The C-Terminal Region of SLIM1 Transcription Factor Is Required for Sulfur Deficiency Response

Sulfur LIMitation1 (SLIM1) transcription factor coordinates gene expression in plants in response to sulfur deficiency (-S). SLIM1 belongs to the family of plant-specific EIL transcription factors with EIN3 and EIL1, which regulate the ethylene-responsive gene expression. The EIL domains consist of DNA binding and dimerization domains highly conserved among EIL family members, while the N- and C-terminal regions are structurally variable and postulated to have regulatory roles in this protein family, such that the EIN3 C-terminal region is essential for its ethylene-responsive activation. In this study, we focused on the roles of the SLIM1 C-terminal region. We examined the transactivation activity of the full-length and the truncated SLIM1 in yeast and Arabidopsis. The full-length SLIM1 and the truncated form of SLIM1 with a deletion of C-terminal 106 amino acids (dC105) transactivated the reporter gene expression in yeast when they were fused to the GAL4 DNA binding domain, whereas the deletion of additional 15 amino acids to remove the C-terminal 120 amino acids (dC120) eliminated such an activity, identifying the necessity of that 15-amino-acid segment for transactivation. In the Arabidopsis slim1-2 mutant, the transcript levels of SULTR1;2 sulfate transporter and the GFP expression derived from the SULTR1;2 promoter-GFP (PSULTR1;2-GFP) transgene construct were restored under -S by introducing the full-length SLIM1, but not with the C-terminal truncated forms dC105 and dC57. Furthermore, the transcript levels of S-responsive genes were restored concomitantly with an increase in glutathione accumulation in the complementing lines with the full-length SLIM1 but not with DC57. The C-terminal 57 amino acids of SLIM1 were also shown to be necessary for transactivation of a S-inducible gene, SHM7/MSA1, in a transient expression system using the SHM7/MSA1 promoter-GUS as a reporter. These findings suggest that the C-terminal region is essential for the SLIM1 activity

Transcriptome Profiling of Sulfur-Responsive Genes in Arabidopsis Reveals Global Effects of Sulfur Nutrition on Multiple Metabolic Pathways

Sulfate is a macronutrient required for cell growth and development. Arabidopsis has two high-affinity sulfate transporters (SULTR1;1 and SULTR1;2) that represent the sulfate uptake activities at the root surface. Sulfur limitation (–S) response relevant to the function of SULTR1;2 was elucidated in this study. We have isolated a novel T-DNA insertion allele defective in the SULTR1;2 sulfate transporter. This mutant, sel1-10, is allelic with the sel1 mutants identified previously in a screen for increased tolerance to selenate, a toxic analog of sulfate (Shibagaki et al., 2002). The abundance of SULTR1;1 mRNA was significantly increased in the sel1-10 mutant; however, this compensatory up-regulation of SULTR1;1 was not sufficient to restore the growth. The sulfate content of the mutant was 10% to 20% of the wild type, suggesting that induction of SULTR1;1 is not fully complementing the function of SULTR1;2 and that SULTR1;2 serves as the major facilitator for the acquisition of sulfate in Arabidopsis roots. Transcriptome analysis of approximately 8,000 Arabidopsis genes in the sel1-10 mutant suggested that dysfunction of the SULTR1;2 transporter can mimic general –S symptoms. Hierarchal clustering of sulfur responsive genes in the wild type and mutant indicated that sulfate uptake, reductive sulfur assimilation, and turnover of secondary sulfur metabolites are activated under –S. The profiles of –S-responsive genes further suggested induction of genes that may alleviate oxidative damage and generation of reactive oxygen species caused by shortage of glutathione

Sulfur Deficiency Increases Phosphate Accumulation, Uptake, and Transport in Arabidopsis thaliana

Recent studies have shown various metabolic and transcriptomic interactions between sulfur (S) and phosphorus (P) in plants. However, most studies have focused on the effects of phosphate (Pi) availability and P signaling pathways on S homeostasis, whereas the effects of S availability on P homeostasis remain largely unknown. In this study, we investigated the interactions between S and P from the perspective of S availability. We investigated the effects of S availability on Pi uptake, transport, and accumulation in Arabidopsis thaliana grown under sulfur sufficiency (+S) and deficiency (−S). Total P in shoots was significantly increased under −S owing to higher Pi accumulation. This accumulation was facilitated by increased Pi uptake under −S. In addition, −S increased root-to-shoot Pi transport, which was indicated by the increased Pi levels in xylem sap under −S. The −S-increased Pi level in the xylem sap was diminished in the disruption lines of PHT1;9 and PHO1, which are involved in root-to-shoot Pi transport. Our findings indicate a new aspect of the interaction between S and P by listing the increased Pi accumulation as part of −S responses and by highlighting the effects of −S on Pi uptake, transport, and homeostasis

