3.6 Drought stress disturbed sucrose and starch metabolism and soluble sugar transport
Consistent with ‘carbohydrate metabolic process’ and ‘photosynthesis’ pathway significantly enriched in drought-repressed genes, 16 DEGs involved in sucrose cleavage, sugar phosphorylation and starch synthesis, and 33 DEGs associated with sugar transport were identified (Table S9). Gene expression analysis showed that the sucrose cleavage genes including cell wall invertase 3-like (SlCWINV3-like) and sucrose synthase (SUS ) TOMSSF were significantly repressed at PMC-MEI stage. Whereas ß-fructofuranosidase and SlSUS6were significantly up-regulated at TED-VUM stage, SlCWINV3-likeand SlSUS6 were significantly repressed at BIN-MP (Figure 6A) stage, suggesting that sucrose hydrolysis is impaired at PMC-MEI and BIN-MP stages but not at TED-VUM stage under drought stress. The sugar transporters, SlSWEET16 and SlSTP8 were notably up-regulated at PMC-MEI stage. SlSWEET16 was markedly induced at both TED-VUM and BIN-MP stages (Figure 6A), suggesting that monosaccharide transport is generally not prevented in tomato anther under drought stress. Further utilization of hexose sugars in metabolic processes requires their phosphorylation, catalyzed by hexokinases that phosphorylate both glucose and fructose, and fructokinases that specifically phosphorylate fructose (Granot et al., 2013). WhileSlhxk1 and Slhxk2 were both significantly repressed at PMC-MEI stage, Slhxk1 was significantly down-regulated at TED-VUM stage. In addition, the rate-limiting enzymes in starch biosynthesis,Slagpl3 and TOMADPGPPs were both significantly down-regulated at PMC-MEI and TED-VUM stages. However, ß-amylase 8, the starch hydrolyzing gene was significantly up-regulated at BIN-MP stage (Figure 5A). Our data suggest hindered anther sink strength and starch biosynthesis while starch degradation is enhanced during pollen development under DS.
Under DS condition, sucrose, fructose and glucose contents were strikingly higher at PMC-MEI and BIN-MP stages, although glucose was significantly lower at PMC-MEI stage whereas their contents remained similar to those in WW anthers at TED-VUM stage (Figure 6B). In drought-stressed anthers, the pattern of starch accumulation in the developing pollen was comparable to that in WW anthers from MEI to VUM stage. However, at BIN and MP stages, starch accumulation in pollen grains of DS anthers was attenuated, whereas more starch accumulated in the pollen grain under WW condition, (Figure 6C). After rewatering, type-2 anthers accumulated more starch granules in the connective and endothecium tissues at PMC-MEI stage, whereas starch accumulation in the pollen grains was decreased at BIN–MP stage, though not to the same extent as in anthers DS 4 d (Figure S3), suggesting partial recovery of carbohydrate metabolic pathway after rewatering. In type-3 anthers, starch accumulation in the pollen and anther walls was abolished (Figure S3) across all the examined stages suggesting that carbohydrate metabolic pathway never recovered after rewatering in the type-3 anthers.