Brassinosteroids promote pollen tube guidance by coordinating gene expression in male and female reproductive tissues

Hormones play an essential role in the survival and maintenance of homeostasis. Steroid hormones are structurally and evolutionarily conserved across animals, insects, and plants. Brassinosteroids (BRs) are plant steroid hormones originally isolated from pollen grains. However, the molecular roles of BRs in plant reproduction remain unclear. Here, we show that biologically active BRs, brassinolide (BL) and castasterone (CS), are detected in pistils and accumulate in the pollen grains of Arabidopsis thaliana. In vitro incubation with BL promotes pollen tube capacitation and attraction to ovules. Furthermore, BL treatment elevates the expression of various reproductive genes in pollen tubes and ovules. Additionally, mutant ovules defective in BRI1 signaling show decreased guidance rates and abnormal development of the filiform apparatus in synergid cells. Overall, this study characterizes the molecular functions of both BL and CS in promoting pollen tube guidance by regulating gene expression in male and female reproductive tissues.