EARLY CELL DEGENERATION IN DEVELOPING ENDOSPERM IS UNIQUE TO THE SHRUNKEN MUTATION IN MAIZE

Evidence is presented to show that the early cell degeneration in the developing endosperm of the sb-runken (sh) mutant is unique to this genotype. Seven non-allelic shrunken or brittle mutants which are more severely starch deficient than the sh endosperm did not show such cell degeneration. The cellular loss in the sh mutant was detected prior to or during the rapid phase of starch biosynthesis in the developing endosperm. Although the previously suggested role of the Sh encoded sucrose synthase 1 (SS1) isozyme in mobilizing sucrose to starch biosynthesis is not precluded, the histological data presented here strongly suggest that the SS protein plays a critical role in maintaining cellular stability in the developing endosperm. We suggest that the brittleness of the sh endosperm cell walls is related to deficient cellulose hemicellulose content due to reduced levels of uridine diphosphate glucose (UDPG) which is the immediate reaction product of the SS1 isozyme present in great abundance in Sh endosperm, but absent in the sh genotype.