Cell death resulted from loss of fumarylacetoacetate hydrolase in Arabidopsis is related to phytohormone jasmonate but not salicylic acid.

Fumarylacetoacetate hydrolase (FAH) catalyzes the final step in Tyr degradation pathway essential to animals but not well understood in plants. Previously, we found that mutation of SSCD1 encoding Arabidopsis FAH causes cell death under short day, which uncovered an important role of Tyr degradation pathway in plants. Since phytohormones salicylic acid (SA) and jasmonate (JA) are involved in programmed cell death, in this study, we investigated whether sscd1 cell death is related to SA and JA, and found that (1) it is accompanied by up-regulation of JA- and SA-inducible genes as well as accumulation of JA but not SA; (2) it is repressed by breakdown of JA signaling but not SA signaling; (3) the up-regulation of reactive oxygen species marker genes in sscd1 is repressed by breakdown of JA signaling; (4) treatment of wild-type Arabidopsis with succinylacetone, an abnormal metabolite caused by loss of FAH, induces expression of JA-inducible genes whereas treatment with JA induces expression of some Tyr degradation genes with dependence of JA signaling. These results demonstrated that cell death resulted from loss of FAH in Arabidopsis is related to JA but not SA, and suggested that JA signaling positively regulates sscd1 cell death by up-regulating Tyr degradation.

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

MAIN CONCLUSION: Sugar negatively regulates cell death resulting from the loss of fumarylacetoacetate hydrolase that catalyzes the last step in the Tyr degradation pathway in Arabidopsis . 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.

Disruption of fumarylacetoacetate hydrolase causes spontaneous cell death under short-day conditions in Arabidopsis.

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 jasmonate-induced anthocyanin accumulation in Arabidopsis seedlings.

Jasmonate (JA) regulates plant development, mediates defense responses, and induces anthocyanin biosynthesis as well. Previously, we isolated the psc1 mutant that partially suppressed coi1 insensitivity to JA, and found that brassinosteroid (BR) was involved in JA signaling and negatively regulated JA inhibition of root growth in Arabidopsis. In this study it was shown that JA-induced anthocyanin accumulation was reduced in BR mutants or in wild type treated with brassinazole, an inhibitor of BR biosynthesis, whereas it was induced by an application of exogenous BR. It was also shown that the 'late' anthocyanin biosynthesis genes including DFR, LDOX, and UF3GT, were induced slightly by JA in the BR mutants relative to wild type. Furthermore, the expression level of JA-induced Myb/bHLH transcription factors such as PAP1, PAP2, and GL3, which are components of the WD-repeat/Myb/bHLH transcriptional complexes that mediate the 'late' anthocyanin biosynthesis genes, was lower in the BR mutants than that in wild type. These results suggested that BR affects JA-induced anthocyanin accumulation by regulating the 'late' anthocyanin biosynthesis genes and this regulation might be mediated by the WD-repeat/Myb/bHLH transcriptional complexes.

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.