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.