Submitted to: Molecular and General Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 30, 1998
Publication Date: N/A
Interpretive Summary: Sucrose is the sole source of carbon entering a developing corn seed where it is utilized in numerous metabolic reactions, and is also stored as starch. The reactions which lead to the appropriate utilization of sucrose in a developing seed are critical to agronomic productivity. ARS scientists in the Crop Genetics and Environmental Research Unit in Gainesville, FL, are conducting long term research to determine such genes and proteins which play important roles in the normal metabolism of sucrose. Previous research at the Unit has identified sucrose synthase (SS) as one such enzyme. The enzyme has two forms (isozymes) SS1 and SS2, which are encoded by two separate genes. Combined use of genetic, biochemical, molecular and cellular approaches have now allowed a clear identification of the functions of each of the two isozymes. Results from this study show, for the first time, that the SS1 and SS2 enzymes release precursors from sucrose for cellulose and starch biosynthesis, respectively, in a developing seed. Thus, a concerted action of the two SS genes is needful for the normal development of a seed - an important unit of crop yield.
In maize, two paralogous genes, Sh1 and Sus1, encode two biochemically similar isozymes of sucrose synthase, SS1 and SS2, respectively. Previous studies have implicated the mild starch-deficiency of the sh1 endosperm to the loss of the SS1 isozyme in the mutant. Here we describe the first mutation of the Sus1 gene, sus1-1, and the isolation of a double recessive genotype, sh1sus1-1. cDNA cloning and characterization of the sus1-1 allele indicate that the mutant gene represented a small deletion at the 3'-end of the gene. Endosperm sucrose synthase activity in sh1sus1-1 was barely 0.5% of the total activity in the Sh1Sus1 genotype. More importantly, comparative analyses of Sh1Sus1, sh1Sus1 and sh1sus1-1 genotypes have, for the first time, allowed us to dissect the relative contributions of each isozyme in endosperm development. Starch contents in endosperm of the three lineage-related genotypes were 100, 74 and 53%, respectively. The anatomical analyses, including the previously described early cell degeneration unique to the sh1Sus1 endosperm, detected no difference between the two sh1 genotypes. We conclude that the SS1 isozyme performed a critical role in providing the substrate for cellulose biosynthesis, whereas, the SS2 protein was needed for generating precursors for starch biosynthesis.