Expression of starch metabolic genes in the developing nectaries of ornamental tobacco plants

Abstract

To understand the mechanisms that control starch metabolism in the nectary, this study combined TEM analysis of floral nectaries in ornamental tobacco with gene expression analysis of a number of starch metabolism genes over the time course of nectary development. Using TEM, we observed an increase in amyloplasts accumulating from early developmental stages until stage 9 (S9), and their subsequent disappearance prior to anthesis (S12). Nectary starch gene expression analysis was initiated by isolating 18 nectary-expressed cDNAs encoding different starch metabolic enzymes. Strong sucrose synthase gene expression was observed early in nectary development at the mRNA and protein levels. ADP-glucose pyrophosphorylase (AGPase) mRNA levels, protein levels, and enzymatic activities also were high at early stages of nectary development then declined as the nectary matured. Starch synthase 3 (SS3), the most strongly expressed SS, followed a similar expression pattern. Quantitative RT-PCR revealed three different regulatory patterns of gene expression, an anabolic pattern, a catabolic pattern and a constitutive pattern. The anabolic genes, including AGPS and SS3, were expressed early in development (S2 and S6), and down regulated after S9. In contrast, catabolic gene expression, including ISA1, AMY and BMY, was detected at S9 and later but not at early stages. The third class of genes was expressed throughout nectary development. The switch from starch anabolism to catabolism seems to be an important key to normal nectary development and function. These combined results provide a better understanding of the importance of starch metabolism to overall nectary activity.