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.
INTRODUCTIONModern agriculture requires adequate fertilization of sulfur to achieve maximum crop yield and performances. Plants use sulfate, the oxidized form of sulfur existing in the soil, as a sulfur source (Crawford et al., 2000;Leustek et al., 2000;Saito, 2004). By contrast, animals, including humans, require sulfur-containing amino acids and proteins as dietary sulfur sources because of their inability to assimilate sulfate into Cys and Met. Significance of plant sulfate assimilatory pathway is manifested by its ability to fill this metabolic gap in the global sulfur cycle in nature (Crawford et al., 2000). In addition to its basic nutritional importance, sulfur is present in numbers of plant metabolites representing important biological activities as redox controllers, vitamins, coenzymes, flavors, and defense chemicals (Crawford et al., 2000;Leustek et al., 2000;Saito, 2004;Grubb and Abel, 2006;Halkier and Gershenzon, 2006). Activation of sulfate transport systems is critical for plant growth under a low-sulfur (-S) environment. When the soil environment is inadequately fertilized with sulfate, plants will sustain their growth by increasing the capacities of sulfate uptake systems in roots (Clarkson et al., 1983;Deane-Drummond, 1987;Smith et al., 1995Smith et al., , 1997. In Arabidopsis thaliana, highaffinity sulfate transporters that facilitate the initial uptake of sulfate serve this purpose (Takahashi et al., 2000;Vidmar et al., 2000;Shibagaki et al., 2002;Yoshimoto et a...