Brassinosteroids promote Arabidopsis pollen germination and growth

Pollen tubes are among the fastest tip-growing plant cells and represent an excellent experimental system for studying the dynamics and spatiotemporal control of polarized cell growth. However, investigating pollen tube tip growth in the model plant Arabidopsis remains difficult because in vitro pollen germination and pollen tube growth rates are highly variable and largely different from those observed in pistils, most likely due to growth-promoting properties of the female reproductive tract. We found that in vitro grown Arabidopsis pollen respond to brassinosteroid (BR) in a dose-dependent manner. Pollen germination and pollen tube growth increased nine- and fivefold, respectively, when media were supplemented with 10 µM epibrassinolide (epiBL), resulting in growth kinetics more similar to growth in vivo. Expression analyses show that the promoter of one of the key enzymes in BR biosynthesis, CYP90A1/CPD, is highly active in the cells of the reproductive tract that form the pathway for pollen tubes from the stigma to the ovules. Pollen tubes grew significantly shorter through the reproductive tract of a cyp90a1 mutant compared to the wild type, or to a BR perception mutant. Our results show that epiBL promotes pollen germination and tube growth in vitro and suggest that the cells of the reproductive tract provide BR compounds to stimulate pollen tube growth.