Mutants starch synthesis

The enzymatic reactions involved in starch synthesis in higher plants and algae and in glycogen synthesis in cyanobacteria are reviewed in this article. Regulation of a-1,4 and a-1,6 glucan synthesis at the enzymic level is discussed, and mutants that cause specific enzymatic deficiencies that affect starch structure are also reviewed. Recent reviews on starch synthesis have been published and are noted (1-6). 

The Kacser—Burns approach has been used in the analysis of starch synthesis by mutants of Arabidopsis thaliana that studied leaf ADP-Glc PPase, and the significance of 3-PGA regulation in vivo A. thaliana mutant strains containing only 7% of the wild-type activity of ADP-Glc PPase and a hybrid of mutant and wild type with 50% activity, had 10% and 61% of the wild type s starch synthetic rate, respectively, at high light intensity. This is a fairly good correlation between the activity of the ADP-Glc PPase and the rate of synthesis of starch. The flux control coefficient was determined to be 0.64. 

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. 

One of these spontaneous mutants gave rise to the precious sticky rice of Thailand. The sticky rice lacks the starch amylose, which constitutes up to 30% of the total starch in nonsticky rice endosperm. The lack of amylose is due to a mutation in a gene called Waxy, which encodes an enzyme required for amylose synthesis (Sano 1984). Sticky rice is an important culinary and cultural component throughout East Asia and is used in festival foods and desserts. In upland regions of Southeast Asia, it is a staple food in many homes. Ten percent of the rice traded each year is sticky rice. 

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