Maize endosperm mutant

Starch is formed in chloroplasts of moss, fern and green algae.18 Chlorophyceae (green algae) starch is similar to that of higher plants, and several species have been used in studies of starch biosynthesis.19,22,29 In a recent set of studies, Ball et al.22 used Chlamydomonas reinhardtii to study starch biosynthesis. They produced several Chlamydomonas mutants which produce starch with characteristics similar to starches produced by maize endosperm mutants.31-34 The various starch mutations of Chlamydomonas will be discussed in Section 3.7. Other classes of algae which produce starch are Prasinophyceae19,35 and Cryptophyceae.35,36... 

Interaction of these mutants further clarifies the biosynthetic pathway. For example, the wx mutant is epistatic to all other known maize endosperm mutants and no amylose accumulates (Table 3.6). Mutants such as sh2, bt2 and sit cause major reductions in starch accumulation, but when in combination with wx, the starch produced is all amylopectin.271 In the double mutant ae wx, wx prevents the production of amylose and ae reduces the degree of branching, resulting in the accumulation of a loosely-branched polysaccharide.88 The su mutant is epistatic to du, su2 and wx relative to accumulation of phytoglycogen, but ae and sh2 are partially epistatic to su, causing a marked reduction in the su stimulated phytoglycogen accumulation (Table 3.6). The addition of du or wx to ae su partially overcomes the ae inhibitory effect, and phytoglycogen accumulates. 

In spite of these limitations to our complete knowledge of starch biosynthesis, information about the pathway of starch biosynthesis gained from studies of maize endosperm mutants can probably be generalized to other plant species because related mutants have occurred in peas, sorghum, barley, rice and Chlamydomonas, and because the same enzymes are found in starch-synthesizing tissues in other plant species. Variation in the number of isozymes and their developmental expression, and variations in cellular compartmentation, however, could result in a range of pathways with significant differences. 

Results obtained with the maize endosperm mutant, amylose extender (ae), suggested that Ae was the structural gene for either BEII or BEIIb (Boyer and Preiss, 1978b, 1981 Preiss and Boyer, 1980). BEI levels were... 

The Chlamydomonas staxch-Ae cient mutant and higher dry weight maize endosperm mutant studies strongly suggest that the in vitro regulatory effects observed with the photosynthetic and nonphotosynthetic plant ADP-Glc PPases are highly functional in vivo and that ADP-Glc synthesis is rate limiting for starch synthesis. 

Obanni M, BeMiller JN. 1995. Identification of starch from various maize endosperm mutants via ghost stmctures. Cereal Chem 72 436 42. 

Robertson, D.S., Survey of the alhino and white-endosperm mutants of maize, J. Hered. 66, 67, 1975. 

Partially purified maize endosperm ADPGlc PPase (34U/mg) was found to be activated by 3PGA and Fru 6-P (25- and 17-fold, respectively) and inhibited by Pi.77 The heterotetrameric endosperm enzyme has been cloned and expressed in E. coli, and its regulatory properties were compared to an isolated allosteric mutant less sensitive to Pi inhibition.117 As indicated above, the increase of starch noted in the mutant maize endosperm ADPGlc PPase insensitive to Pi inhibition supports the importance of the allosteric effects of 3-PGA and Pi in vivo. Also as indicated above, it is believed that the major endosperm ADP-Glc PPase isoform is located in the cytosol.141... 

The first enzymic studies done on the dul mutant were carried out in 1981, and both SSII and starch-branching enzyme Ha (SBEIIa) were found to have reduced activity in the endosperm compared to normal maize endosperm.204 SSII was shown to be different from SSI.173,205 SSII requires a primer for activity, and could not catalyze an unprimed reaction even in the presence of 0.5 M citrate, it also has less affinity for amylopectin than does SSI. However, 0.5 M citrate lowered the Km for amylopectin 17-fold. The activity with glycogen as a primer is one-half that observed with amylopectin. Therefore, glycogen is not as effective as a primer as is amylopectin, which differs from what was observed for starch synthase I. Both maize endosperm SSI and SSII had a Km for ADPGlc of 0.1 mM.196,205... 

In further experiments,185 it was found that DU1 was one of the two major soluble starch synthases, and when the C-terminal 450 residues of DU1 were expressed in E. coli, it was shown to have SS activity. Of interest was that antisera prepared against DU1 detected a soluble protein in endosperm extracts of molecular size greater than 200 kDa that was absent in dul mutants.185 The antisera reduced starch synthase activity by 20-30% in kemal extracts. In the same study, antisera prepared against SSI reduced starch synthase activity by 60%. In dul mutants, antisera prepared against SSI reduced the SS activity essentially to zero, suggesting that SSI and SSII were the only maize endosperm soluble starch synthases. Because of the high similarity in sequence of the DU1 starch synthase II to the potato SSIII, and because both are exclusively soluble, it is argued that DU1 is the evolutionary counterpart of potato SSIII.185,188 206 It is proposed that DU1 and maize SSII should be known as maize SSIII.184... 

Amylose extender mutants have been found in rice.272 The alteration of the starch structure is very similar to that reported for maize endosperm ae mutants. The defect is in the BE3 isozyme, BE3 of rice being more similar in amino acid sequence to maize BE11 than to BEl.36,272 Thus, rice BE3 may catalyze the transfer of small chains, rather than long chains. 

Nelson, O. E., Mutant genes that change the composition of maize endosperm proteins. Federation Proceedings 1966,25 (6), 1676-1678. 

An ADPGlc PPase cDNA clone, isolated from a maize endosperm library (Barton et al., 1986), hybridized with the small subunit cDNA clone from rice (Anderson etal., 1989). This maize ADPGlc PPase cDNA clone hybridizes to a transcript that is present in maize endosperm but absent in bt 2 endosperm. Thus, the bt 2 mutant appears to be the structural gene of the 55-kDa subunit of the ADPGlc PPase. These data also indicate that the nonphotosynthetic tissue ADPGlc PPase is also composed of two subunits and, on the basis of immunoreactivity, there is homology between the large and small subunits in the leaf enzyme with the subunits of a reserve tissue enzyme, respectively. 

Dickinson, D. B., and Preiss, J. 1969b. Presence of ADP-glucose pyrophosphorylase in Shrunken-2 and Brittle-2 mutants of maize endosperm. Plant Physiol. 44, 1058-1062. 

Allosteric mutants have been found for C. reinhardtii and for maize endosperm. A significant finding was made by Ball et al. who isolated a starch-deficient mutant of C. reinhardtii having an ADP-Glc PPase, which could not be effectively activated by 3-PGA. The inhibition by P was similar to the wild type. The starch deficiency was observed in the mutant whether the organism was grown photoautotrophically with CO2 or in the dark with acetate as the carbon source. Thus, the allosteric mechanism seems to be operative for photosynthetic or nonphotosynthetic starch biosynthesis. 

Another putative ADP-Glc PPase allosteric mutant from maize endosperm, which has 15% more dry weight (in addition to starch) than the normal endosperm, has been isolated and described by Giroux et al The allosteric mutant ADP-Glc PPase was less sensitive to Pi inhibition than the normal enzyme. 

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