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Plant Physiol, December 2001, Vol. 127, pp. 1798-1807

The ram1 Mutant of Arabidopsis Exhibits Severely Decreased beta -Amylase Activity1

Ron J. Laby, Donggiun Kim,2 and Susan I. Gibson*

Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005-1892

Despite extensive biochemical analyses, the biological function(s) of plant beta -amylases remains unclear. The fact that beta -amylases degrade starch in vitro suggests that they may play a role in starch metabolism in vivo. beta -Amylases have also been suggested to prevent the accumulation of highly polymerized polysaccharides that might otherwise impede flux through phloem sieve pores. The identification and characterization of a mutant of Arabidopsis var. Columbia with greatly reduced levels of beta -amylase activity is reported here. The reduced beta -amylase 1 (ram1) mutation lies in the gene encoding the major form of beta -amylase in Arabidopsis. Although the Arabidopsis genome contains nine known or putative beta -amylase genes, the fact that the ram1 mutation results in almost complete loss of beta -amylase activity in rosette leaves and inflorescences (stems) indicates that the gene affected by the ram1 mutation is responsible for most of the beta -amylase activity present in these tissues. The leaves of ram1 plants accumulate wild-type levels of starch, soluble sugars, anthocyanin, and chlorophyll. Plants carrying the ram1 mutation also exhibit wild-type rates of phloem exudation and of overall growth. These results suggest that little to no beta -amylase activity is required to maintain normal starch levels, rates of phloem exudation, and overall plant growth.

1 This work was supported by the U.S. Department of Energy, Energy Biosciences Program (grant no. DE-FG03-98ER20300).

2 Present address: Department of Biochemistry, University of Missouri, 117 Schweitzer Hall, Columbia, MO 65211.

* Corresponding author; e-mail sig{at}bioc.rice.edu; fax 713-348-5154.

© 2001 American Society of Plant Physiologists


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