| Nitrate reductase (NR), the first enzyme in the nitrate assimilation pathway is highly regulated. The initial goal of this research was to dissect genetically the role of sugars in NR gene regulation. In a selection for weak resistance to chlorate on medium containing 3% sucrose, 5 of the 8 new mutants isolated, exhibited a modest decrease in NR activity. Interestingly, the remaining 3 mutants belonged to a new class of mutants with NR activity similar to, or higher than, wild type. It was discovered that 3% sucrose hastens the yellowing of rosette leaves in the wild type and the sds1 (s&barbelow;ugars and d&barbelow;elayed s&barbelow;enescence) mutant, which belongs to the new class of chlorate-resistant mutants, was the only one in this group whose leaves stayed green longer than the wild type under this condition. The sds1 mutant showed decreased sensitivity to glucose inhibition of seed germination, seedling development, and repression of photosynthetic gene expression. sds1 was also delayed in natural leaf senescence and in senescence induced artificially by placing detached leaves in the dark or treating with hormones. The phenotypic delay in senescence under all conditions correlated with a delay in the decline in the expression of photosynthesis-associated genes (PAGs) and with an increase in the expression of some senescence-associated genes (SAGs). The delay in natural senescence correlated with elevated levels of starch and soluble sugars in sds1 plants.; Although sds1 seedlings retained their responses to ethylene and auxin, the ethylene-insensitive mutant, ein2, and the auxin-response mutant, axr1-12, exhibited a similar delay in sugar-induced yellowing of rosette leaves as sds1 exhibited. In contrast to sds1, these mutants exhibited enhanced sensitivity to glucose inhibition of seed germination and hypocotyl elongation. Other ethylene and auxin mutants also exhibited enhanced sensitivity to glucose. While a connection between sugar- and ethylene-signaling was consistent with previous work, a connection between sugar- and auxin-signaling has not yet been reported. Thus, the identification and characterization of sds1 may help link the pathways for sugar-signaling and hormone-signaling to early stages of development and to senescence in mature plants. |
