Abstract
The aim of this study was to characterize hormonal crosstalk with the sugar signaling and metabolic pathway based on a time course analysis of drought intensity. Drought intensity-responsive changes in the assimilation of newly fixed carbon (C) into soluble sugar, the content of sugar and starch, and expression of genes involved in carbohydrate metabolism were interpreted as being linked to endogenous abscisic acid (ABA) and salicylic acid (SA) levels and their signaling genes. The ABA and SA levels in the drought-stressed leaves increased together during the early drought period (days 0–6), and additional ABA accumulation occurred with depressed SA during the late period (days 6–14). Although drought treatment decreased the assimilation of newly fixed C into soluble sugar, representing a 59.9%, 33.1%, and 62.9% reduction in 13C-glucose, 13C-fructose, and 13C-sucrose on day 14, respectively, the drought-responsive accumulation of soluble sugars was significant. During the early period, the drought-responsive accumulation of hexose and sucrose was concurrent with the upregulated expression of hexokinase 1 (HXK1), which, in turn, occurred parallel to the upregulation of ABA synthesis gene 9-sis-epoxycarotenoid dioxygenase (NCED3) and SA-related genes (isochorismate synthase 1 (ICS1) and non-expressor of pathogenesis-related gene (NPR1)). During the late period, hexose accumulation, sucrose phloem loading, and starch degradation were dominant, with a highly enhanced expression of the starch degradation-related genes β-amylase 1 (BAM1) and α-amylase 3 (AMY3), which were concomitant with the parallel enhancement of sucrose non-fermenting−1 (Snf1)-related protein kinase 2 (SnRK2).2 and ABA-responsive element binding 2 (AREB2) expression in an ABA-dependent manner. These results indicate that the drought-responsive accumulation of sugars (especially SA-mediated sucrose accumulation) is part of the acclamatory process during the early period. Conversely, ABA-responsive hexose accumulation and sucrose phloem loading represent severe drought symptoms during the late drought period.
Highlights
The pool size of sugars under drought stress is determined by hexose biosynthesis via hexokinase phosphorylation activity [8], sucrose synthesis catalyzed by sucrose phosphate synthase, and sucrose hydrolysis catalyzed by intracellular invertase [8], as well as starch degradation and sugar transport [1,9]
The drought-induced accumulation of soluble sugars has often been observed in previous studies [1,6,7,10]; the photosynthesis and carbon assimilation to hexose and sucrose are notably decreased [2,11]
The drought treatment was created by decreasing the daily irrigation volume gradually decreased the leaf osmotic potential, relative water content, and leaf dry mass (Table 1)
Summary
The dehydration stress-induced decrease in leaf water potential is responsible for the reduction of photosynthesis and C assimilation [1,2]. The decrease in the rate of photosynthesis is attributed mainly to stomatal closure, which is the earliest response to water deficit stress [3]. The pool size of sugars under drought stress is determined by hexose biosynthesis via hexokinase phosphorylation activity [8], sucrose synthesis catalyzed by sucrose phosphate synthase, and sucrose hydrolysis catalyzed by intracellular invertase [8], as well as starch degradation and sugar transport [1,9]. The drought-induced accumulation of soluble sugars has often been observed in previous studies [1,6,7,10]; the photosynthesis and carbon assimilation to hexose and sucrose are notably decreased [2,11]. Especially sucrose, are important regulatory signals of stress responses and tolerance mechanisms with extensive interactions between sugars and plant hormones; for example, the sugar involvement in the regulation of hormone-responsive pathways [13,14] and, the hormone-mediated sugar accumulation and sucrose metabolism [7,15,16], as well as the hormonal regulation of leaf starch degradation [7,14,17]
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