3.7 Effect of drought on phytohormones metabolism and signaling
The expression profiles of IAA, ABA and JA metabolic and signaling genes based on RNA-Seq data and their corresponding endogenous contents were analyzed. A total of 38 DEGs encoding enzymes involved in auxin metabolism (17) and signaling (21) were at least regulated at one stage (Table S10). The IAA biosynthesis gene, SlTAR2, was significantly down-regulated at PMC-MEI stage, whereas SlTAR1-like ,SlTAR 2, ToFZY1 and ToFZY2 were significantly up-regulated at TED-VUM stage (Figure 7C). Consistently, the content of endogenous IAA was significantly reduced at PMC-MEI stage whereas it was moderately increased at TED-VUM under DS condition (Figure 7B).
A total of 19 DEGs associated with ABA metabolism and signaling were identified (Table S10). At PMC-MEI stage, two dehydrogenase/reductase genes, 3-oxoacyl-CoA reductase 1 and SlSDR12-like involved in ABA biosynthesis were significantly up-regulated, whileSlcyp707a1, encoding ABA 8สน-hydroxylase involved in ABA catabolism, was markedly induced. Correspondingly, the core components of ABA signaling genes including one ABA receptor, SlPYL9 , twoprotein phosphatase 2C (PP2C ) genes (SlPP2C37 andSlDIG3), and two SNF1-RELATED PROTEIN KINASE2 (SnRK2 ) genes (SlSRK2C and AY222455 ) were significantly up-regulated (Figure 7). Additionally, two known direct downstream transcription factor targets of SnRK2, SlAREB , homologue ofAtAREB2 and SlABF , homologue of AtABF3 (Figure S2B) which are regulators of ABA-responsive genes, were exclusively significantly up-regulated at PMC-MEI stage, suggesting that ABA signaling is induced early in anther development under drought stress. At TED-VUM stage, only SlSDR12-like and SlDIG3 were significantly up-regulated and only a few genes were moderately induced at BIN-MP stage such as Slcyp707a1 , SlPP2C37 andSlSRK2C (Figure 7), suggesting that ABA signaling is impaired under drought condition at TED-VUM and BIN-MP stages. Consistently, endogenous ABA content was significantly increased at PMC-MEI and TED-VUM stages with an increase of 205% and 395%, respectively (Figure 7B). Together, these results suggest that ABA, in conjunction with its catabolic gene might play a significant role in drought tolerance mechanism in the early stages of anther development.
A total of 21 DEGs associated with JA metabolism and signaling were recognized in at least one stage (Table S10). RNA-Seq analysis of four genes involved in JA biosynthesis revealed that at PMC-MEI stage,SlLOX5 exhibited significant increase. At TED-VUM and BIN-MP stages, all four genes: SlLOX5 , SlOPR3 ,Phospholipase A2 and SlAOC were down-regulated under drought stress although none was significant at BIN-MP stage (Figure 7C). Invariably, under DS, endogenous JA contents were significantly decreased at TED-VUM and BIN-MP stages (Figure 7B).