GENETIC-EVIDENCE THAT THE 2 ISOZYMES OF SUCROSE SYNTHASE PRESENT IN DEVELOPING MAIZE ENDOSPERM ARE CRITICAL, ONE FOR CELL-WALL INTEGRITY AND THE OTHER FOR STARCH BIOSYNTHESIS

In maize, two paralogous genes, Sh1 and Sus1, encode two biochemically similar isozymes of sucrose synthase, SSI and SS2, respectively. Prev ious studies have attributed the mild starch deficiency of the shrunke n1 (sh1) endosperm to the loss of the SSI isozyme in the mutant. Here we describe the first mutation in the sucrose synthase1 (Sus1) gene, s us1-1, and the isolation of a double recessive genotype, sh1 sus1-1. C ombined data from diverse studies, including Northern and Western anal yses, RT-PCR and genomic FCR, cloning and sequencing data for the 3' r egion, show that the mutant sus1-1 gene has a complex pattern of expre ssion, albeit at much reduced levels as compared to the Sus1 gene. End osperm sucrose synthase activity in sh1 sus1-1 was barely 0.5% of the total activity in the Sh1 Sus1 genotype. Significantly, comparative an alyses of Sh1 Sus1, sh1 Sus1 and sh1 sus1-1 genotypes have, for the fi rst time, allowed us to dissect the relative contributions of each iso zyme to endosperm development. Starch contents in endosperm of the thr ee related genotypes were 100, 78 and 53%, respectively. Anatomical an alyses, which confirmed the previously described early cell degenerati on phenotype unique to the sh1 Sus1 endosperm, revealed no detectable difference between the two sh1 genotypes. We conclude that the SS1 iso zyme plays the dominant role in providing the substrate for cellulose biosynthesis, whereas the SS2 protein is needed mainly for generating precursors for starch biosynthesis.