutant of SSCD1 and HOMOGENTISATE DIOXYGENASE, in which intermediates of the Tyr degradation pathway could not be produced, displayed a normal response to salt stress. Furthermore, the sscd1 mutant showed more accumulation of reactive oxygen species (ROS) and less up-regulation of some ROS-scavenging genes such as ASCORBATE PEROXIDASE 2 and COPPER/ZINC SUPEROXIDE DISMUTASE 1 compared with wild type under salt stress. In addition, SSCD1 expression was also up-regulated by H2O2, and the sscd1 mutant exhibited hypersensitivity to oxidative stress compared with wild type. Taken together, we concluded that loss of FAH in sscd1 leads to the accumulation of Tyr degradation intermediates, which impairs the up-regulation of some ROS-scavenging genes under salt stress, causing more accumulation of ROS, resulting in the hypersensitivity of sscd1 to salt stress.

Sugar suppresses cell death caused by disruption of fumarylacetoacetate hydrolase in Arabidopsis

AbstractMain conclusionSugar negatively regulates cell death resulting from the loss of fumarylacetoacetate hydrolase that catalyzes the last step in the Tyr degradation pathway inArabidopsis. Fumarylacetoacetate hydrolase (FAH) hydrolyzes fumarylacetoacetate to fumarate and acetoacetate, the final step in the tyrosine (Tyr) degradation pathway that is essential to animals. Previously, we first found that the Tyr degradation pathway plays an important role in plants. Mutation of the SSCD1 gene encoding FAH in Arabidopsis leads to spontaneous cell death under short-day conditions. In this study, we presented that the lethal phenotype of the short-day sensitive cell death1 (sscd1) seedlings was suppressed by sugars including sucrose, glucose, fructose, and maltose in a dose-dependent manner. Real-time quantitative PCR (RT-qPCR) analysis showed the expression of Tyr degradation pathway genes homogentisate dioxygenase and maleylacetoacetate isomerase, and sucrose-processing genes cell-wall invertase 1 and alkaline/neutral invertase G, was up-regulated in the sscd1 mutant, however, this up-regulation could be repressed by sugar. In addition, a high concentration of sugar attenuated cell death of Arabidopsis wild-type seedlings caused by treatment with exogenous succinylacetone, an abnormal metabolite resulting from the loss of FAH in the Tyr degradation pathway. These results indicated that (1) sugar could suppress cell death in sscd1, which might be because sugar supply enhances the resistance of Arabidopsis seedlings to toxic effects of succinylacetone and reduces the accumulation of Tyr degradation intermediates, resulting in suppression of cell death; and (2) sucrose-processing genes cell-wall invertase 1 and alkaline/neutral invertase G might be involved in the cell death in sscd1. Our work provides insights into the relationship between sugar and sscd1-mediated cell death, and contributes to elucidation of the regulation of cell death resulting from the loss of FAH in plants.