Leucoplast is the name given to a general group of plastids, which lack any pigment and are often referred to as non-green plastids. Leucoplasts are very widely distributed in different plant tissues and have a wide range of morphologies and content, the latter being primarily a variation in the type of storage molecules that they accumulate. In fact amyloplasts could be considered a form of leucoplast that has specialised in storing starch. Whilst they are widespread in plant tissues, the general cell biology of leucoplasts has not been extensively investigated although many aspects of their biochemistry have been examined under the umbrella of non-green plastids (Emes and Neuhaus 1997; Eastmond et al. 1997). Leucoplasts can be isolated in clean populations from seed endosperm tissues and their biochemical characteristics examined (Negm et al. 1995).
A major class of leucoplasts are those found in different types of root cells of and often referred to as root plastids. These undoubtedly play a central role in root metabolism and function and many aspects of their biochemistry and metabolism have been described in detail (Emes and Neuhaus 1997; Debnam and Emes 1999; Fox et al. 2001). Early work examining electron micrographs of root plastids showed that proplastids in cells leaving the root apical meristem lose any thyla-koid-like structures and pass through a transient phase of starch accumulation before becoming highly amoeboid in shape and then finally discoid with significant amounts of pregranal structures (Whatley 1983).
As with chromoplasts, the targeting of GFP to the plastid compartment has allowed the direct observation of root plastids in living tissue and they appear highly variable in morphology and exhibit many structures, which are reminiscent of stromules (Fig. 1). Indeed it is difficult to separate the presence of stromules on these plastids with variation in their morphology to the extent that stromules might be regarded as the most extreme characteristic of their morphological form. The cellular distribution of root plastids and leucoplasts in other tissues appears to be directed and non-random, since targeting of GFP to the plastid compartment reveals that leucoplasts commonly associate with the nucleus in an intimate manner, in that they surround the nucleus and are even found to lie within grooves in the nuclear membrane (Kwok and Hanson 2004). Such an association would seem to facilitate efficient signalling between plastid and nucleus and may also be a strategy for ensuring correct plastid segregation at cytokinesis (see Section 2).
Recent studies have revealed a novel role for plastids in directing the interaction of the root cells with symbiotic fungi and bacteria. Firstly, extensive plastid stromule networks develop in cells in arbscules where they interact with the fungal surface (Fester et al. 2001; Hans et al. 2004). Secondly, there is a major upregula-tion in plastid metabolic activity in these cells, as shown by transcript and metabolite profiling which provides a variety of metabolites which are central to the symbiotic interaction with the invading symbiont and the synthesis of the symbiotic structures such as the peri-arbuscular membrane (Lohse et al. 2005). Moreover two plastid membrane proteins, CASTOR and POLLUX, are crucial to the micro-bial admission into root cells, which forms the very first stage of the symbiotic relationship (Imaizumi-Anraku et al. 2005). Thus, it appears that a pre-existing en-dosymbiont in root cells, the plastid, and aids the integration of free-living soil bacteria into a symbiotic relationship with plants.
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