ABSTRACT

The biology of plastids in green plants has a long history, and the literature in the area is vast. A book by Wise and Hoober (2006) provides relevant information on different forms of plastids, their origin, specific structural features, functions, and locations in plants. Chloroplast is recognized as the most important form of plastid in green plants and algae because of its function in photosynthesis, which sustains life on the planet Earth. In addition to synthesis of pigments and proteins, chloroplasts in higher plants also participate in the biosynthesis of several essential cellular metabolites, including amino acids, fatty acids, and phytohormones. The organelle also plays a role in the network of sulfur metabolism (Biswal et al. 2008). Existing literature suggests chloroplast as a sensor of abiotic stress and modulator of plant stress response (Biswal et al. 2011). The organelle is also known to play a dominant role in regulating leaf and whole plant development (López-Zuez and Pyke 2005; Inaba and Ito-Inaba 2010). Importantly, the development of the plastid is tightly coupled to the developmental program of the plants, specifically in monocarpic plants (Krupinska et al. 2013). In this context, the mechanism of development of chloroplast and the regulatory system associated with it are emerging as a fascinating area of study in 78plant science. As such, the development of different plastid forms in plants is complex. The diversity of different plastid forms is primarily because of the diversity in their developmental programs as determined by their position; location (e.g., cells, tissues, and organs); environmental condition; and interaction with neighboring cells (Biswal et al. 2003; Jarvis and López-Juez 2013). Currently, excellent reviews are available on the nature and mechanism of chloroplast development and existing knowledge gaps in our understanding of the regulatory systems associated with the developmental programs (Croce and Amerongen 2011; Fischer 2012; Flores-Pérez and Jarvis 2013; Jarvis and López-Juez 2013; Juvany et al. 2013; Nickelsen and Rengstl 2013; Rochaix 2014; Pogson et al. 2015; Suga et al. 2015). This review will focus only on the pathway of development of chloroplast from proplastid with subsequent transformation of chloroplast to gerontoplast (senescing chloroplast). Gerontoplast is rather a new entry to the plastid family in green plants (Sitte 1977). This plastid with characteristic structural features plays a role in nutrient recycling (Thomas 2013). Figure 5.1 provides a quick introduction to the theme of the review. The development of chloroplast covers three major phases, namely, the buildup phase in emerging leaves with gradual transformation of proplastids to young chloroplasts, which are subsequently transformed to mature chloroplasts. The chloroplasts in fully green mature leaves are capable of capture, assimilation, and storage of carbon with high efficiency. Transformation of mature chloroplast to gerontoplast is the terminal phase of the developmental program, leading to leaf yellowing followed by necrosis and death (Thomas 2013). The genetic regulation and the regulatory network with several internal and environmental factors that determine these developmental transitions are complex and are not fully resolved. Among the environmental factors, light is recognized as the most important one that modulates the plastid development in the photomorphogenic network, mediated by well-identified photoreceptors (Biswal et al. 2003). Reports available on several internal factors modulating plastid development are plenty (Wise and Hoober 2006; Biswal et al. 2013). Phytohormones are well reported to play a critical role during plant development (Biswal et al. 2003). Development-dependent signaling and organellar interactions also influence plastid biogenesis and senescence during leaf development (Biswal et al. 2013). Undoubtedly, the nucleus is one of the important organelles, which codes for most of the plastid proteins, both structural and regulatory. The other organelles, including mitochondria, peroxisomes, and the endoplasmic reticulum, are, in recent years, known to play a no less important role in regulating plastid development, through an interconnected metabolic network (Pogson and Albrecht 2011; Biswal et al. 2013).