Co-occurring elevated temperature and drought stresses during cotton fiber thickening stage inhibit fiber biomass accumulation and cellulose synthesis

In order to investigate the effect of simultaneous elevated temperature and drought on Gossypium hirsutum fiber development under the background of global climate change, cotton (cv. Sumian 15 and cv. PHY370WR) plants were grown under ambient temperature (AT) or elevated temperature (ET, 2.5–2.7 °C higher than AT) conditions from 2016 to 2018. Meanwhile, three soil relative water content (SRWC) levels including (75 ± 5) % (optimum soil moisture), (60 ± 5) % (moderate drought) and (45 ± 5) % (severe drought) were established under each temperature condition. Results showed that elevated temperature combined with drought more negatively impacted the theoretical maximum of fiber biomass by further reducing the duration of rapid increase and the average increase rate of fiber biomass than any single stress. Elevated temperature and drought had opposite effects on sucrose synthesis in fibers by differently altering sucrose phosphate synthase (SPS) activity and GhSPS isoforms ( GhSPS-1 , GhSPS-2 , and GhSPS-3 ) expressions, and the two stresses had an antagonistic effect on SPS activity when they were coupled. However, lower sucrose content was observed under all single and coupled stresses, meaning that less carbon was available for cellulose synthesis. Elevated temperature or drought alone also limited cellulose synthesis by decreasing the activities of sucrose synthase (SuSy), soluble acid invertase (VIN) and alkaline invertase (CIN) involved in sucrose degradation. The decrease of SuSy activity should be the result of down-regulation of GhSuSyA , GhSuSyB and GhSuSyC expressions, and the decrease of VIN activity should be the result of down-regulation of GhInv-1 and GhInv-2 expressions under high temperature or drought alone. The combination of the two stresses had more negatively effects on SuSy activity by further down-regulating the iso-forms GhSuSyA and GhSuSyB expressions, on VIN activity by further decreasing GhInv-1 and GhInv-2 expressions, and on CIN activity than any single stress, which would further inhibit sucrose degradation to form UDPG for cellulose synthesis. Moreover, elevated temperature or drought alone enhanced callose accumulation by highly up-regulating GhCalS-5 expression and down-regulating β-1,3-glucanase expression, and the combined stress further increased the callose accumulation, suggesting that more UDPG from sucrose decomposition was allocated to callose synthesis rather than cellulose synthesis. All these confirmed that elevated temperature combined with drought exhibited synergistic negative effects on cotton fiber biomass accumulation and cellulose synthesis during fiber thickening. • The combination of elevated temperature and drought damages more on fiber biomass. • Combined stress more negatively impacts cellulose content than any single stress. • Elevated temperature and drought have antagonistic effects on sucrose synthesis. • Combined stress further limits sucrose hydrolysis to form UDPG for cellulose synthesis. • UDPG from sucrose hydrolysis is more transformed to callose under combined stress.