Analysis of the quadruple lsu mutant reveals molecular determinants of the role of LSU proteins in sulfur assimilation in Arabidopsis
Because plants are immobile, they have developed intricate mechanisms to sense and absorb nutrients, adjusting their growth and development accordingly. Sulfur is an essential macroelement, but our understanding of its metabolism and homeostasis is limited. LSU (RESPONSE TO LOW SULFUR) proteins are plant-specific proteins with unknown molecular functions and were first identified during transcriptomic studies on sulfur deficiency in Arabidopsis. These proteins are crucial hubs that integrate environmental signals and are involved in the response to various stressors. Herein, we report the direct involvement of LSU proteins in primary sulfur metabolism for the first time. Our findings revealed that the quadruple lsu mutant, q-lsu-KO, which was grown under nonlimiting sulfate conditions, exhibited a molecular response resembling that of sulfur-deficient wild-type plants. This led us to explore the interactions of LSU proteins with sulfate reduction pathway enzymes. We found that all LSU proteins interact with ATPS1 and ATPS3 isoforms of ATP sulfurylase, all three isoforms of adenosine 5´phosphosulfate reductase (APR), and sulfite reductase (SiR). Additionally, in vitro assays revealed that LSU1 enhances the enzymatic activity of SiR. These results highlight the supportive role of LSU proteins in the sulfate reduction pathway. Overall design: To dissect the role of LSU proteins in plant metabolism, we investigated the impact of knocking out all four LSU genes on the plant global transcriptome under both normal and sulfur deficiency conditions using 3'RNA-seq. Libraries were generated from total RNA extracted from either the shoots or roots of 15-day-old q-lsu-KO mutant and Col-0 (wild-type) plants. The plants were grown under normal sulfur or sulfur deficiency conditions.