All chloroplasts seem to have the ability to accumulate starch grains within the stroma as a transient store of photosynthetic assimilate. Normally these starch grains are degraded through the dark part of the photoperiod and the products exported. Amyloplasts, however, are a plastid type in which starch accumulation is long term and are mostly found in storage tissues such as tubers and seed endosperm where they are highly abundant. All plant starch is synthesised in the plastid and produced either directly from photosynthate, as occurs in leaf plastids or indirectly from photosynthate transported to heterotrophic tissues within the plant. The latter process occurs in amyloplasts, which are the dominant organelle in storage tissues and are of great agricultural and economic significance since 75% of the energy contained in the average human diet is derived from starch (Duffus 1984). Starch is an insoluble, complex, semi-crystalline polymer of glucose synthesised in amyloplasts by the polymerisation of ADP glucose, producing highly branched amylopectin and relatively unbranched amylose in proportions of 70:30% (Smith et al. 1997). Starch is present within amyloplasts as grains, which have a distinct structure consisting of a series of concentric rings alternating between semi-crystalline and amorphous zones. These zones are a result of differ ences in organisation of the amylopectin chains (Smith et al. 1997). Two different size classes of starch grains are present in endosperm amyloplasts; the A-type, of up to 45 |im in diameter, and the smaller B-type of up to 10 |im in diameter. The ratio of these two types is variable and under environmental control but has a major effect on the processing qualities of the resultant starch in the food industry (Langeveld et al. 2000).
Amyloplasts most often form from proplastids during the early development of storage organs such as tubers or seed endosperm. In red winter wheat, proplastids are present within the coencytic endosperm but when cellularisation is complete, starch deposition commences and amyloplasts are formed (Bechtel and Wilson 2003). Some understanding of the way in which amyloplast differentiation is controlled has come mostly from exploiting cell cultures, in which amyloplast differentiation can be induced by adding phytohormones. Tobacco BY-2 cells grown in the dark are undifferentiated and contain proplastids. The presence of auxin causes these cells to proliferate. When auxin is replaced with benzyladenine, rates of cell division decline and proplastids accumulate starch and form amyloplasts (Sakai et al. 1992, 1999).
Amyloplasts are also present in a specialised cell type in the root tip; the collu-mella cells. Collumella cells form the gravisensing system in roots enabling gravi-tropic responses in growth in relation to the gravity vector. The sinking of amy-loplasts, called statoliths in these cells, in the cytoplasm under the influence of gravity is thought to initiate a signal transduction pathway involving auxin redistribution (Swarup et al. 2005), which results in differential cell expansion and downward growth in a positively gravitropic manner. The reverse happens in stems of shoots where statoliths are present in a sheath of cells around the vascula-ture and cause upward growth of shoots in negatively gravitropic manner (Yama-moto et al. 2002). How these amyloplasts form specifically in these two cell types whilst cells around them contain different plastid types is unclear but undoubtedly the control is more complex than a simple change in the type of phytohormone as suggested by differentiation in cell cultures.
Since chloroplasts in many plant species accumulate significant amounts of starch during the light period, it may be pertinent to consider how these starch-laden chloroplasts differ from amyloplasts where starch storage is more long term. Transient starch in chloroplasts has a lower amylose content, forms as flattened plate-like structures rather than the more spherical grains of amyloplasts and does not possess the growth rings of amyloplast starch (A. Smith, personal communication). In some species such a tobacco and cotton, starch breakdown in older leaves is not complete by dawn and starch accumulates in these older leaf cells, taking on some properties of long-term storage starch. Conversely some species synthesise very little starch in the chloroplast at all during the light period and export their photosynthate and synthesise sucrose in the cytosol (Zeeman et al. 2004).
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