Specialized Vacuoles of Myrosin Cells: Chemical Defense Strategy in Brassicales Plants

Plant vacuoles display many versatile functions. Vacuoles in vegetative tissues are generally involved in protein degradation, and are called lytic vacuoles. However, vegetative vacuoles in specialized cells can accumulate large concentrations of proteins, such as those in idioblast myrosin cells along veins in the order Brassicales, which store large amounts of myrosinases (thioglucoside glucohydrolase and thioglucoside glucohydrolase). Myrosinases cleave the bond between sulfur and glucose in sulfur-rich compounds (glucosinolates) to produce toxic compounds (isothiocyanates) when plants are damaged by pests. This defense strategy is called the myrosinase-glucosinolate system. Recent studies identified atypical myrosinases, PENETRATION 2 (PEN2) and PYK10, along with key components for development of myrosin cells. In this review, we discuss three topics in the myrosinase-glucosinolate system. First, we summarize the complexity and importance of the myrosinase-glucosinolate system, including classical myrosinases, atypical myrosinases and the system that counteracts the myrosinase-glucosinolate system. Secondly, we describe molecular machineries underlying myrosin cell development, including specific reporters, cell lineage, cell differentiation and cell fate determination. The master regulators for myrosin cell differentiation, FAMA and SCREAM, are key transcription factors involved in guard cell differentiation. This indicates that myrosin cells and guard cells share similar transcriptional networks. Finally, we hypothesize that the myrosinase-glucosinolate system may have originated in stomata of ancestral Brassicales plants and, after that, plants co-opted this defense strategy into idioblasts near veins at inner tissue layers.