Role of Arabidopsis RabG3b and autophagy in tracheary element differentiation

The vascular system of plants consists of two conducting tissues, xylem and phloem, which differentiate from procambium cells. Xylem serves as a transporting system for water and signaling molecules and is formed by sequential developmental processes, including cell division/expansion, secondary cell wall deposition, vacuole collapse and programmed cell death (PCD). PCD during xylem differentiation is accomplished by degradation of cytoplasmic constituents, and it is required for the formation of hollow vessels, known as tracheary elements (TEs). Our recent study revealed that the small GTPase RabG3b acts as a regulator of TE differentiation through its autophagic activation. By using an Arabidopsis in vitro cell culture system, we showed that autophagy is activated during TE differentiation. Overexpression of a constitutively active RabG3b (RabG3bCA) significantly enhances both autophagy and TE differentiation, which are consistently suppressed in transgenic plants overexpressing a dominant negative form (RabG3bDN) or RabG3b RNAi (RabG3bRNAi), a brassinosteroid- insensitive mutant bri1-301 and an autophagy mutant atg5-1. On the basis of our results, we propose that RabG3b functions as a component of autophagy and regulates TE differentiation by activating the process of PCD.