Transcriptome and metabolite reveal the inhibition induced by combined heat and drought stress on the viability of silk and pollen in summer maize

With the influence of climate warming, the frequency of combined high temperature and water deficit events is increasing, and the reproductive organs of maize are extremely sensitive to heat or drought stress. However, the difference between maize pollen and silk in response to combined stress is unclear. In this study, single high temperature (TS) or drought (DS) and combined stress (TDS) were conducted at the reproductive development stage from the 10-leaf vegetative stage to anthesis and the molecular and biochemical responses of reproductive organs to stress were analyzed using an integrated transcriptomic and metabolomic approach. Results showed that TDS decreased by 45.65 % pollen viability and inhibited apical silk elongation and floret abortion, leading to 19.01 % silks unfertilized. Metabolome analysis of pollen and silk revealed that under TS, DS, and TDS, soluble sugar, including sucrose, D-fructose, D-galactose, and D-glucose of pollen were changed dramatically, and cytokinin was the sole phytohormone significantly positively correlated with silk viability. Transcriptome profiling in the crosstalk pathway between zeatin and sugar biosynthesis showed that the lower expression of Zm00001d047253-encoded sucrose synthase enzyme suppressed the transformation of sucrose into D-fructose, possibly resulting in decreased pollen viability under stress. Moreover, low transcript levels of ZmIPT8 and ZmIPT6 , which encode isopentenyl transferase, in silk restricted cytokinin biosynthesis under stress. Low isopentenyl transferase activity and cytokinin content were decreased by 30.62 % and 53.18 %, respectively under TDS, and further experiment verified the significant remedial effect of exogenous cytokinin on the activity of silks under stress. These findings provide new insights for breeding maize that is better adapted to climate change to maintain global food security. • Heat and drought stress resulted in apical silks being unexserted and unfertilized. • Lower SUS suppressed sucrose transforming to D-fructose in pollen under stress. • Lower IPT activity triggered cytokinin deficit leading to dwarf silks under stress. • Exogenous application of cytokinin significantly restored the silks under stress.