Autophagy and Non-Classical Vacuolar Targeting in Tobacco BY-2 Cells

Abstract

The vacuole is an organelle that occupies nearly 90% of the plant cell volume and performs various functions that are essential for plant growth, development and adaptation to environmental changes. The vacuoles are categorized into lytic vacuoles and storage vacuoles based on their biochemical function. Lytic vacuoles contain various catabolic enzymes whereas storage vacuoles contain storage proteins, sugars or pigments. In some cases, pigmentaccumulating vacuoles containmany hydrolases that are predominantly found in lytic vacuoles. Thus vacuoles are organelles with diverse functions. Proteins that are stored in nutrient-storage organs, such as seeds or tubers, are, in most cases, stored in storage vacuoles. Defense-related proteins, such as chitinase and protease inhibitors, are also accumulated in vacuoles in many plant cells. Using some of these storage or defense-related proteins vacuolar targeting machineries have been characterized using both protoplasts from plant tissues and cultured cells of plants including tobacco BY-2 cells. In particular, the transport pathway that starts at the endoplasmic reticulum (ER) and passes through the Golgi apparatus has been characterized in depth in BY-2 cells (Matsuoka 2004). The flow path to the vacuole is a branch of the secretory pathway and is known as a signaland receptor-mediated process. In addition to these classical vacuolar targeting pathways, many researchers have recently reported various non-classical vacuolar transport pathways to both vacuoles. Among them, two major routes to the vacuoles have been characterized in plant cells. One is autophagy, which is a bulk degradation system of cellular components. The other is a non-classical vacuolar targeting pathway, in which ER-derived vesicles are directly transported to the vacuoles. In this chapter, we summarize the recent findings concerning autophagic routes and non-classical routes in plant cells and knowledge concerning these pathways as obtained from tobacco BY-2 cells.