Interallelic complementation at the sh locus in maize

Structure-Function Analysis of Interallelic Complementation in ROOTY Transheterozygotes

Auxin is a crucial plant growth regulator. Forward genetic screens for auxin-related mutants have led to the identification of key genes involved in auxin biosynthesis, transport, and signaling. Loss-of-function mutations in genes involved in glucosinolate biosynthesis, a metabolically related route that produces defense compounds, result in auxin overproduction. We identified an allelic series of fertile, hypomorphic Arabidopsis (Arabidopsis thaliana) mutants for the essential glucosinolate biosynthetic gene ROOTY (RTY) that exhibit a range of phenotypic defects characteristic of enhanced auxin production. Genetic characterization of these lines uncovered phenotypic suppression by cyp79b2 cyp79b3, wei2, and wei7 mutations and revealed the phenomenon of interallelic complementation in several RTY transheterozygotes. Structural modeling of RTY elucidated the relationships between structure and function in the RTY homo- and heterodimers, and unveiled the likely structural basis of interallelic complementation. This work underscores the importance of employing true null mutants in genetic complementation studies.

Electrophoretic analysis of sucrose synthetase proteins in the complementing heterozygotes at the Shrunken locus in maize

Electrophoretic comparisons of sucrose synthetase (SS) proteins in complementing heterozygotes and the corresponding in vitro mixtures of extracts from the homozygotes are described. The latter revealed two protein bands in the expected fashion. The SS protein pattern in the hybrid was different from that of the mixtures. The possibility of heteromeric SS molecules, formed by random polymerization of subunits of the tetramer coded by each allele in the heterozygote, was considered. Such an interaction was expected to form a multiple of five SS proteins that could be visualized after gel electrophoresis. However, only two SS bands were seen in the hybrids. The basis of this marked deviation remains to be explained.

The enzymatic deficiency conditioned by the shrunken-1 mutations in maize

Evidence is presented to show that the Sh locus specifies sucrose synthetase in the developing endosperm of maize. The sh/sh/sh endosperm possesses less than 10% sucrose synthetase activity as compared to the normal Sh/sh/sh endosperm. The residual enzyme activity in five independently derived mutant genotypes is attributable to a protein molecule of different electrophoretic and immunochemical specificities that is presumably independent of the sh locus. Sucrose synthetase activity in the embryo in both the genotypes is electrophoretically indistinguishable from the one present in the mutant endosperm. Mutant endosperm has a reduced starch content as compared to the normal. This observation constitutes genetic evidence supporting a critical role for sucrose synthetase in starch biosynthesis.