Contribution of Root Hair Development to Sulfate Uptake in <i>Arabidopsis</i>

Root hairs often contribute to nutrient uptake from environments, but the contribution varies among nutrients. In Arabidopsis, two high-affinity sulfate transporters, SULTR1;1 and SULTR1;2, are responsible for sulfate uptake by roots. Their increased expression under sulfur deficiency (&#8722;S) stimulates sulfate uptake. Inspired by the higher and lower expression, respectively, of SULTR1;1 in mutants with more (werwolf [wer]) and fewer (caprice [cpc]) root hairs, we examined the contribution of root hairs to sulfate uptake. Sulfate uptake rates were similar among plant lines under both sulfur sufficiency (+S) and &#8722;S. Under &#8722;S, the expression of SULTR1;1 and SULTR1;2 was negatively correlated with the number of root hairs. These results suggest that both &#8722;S-induced SULTR expression and sulfate uptake rates were independent of the number of root hairs. In addition, we observed (1) a negative correlation between primary root lengths and number of root hairs and (2) a greater number of root hairs under &#8722;S than under +S. These observations suggested that under both +S and &#8722;S, sulfate uptake was influenced by the root biomass rather than the number of root hairs

Arabidopsis SLIM1 Is a Central Transcriptional Regulator of Plant Sulfur Response and Metabolism

Sulfur is an essential macronutrient required for plant growth. To identify key transcription factors regulating the sulfur assimilatory pathway, we screened Arabidopsis thaliana mutants using a fluorescent reporter gene construct consisting of the sulfur limitation-responsive promoter of the SULTR1;2 sulfate transporter and green fluorescent protein as a background indicator for monitoring plant sulfur responses. The isolated mutant, sulfur limitation1 (slim1), was unable to induce SULTR1;2 transcripts under low-sulfur (–S) conditions. Mutations causing the sulfur limitation responseless phenotypes of slim1 were identified in an EIL family transcription factor, ETHYLENE-INSENSITIVE3-LIKE3 (EIL3), whose functional identity with SLIM1 was confirmed by genetic complementation. Sulfate uptake and plant growth on –S were significantly reduced by slim1 mutations but recovered by overexpression of SLIM1. SLIM1 functioned as a central transcriptional regulator, which controlled both the activation of sulfate acquisition and degradation of glucosinolates under –S conditions. Metabolite analysis indicated stable accumulation of glucosinolates in slim1 mutants, even under –S conditions, particularly in the molecular species with methylsulfinylalkyl side chains beneficial to human health. Overexpression of SLIM1 and its rice (Oryza sativa) homologs, but no other EIL genes of Arabidopsis, restored the sulfur limitation responseless phenotypes of slim1 mutants, suggesting uniqueness of the SLIM1/EIL3 subgroup members as sulfur response regulators

Glucosinolate Distribution in the Aerial Parts of sel1-10, a Disruption Mutant of the Sulfate Transporter SULTR1;2, in Mature Arabidopsis thaliana Plants

Plants take up sulfur (S), an essential element for all organisms, as sulfate, which is mainly attributed to the function of SULTR1;2 in Arabidopsis. A disruption mutant of SULTR1;2, sel1-10, has been characterized with phenotypes similar to plants grown under sulfur deficiency (&minus;S). Although the effects of &minus;S on S metabolism were well investigated in seedlings, no studies have been performed on mature Arabidopsis plants. To study further the effects of &minus;S on S metabolism, we analyzed the accumulation and distribution of S-containing compounds in different parts of mature sel1-10 and of the wild-type (WT) plants grown under long-day conditions. While the levels of sulfate, cysteine, and glutathione were almost similar between sel1-10 and WT, levels of glucosinolates (GSLs) differed between them depending on the parts of the plant. GSLs levels in the leaves and stems were generally lower in sel1-10 than those in WT. However, sel1-10 seeds maintained similar levels of aliphatic GSLs to those in WT plants. GSL accumulation in reproductive tissues is likely to be prioritized even when sulfate supply is limited in sel1-10 for its role in S storage and plant defense

Strigolactones Modulate Salicylic Acid-Mediated Disease Resistance in Arabidopsis thaliana

Strigolactones are low-molecular-weight phytohormones that play several roles in plants, such as regulation of shoot branching and interactions with arbuscular mycorrhizal fungi and parasitic weeds. Recently, strigolactones have been shown to be involved in plant responses to abiotic and biotic stress conditions. Herein, we analyzed the effects of strigolactones on systemic acquired resistance induced through salicylic acid-mediated signaling. We observed that the systemic acquired resistance inducer enhanced disease resistance in strigolactone-signaling and biosynthesis-deficient mutants. However, the amount of endogenous salicylic acid and the expression levels of salicylic acid-responsive genes were lower in strigolactone signaling-deficient max2 mutants than in wildtype plants. In both the wildtype and strigolactone biosynthesis-deficient mutants, the strigolactone analog GR24 enhanced disease resistance, whereas treatment with a strigolactone biosynthesis inhibitor suppressed disease resistance in the wildtype. Before inoculation of wildtype plants with pathogenic bacteria, treatment with GR24 did not induce defense-related genes; however, salicylic acid-responsive defense genes were rapidly induced after pathogenic infection. These findings suggest that strigolactones have a priming effect on Arabidopsis thaliana by inducing salicylic acid-mediated disease resistance