Abstract
The plant response to sulfur deficiency includes extensive metabolic changes which can be monitored at various levels (transcriptome, proteome, metabolome) even before the first visible symptoms of sulfur starvation appear. Four members of the plant-specific LSU (response to Low SUlfur) gene family occur in Arabidopsis thaliana (LSU1-4). Variable numbers of LSU genes occur in other plant species but they were studied only in Arabidopsis and tobacco. Three out of four of the Arabidopsis LSU genes are induced by sulfur deficiency. The LSU-like genes in tobacco were characterized as UP9 (UPregulated by sulfur deficit 9). LSU-like proteins do not have characteristic domains that provide clues to their function. Despite having only moderate primary sequence conservation they share several common features including small size, a coiled–coil secondary structure and short conserved motifs in specific positions. Although the precise function of LSU-like proteins is still unknown there is some evidence that members of the LSU family are involved in plant responses to environmental challenges, such as sulfur deficiency, and possibly in plant immune responses. Various bioinformatic approaches have identified LSU-like proteins as important hubs for integration of signals from environmental stimuli. In this paper we review a variety of published data on LSU gene expression, the properties of lsu mutants and features of LSU-like proteins in the hope of shedding some light on their possible role in plant metabolism.
Highlights
The first global analyzes of gene expression profiles under sulfur deficiency stress in Arabidopsis appeared in Hirai et al (2003), Maruyama-Nakashita et al (2003) and Nikiforova et al (2003), these studies focused on genes encoding proteins with known functions
Two years later LSU1 (At3g49580) and LSU2 (At5g24660) were identified as two out of 15 sulfurresponsive genes which were significantly up-regulated in roots as early as 2 h (LSU1) or 4 h (LSU2) after plants were transferred to sulfur-free medium; a sulfur-responsive element (SURE) was identified in their promoter regions (MaruyamaNakashita et al, 2005)
LSU1, LSU2, and LSU3 genes from Arabidopsis are induced by sulfur deficiency; only LSU2 has been shown to be involved in retrograde signaling associated with chloroplast malfunction
Summary
The first global analyzes of gene expression profiles under sulfur deficiency stress in Arabidopsis appeared in Hirai et al (2003), Maruyama-Nakashita et al (2003) and Nikiforova et al (2003), these studies focused on genes encoding proteins with known functions. It has recently been reported that expression of LSU1 (and BGLU28 [At2g44460], SDI1 [At5g48850] and SULTR4;2 [At3g12520]) is much less affected by S availability in the sultr1;2 mutants than in the wild type (Zhang et al, 2014) This observation is not strictly related to the function of LSU/UP9 proteins, it is worth noting because it makes an important contribution to understanding the plant mechanisms responsible for sensing S availability and S status-dependent regulation of gene expression. An Arabidopsis homolog of NtEIL2, SLIM1, which has been identified earlier as a critical transcriptional regulator of plant sulfur response and sulfur metabolism (Maruyama-Nakashita et al, 2006), was able to bind to UP9C promoter containing UPEbox. The set of genes containing UPE-box in promoter appears to be very similar to the OAS cluster genes (Hubberten et al, 2012b)
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