Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/4/2001
Publication Date: N/A
Citation: Interpretive Summary: Sucrose synthase (SuSy) is an essential enzyme for proper utilization of sucrose in developing seeds, an important unit of crop yield. Yet, we have very limited knowledge on the number of genes that encode the enzyme, plant parts where these genes are expressed and possible consequences if these genes are non-functional. Such basic knowledge is critical for both conventional and non-conventional breeding, and for metabolic engineering through recombinant DNA technologies. Based on our past and current research at Crop Genetics & Environmental Research Unit in Gainesville, FL, and in collaboration with scientists at Pioneer Hi- Bred International Inc., Iowa, and University of Florida, we report here significant new knowledge on physiology and genetics of this enzyme. Specifically, we show for the first time, using biochemical and molecular genetic approaches that there are three SuSy genes in corn; all are expressed in the storage cells of the seed but only two of the three are expressed in the germinal tissue and several other parts of the plant. Simultaneous loss of function due to mutations in two genes leads to a loss of ~99% of the total enzyme activity, reduced levels of starch and much loss of cellular strength in developing seeds. Obviously, these two SuSy genes are critical for normal seed development. No mutants are found for the 3rd gene thus far. Collectively, these results provide new insights on the function of this enzyme and point to potential difficulties in its metabolic manipulation due to a three gene control (and not one or two genes) of this enzyme in corn plant.
Technical Abstract: Previous studies have identified two tissue- and cell-specific, yet functionally redundant, sucrose synthase genes, Sh1 and Sus1, that encode biochemically similar isozymes of SuSy, SS1 and SS2. We report here, evidence of a third SuSy gene in maize (Zea mays L.). RNA and/or protein blot analyses on developing kernels of various genotypes, representing mutants in one or both genes (a double mutant), have led us to show expression of a third SuSy gene, designated here as Sus2. Sus2 transcripts are detectable in developing endosperm of the three genotypes, including a double mutant, sh1sus1-1, at all developmental stages tested. The Sus2 transcript is also present in embryo RNA from both Sh1Sus1 and sh1sus1-1 genotypes. Finally, we study the expression of other genes related to sugar metabolism in developing seeds. The double mutant sh1sus1-1, which is missing 99.5% of SuSy enzyme activity, provides the genetic background for studying possible coordinate changes in the expression of Sucrose phosphate synthase (Sps), Cell wall invertase (Incw2), UGPase, AGPase, and Sucrose transporter (Sut). Surprisingly, we find no evidence of cross- regulation of these genes.