Solanum lycopersicum RNA-Seq: Flavonols improve thermotolerance in tomato pollen during germination and tube elongation
Temperatures above the optimum can result in lower crop yields with substantial reductions in reproductive success. Temperature stress increases the levels of reactive oxygen species (ROS) in pollen of Solanum lycopersicum (tomato) resulting in impaired pollen performance. Plants have multiple mechanisms to maintain ROS homeostasis including the synthesis of antioxidants, including flavonol metabolites. The tomato mutant, anthocyanin reduced (are) has a defect in the gene encoding flavanone 3 hydroxylase (F3H), which results in the reduced synthesis of flavonols. We examined pollen production, viability, germination, and pollen tube elongation in the are mutant and found significant reductions in each of these parameters relative to the VF36 parental line, all of which were accentuated by heat stress. Genetic complementation with a F3H transgene and chemical complementation with exogenous flavonols reversed the effect of elevated temperature on pollen germination in are to levels similar to the parental line, VF36. Transformation of the VF36 line with a F3H transgene was able to prevent the impaired pollen performance at elevated temperatures. Consistent with flavonol modulated ROS defining pollen performance, the ROS levels in these tomato lines are inversely proportional to flavonol levels and inhibitors of ROS synthesis improved pollen performance. RNA-Seq was performed with pollen grains during germination and tube extension in are, VF36, and VF36 transformed with the F3H transgene at optimal and stress temperatures. This analysis revealed a robust set of genes whose transcript abundance increased at elevated temperature, with the greatest number of differentially expressed transcripts found in are. These results demonstrated that heat-induced ROS limit reproductive success, and flavonols protect pollen from heat-induced ROS and the resulting reproductive impairments. We identified changes in transcripts whose expression are inversely correlated with reproductive success providing insight into genetic targets for future development of plants with enhanced tolerance to the temperature stress associated with climate change.