Brassinosteroids regulate glucosinolate biosynthesis in Arabidopsis thaliana.

Plants must constantly adjust their growth and defense responses to deal with the wide variety of stresses they encounter in their environment. Among phytohormones, brassinosteroids (BRs) are an important group of plant steroid hormones involved in numerous aspects of the plant lifecycle including growth, development and responses to various stresses including insect attacks. Here, we show that BRs regulate glucosinolate (GS) biosynthesis and function in insect herbivory. Preference tests and larval feeding experiments using the generalist herbivore, diamondback moth (Plutella xylostella), revealed that the larvae prefer to feed on Arabidopsis thaliana brassinosteroid insensitive 1 (bri1-5) plants over wild-type Ws-2 or BRI1-Flag (bri1-5 background) transgenic plants, which results in an increase in larval weight. Analysis of GS contents showed that 3-(methylsulfinyl) propyl GS (C3) levels were higher in bri1-5 than in Ws2 and BRI1-Flag transgenic plants, whereas sinigrin (2-propenylglucosinolate), glucoerucin (4-methylthiobutylglucosinolate) and glucobrassicin (indol-3-ylmethylglucosinolate) levels were lower in this mutant. We investigated the effect of brassinolide (BL) on GS biosynthesis in Arabidopsis and radish (Raphanus sativus L.) by monitoring the expression levels of GS biosynthetic genes, including MAM1, MAM3, BCAT4 and AOP2, which increased in a BL-dependent manner. These results suggest that BRs regulate GS profiles in higher plants, which function in defense responses against insects.