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Abstract

The globally cultivated Brassica species possess diverse aliphatic glucosinolates, which are important
for plant defense and animal nutrition. The committed step in the side-chain elongation of
methionine-derived aliphatic glucosinolates is catlyzed by methylthioalkylmalate synthase, which
likely evolved from the isopropylmalate synthases of leucine biosynthesis. However, the molecular
basis for the evolution of methylthioalkylmalate synthase and its generation of natural product
diversity in Brassica is poorly understood. Here we show that Brassica genomes encode multiple
methylthioalkylmalate synthases that have differences in expression profiles and 2-oxo substrate
preference, which account for the diversity of aliphatic glucosinolates across Brassica accessions.
Analysis of the 2.1 Å resolution x-ray crystal structure of B. juncea methylthioalkylmalate synthase
identified key active site residues responsible for controlling the specificity for different 2-oxo
substrates and the determinants of side-chain length in aliphatic glucosinolates. Overall, these results
provide the evolutionary and biochemical foundation for the diversification of glucosinolates profiles
across globally cultivated Brassica species, which could be used with ongoing breeding strategies
towards the manipulation of beneficial glucosinolates compounds for animal health and plant protection.