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
Ps. Chourey et al., 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, MGG. Molecular & general genetics, 259(1), 1998, pp. 88-96
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.