Amyloplasts

Starch is stored in plant cells in the form of granules in the stroma of plas-tids (plant cell organelles) of two types chloroplasts, in which photosynthesis takes place, and amyloplasts, plastids that are specialized starch accumulation bodies. When starch is to be mobilized and used by the plant that stored it, it must be broken down into its component monosaccharides. Starch is split into its monosaccharide elements by stepwise phosphorolytic cleavage of glucose units, a reaction catalyzed by starch phosphorylase (Figure 7.23). This is formally an a(1 4)-glucan phosphorylase reaction, and at each step, the prod-... 

The carotenoids are located in photosynthetic pigment-protein complexes (PPCs) in the thylakoid membranes (Young, 1993), with minor amounts in the chloroplast envelope (Joyard et al, 1991) and the envelope of amyloplasts (Fishwick and Wright, 1980). In all plastid envelope membranes, violaxanthin is the major carotenoid. Carotenes are also found in plastoglobuli (Lichtenthaler and Peveling, 1966). 

FisHwicK M J and wright a j (1980) Isolation and characterisation of amyloplast envelope membranes from Solanum tuberosum. Phytochemistry, 19, 55-9. 

TAUBERGER, E., FERNIE, A.R., EMMERMANN, M., KOSSMANN, J., WILLMITZER, L., TRETHEWEY, R.N., Antisense inhibition of plastidial phosphoglucomutase provides compelling evidence that potato tuber amyloplasts import carbon from the cytosol in the form of glucose-6-phosphate, Plant J., 2000, 23, 43-53. 

In these plant organs, starch is present in the form of microscopically small granules in special organelles known as amyloplasts. Starch granules are virtually insoluble in cold water, but swell dramatically when the water is heated. Some 15-25% of the starch goes into solution in colloidal form when the mixture is subjected to prolonged boiling. This proportion is called amylose ( soluble starch ). 

Sowokinos, J. R., Orr, P. H., Knoper, J. A., Yarns, J. L. (1987). Influence of potato storage and handling stress, sugars, chip quality and integrity of the starch (amyloplast) membrane. Am. Potato J., 64,213-225. Spychalla,N. R, Desborough, S. L. (1990). Fatty acids, membrane permeability, and sugars of stored potato tubers. Plant Physiol., 94, 1207-1213. 

FIGURE 20-2 Amyloplasts filled with starch (dark granules) are stained with iodine in this section of Ranunculus root cells. Starch granules in various tissues range from 1 to 100 /cm in diameter. 

Starch synthase in chloroplasts and amyloplasts catalyzes the addition of single glucose residues, donated by ADP-glucose, to the reducing end of a starch molecule by a two-step insertion mechanism. Branches in amylopectin are introduced by a second enzyme. 

The individual pathways of carbohydrate metabolism in plants overlap extensively they share pools of common intermediates, including hexose phosphates, pentose phosphates, and triose phosphates. Transporters in the membranes of chloroplasts, mitochondria, amyloplasts, and peroxisomes mediate the movement of sugar phosphates between organelles. The direction of metabolite flow through the pools changes from day to night. 

Calvin cycle 752 plastids 752 chloroplast 752 amyloplast 752 carbon-fixation reaction 753 ribulose 1,5-bisphosphate 753 3-phosphoglycerate 753 pentose phosphate pathway 753 reductive pentose phosphate pathway 753 C3 plants 754 ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) 754 rubisco activase 757... 

Reserve starch is usually formed in amyloplasts, although it is occasionally formed in chloroamyloplasts. These are chloroplasts that have lost their lamellar structure and subsequently start producing fairly large reserve starch granules.17 Chloroamyloplasts form starch independent of photosynthesis. They have been described in tobacco (Nicotiana tabacum L.) leaves, Aloe leaves and flowers, central pith of potato (Solanum tuberosum L.) fruit, Pellionia and Dieffenbachia stems, and other tissues.17,18 Such sources of reserve starch are insignificant, however, when compared to the reserve starch formed in roots, tubers and seeds. 

They concluded that these data supported Badenhuizen s17,18 suggestion that starch molecules are completely synthesized in the amyloplast stroma and are then deposited on the granule surface. Once the polysaccharides are part of the granule, there was no evidence of subsequent conversion of amylose to amylopectin.166 This is in contrast to conclusions drawn from long-term 14C-labeling studies of wheat starch.167,168 In results of these studies, amylose appeared to be synthesized first, and then transformed into amylopectin. These differences may be due to the different species used or to the widely different sampling times. 

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