Waxy mutants starch synthesis

Isozymes of plant starch synthases (2, 16-20) and branching enzymes (2, 16, 21-26) have been characterized. Presumably, they play distinct roles in amylopectin and amylose synthesis. Also, plant and Chlamydomonas reinhardtii granule-bound starch synthases (27-33) are responsible for synthesis of amylose. Mutants defective in this enzyme are known as waxy mutants and contain starch granules that have little or no amylose. 

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. 

S9, with higher starch synthase activity, had 36% of the amylose content observed in the nonmutant endosperm, whereas mutant S5, with an even lower starch synthase activity of 32%, had only 21% of the nonmutant maize amylose content. These data further support the view that the waxy protein is involved in amylose synthesis. 

Delrue, B., Fontaine, T., Routier, F., Decq. A., Wieruszeski, J.-M., Van Den Koomhuyse, N., Maddelein, M.-L., Fournet, B., and Ball, S. 1992. Waxy Chlamydomonas reinhardtii Monocellular algal mutants defective in amylose synthesis and granule-bound starch synthase activity accumulate a structurally modified amylopectin. J. BacterioL 174,3612-3620. 

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