Ph.D. Student University of Florida Gainesville, Florida
Body of Abstract: Maize kernels rapidly synthesize and metabolize sorbitol via sorbitol dehydrogenase (SDH), which catalyzes the reversible conversion of fructose + NADH ↔ sorbitol + NAD+. The reaction occurs predominantly in the endosperm, where the high-sugar, low-oxygen microenvironment favors synthesis of sorbitol and NAD+. A single copy Sdh1 gene encodes maize SDH and is most strongly expressed during the grain-filling stage of kernel development. However, physiological roles of sorbitol in the maize kernel remain elusive. An Ac/Ds-induced sdh1 mutation lacks detectable SDH activity and accumulates little to no sorbitol in kernels. Ears of sdh1 mutants bear small kernels with 13-17% less dry weight, suggesting an important role for sorbitol in kernel filling. We hypothesize that SDH aids endosperm development in at least two ways: 1) by enhancing sink strength through metabolism of fructose and indirectly, sucrose, and 2) by regenerating NAD+ that sustains redox balance and glycolytic flux in the low-oxygen kernel interior. Support for this scenario comes from rising levels of fructose and sucrose revealed in sdh1 mutants (100% and 30%, respectively). Another potential role of SDH is the provision of sorbitol to developing embryos. The wild-type, but not sdh1 embryos, show an unusual capacity to use sorbitol as their sole C source for maturation and sustained growth during in vitro culture. In these structures, SDH would operate in a fructose-forming direction. To further test hypotheses for roles of SDH in kernel development, Sdh1 over-expression lines were generated and are being characterized. Understanding the physiological impacts of SDH will help pave the way for new genetic and metabolic approaches to improving the quality and quantity of maize kernels.