Differential induction of glyoxylate cycle enzymes by stress as a marker for seedling vigor in sugar beet (Beta vulgaris)

Significant differences in seedling vigor exist among sugar beet (Beta vulgaris) hybrids; however, traditional approaches to breeding enhanced vigor have not proven effective. Seedling vigor is a complex character, but presumably includes efficient mobilization of seed storage reserves during germination and efficient seedling growth in diverse environments. The involvement of lipid metabolism during germination. of sugar beet under stress conditions was suggested by the isolation at high frequency of Expressed Sequence Tags (ESTs) with similarity to isocitrate lyase (EC 4.1.3.1). High-level expression of this glyoxylate cycle enzyme during germination and seedling emergence was also suggested by nucleotide sequencing of cDNA libraries obtained from a well emerging sugar beet hybrid during germination under stress. Genes involved in carbohydrate and lipid catabolism were differentially expressed in a strongly emerging hybrid, relative to a weakly emerging hybrid, during stress germination. Stress markedly reduced the levels of a-amylase transcripts in the weakly emerging hybrid. In contrast, the strongly emerging hybrid exhibited only a moderate reduction in a-amylase transcript levels under the same conditions, and showed large increases in the expression of genes involved in lipid metabolism, suggesting compensation by lipid for carbohydrate metabolism in the better emerging hybrid. Differential activity of the glyoxylate cycle thus appears to be a physiological marker that distinguishes between high- and low-vigor sugar beet cultivars. This finding suggests, for the first time, a biochemical target for selection for enhanced germination and improved emergence in sugar beet.