Synthesis and degradation of dhurrin. (a) Pathway of synthesis and degradation of dhurrin. Dhurrin is synthesized from tyrosine through a p-hydroxymandelonitrile intermediate; this is catalyzed by CYP79A1 and CYP71E1. Subsequently, dhurrin content is regulated in synthesis and degradation. In the synthesis pathway, the UDP-Glc p-hydroxymandelonitrile glycosyltransferase UGT85B1 converts p-hydroxymandelonitrile into dhurrin. In the degradation pathway, dhurrinase and p-(S)-hydroxymandelonitrile lyase sequentially degrade dhurrin and release hydrogen cyanide (HCN) irreversibly. (b) Downregulation of genes for degradation of dhurrin. RPKMs for each gene were compared in mock-infected (gray bars) and pathogen-infected (black bars) leaves. (c) Region of the p-hydroxymandelonitrile gene and the corresponding region in rice. Sorghum genes encoding vacuolar protein sorting 55 protein–like protein (Vps55), 26 S protease regulatory subunit 6A (Pro6A), or heat shock protein 40 (HSP40) had corresponding genes on rice chromosome 2. Genes in the rice corresponding region encoded UDP-glycosyltransferase 91D1 (UGT; Os02t08039000), peptidase S8 (PepS8; Os02t0803900), methyltransferase type 11 (MT11; Os02t0804300), or an unknown protein (Os02t0804100, Os02t0804400), or were non-coding (Os02t0804000). Genes neighboring p-(S)-hydroxymandelonitrile lyase in sorghum, encoding synaptobrevin 1(Syn1), 40 S ribosomal protein S30 (RPS30), or 50 S ribosomal protein L9 (RPL9), had no corresponding genes in rice.