Chloroplast structure and morphology

Chloroplasts are the most prominent form of plastid occurring in all green plant tissues and enable photosynthetic carbon fixation to occur in addition to a variety of other biochemical processes central to cellular metabolism. Like all plastids, they are bounded by a double plastid envelope membrane, which acts as a major control point for chloroplast import and export as well as being a major site for biochemical synthesis (Joyard et al. 1998). Chloroplasts in leaf mesophyll cells are typically ellipsoidal in shape but with defined poles, a feature that is crucial to their division. However, chloroplasts can also be highly pleiomorphic and can take up irregular morphologies in different cell types. Indeed, the potential plasticity in plastid shape has become clear in recent years with the analysis of giant plastids, which occur when plastid division is perturbed. In these giant plastids, which are up to 50-fold larger than normal chloroplasts, the morphology is highly irregular (Pyke et al. 1994) yet apparently stable when osmotically challenged (Pyke 2006) suggesting that a mechanism exists which controls and exerts stability on plastid morphology. A suggestion that an FtsZ-based internal plastoskeleton might function in controlling plastid morphology (Reski 2002) needs further experimentation since most of the FtsZ molecules within the plastid appear to be involved in division rather than morphological control. The recent discovery of mechanosensory proteins within the plastid envelope (Haswell and Meyerowitz 2006) showed that perturbation of such proteins by mutation affects plastid morphology, implying that tension monitoring in the plastid envelope somehow plays a role in morphological control.

A major structural component, which typifies the chloroplast, is the extensive thylakoid membrane system, which extends throughout the body of the chloroplast and dominates its internal architecture. Thylakoid membranes are the site of pho-tosynthetic electron transport and ATP synthesis and delimit a distinct compart ment within the chloroplasts: the thylakoid lumen. Thylakoids are composed of lamellae, which are arranged into a highly complex system of stacked lamellae called grana interconnected by single lamellae called stromal lamellae.

Models for thylakoid membrane structure have been developed largely from analysis of electron micrographs of sectioned chloroplasts, a system that is fraught with difficulty in interpretation in generating three-dimensional models from two-dimensional images. Three different models have been proposed (Arvidsson and Sundby 1999; Mustardy and Garab 2003; Shimoni et al. 2005) but differ in their conclusions, although all show the highly complex nature of thylakoid membrane arrangement within the grana. The model of Mustardy and Garab (2003) shows the grana as fused stacks of membrane which look like fan blades, with stromal lamellae joining stacks together at alternating levels within the stack, and the whole structure forming a right handed helix. The reason for this complex thyla-koid membrane morphology is to provide a large surface within the plastid on which light capture by chlorophyll and electron transport can occur. Consequently, the area of thylakoid membrane within a mature plastid is large and much greater than simple invaginations from the plastid envelope membrane.

Surprisingly, the mechanisms by which the construction of the thylakoid membrane system is initiated, synthesised in large amounts and then built into a complex three-dimensional architecture is poorly understood. Electron micrographs showing invaginations of the inner plastid envelope into the stroma gave credence to the hypothesis that thylakoid membrane is derived, at least initially, from such invaginations as proplastids differentiate into chloroplasts. Proplastids usually contain small amounts of thylakoid membrane and the extensive biogenesis of more thylakoid membrane may simply involve building off of extant membrane. However, recent studies have clearly shown that both chloroplasts and proplastids contain vesicles within the stroma (Westphal et al. 2003; Gunning 2004) and that a vesicle trafficking system occurs in plastids primarily between the plastid envelope and the stroma (Westphal et al. 2003). Vesicles are budded from the inner plastid envelope and accumulate close to the inner membrane, particularly when fusion processing at the thylakoid membrane is curtailed by low temperature (Morre et al. 1991). The main purpose of plastid vesicle transport is probably that of providing galactolipids, which are synthesised in the plastid envelope membranes (Joyard et al. 1998), for continued synthesis of thylakoid membrane, although they could also deliver hydrophobic proteins, which reside in the thylakoid membrane. Plastid vesicle trafficking appears to utilize several homologous components of the cytosolic ER Golgi trafficking system, encoded by nuclear genes, in that the chloroplast contains both ARF1 and Sar1 GTPases (Andersson and Sande-lius 2004), which are involved in vesicle assembly. In addition, the chloroplast also contains dynamin (Park et al. 1998) and proteins required for vesicle fusion (Hugeney et al. 1995). Two other nuclear-encoded proteins involved in the vesicle directed thylakoid biogenesis are VIPP1 (Kroll et al. 2001) and Thf1 (Wang et al. 2004). Mutations in either gene result in abolition of vesicles and perturbed synthesis of the thylakoid membrane. VIPP1 forms a high molecular weight complex on the inner envelope membrane, which could conceivably be involved in vesicle production (Aseeva et al. 2004). An intriguing problem for the future will be to understand how vesicle directed thylakoid synthesis is controlled to facilitate the construction of thylakoid architecture and biogenesis of the correct three-dimensional arrangement of the thylakoid membrane network. FZL is a dynamin-related membrane remodelling protein and is located inside the chloroplast in punctate foci on the plastid envelope and on the thylakoid membrane (Gao et al. 2006). Perturbation of this protein results in altered thylakoid morphology and changes in patterns of granal stacking suggesting that it plays an important role in thylakoid organisation and possibly in the dynamic continuum of membrane synthesis between the plastid envelope and the thylakoid.

During chloroplast development there is a significant increase in size of the plastid organelle from proplastid to mature chloroplast. There is also significant variation in mature chloroplast size in different cell types and also within the population of chloroplasts within individual leaf mesophyll cells. An important question yet to be addressed is what mechanisms control chloroplast size? Within a population of chloroplasts in a leaf mesophyll cell there is a trade-off between plastid density and size such that permutations of more small ones or fewer larger ones can be observed in cells of differing sizes and in different species where average leaf mesophyll cell size varies (Ellis and Leech 1983; Pyke 1999). However, the expansion process by which chloroplasts increase the surface area of their envelope membrane and the extent of the stroma and the thylakoid membrane must have a control system which shuts down further expansion at maturity. Conceivably a mechanosensing mechanism (Haswell and Meyerowitz 2006) could achieve this so that as chloroplasts become more densely packed and start squashing each other, as happens in leaf mesophyll cells, mechanosensing feedback shuts down further plastid replication and plastid expansion.

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