Brassinosteroids and Flavonols Confer Temperature Stress Tolerance to Pollen Tube Germination and Growth

Successful reproduction is a prerequisite for the maintenance of species in the era of climate change. In agriculture, temperature has a significant impact on pollen tube elongation and attraction, and consequently, on the efficiency of fruiting and crop yields. Brassinosteroids (BRs) are plant steroid hormones involved in pollen tube germination, elongation, and capacitation. To elucidate the effect of BR signaling in pistils during the reproductive process, transcriptome analysis of BR receptor (BRI1) mutant pistils was performed, which indicated that genes involved in flavonol biosynthesis were downregulated in bri1 pistils. Flavonols are essential for pollen tube growth in some plants and facilitate pollen tube elongation under high-temperature conditions. To investigate the effects of BR and flavonols on pollen tubes under temperature stress, in vitro assays were performed using both the compounds. Brassinolide (BL), which is the most active BR, promoted pollen tube germination at high, low, and optimal temperatures. Germination was further enhanced by the flavonol, quercetin, in the presence of BL especially at low temperatures. in vivo analysis supported the observation that when BL and quercetin were supplied concurrently, the pollen tube guidance rate was rescued at low temperatures. Quercetin enhanced pollen tube elongation under all the conditions tested at an optimal concentration of 15 μM. BL elevated reactive oxygen species (ROS) under all temperature conditions, whereas quercetin regulated ROS synergistically with BR. Thus, BRs and flavonols supplied by pistils are suggested to function in pollen tube germination and elongation through the maintenance of ROS homeostasis.