Photosynthesis and the regulatory role of sucrose and hexose in sugarcane leaves

Abstract

IN CROPS other than sugarcane, there is good evidence that the size and activity of carbon sinks influence source photosynthetic activity via regulation of photosynthesisrelated enzymes, an effect that is partly mediated through coarse regulation of gene expression. The existence in sugarcane of a robust sugar-dependent relationship between leaf and sink tissues could represent a potentially fundamental limiting factor for sucrose accumulation in the stalk and, consequently, play a major role in overall sucrose yield. Previous work in our laboratories has demonstrated that increased culm sink demand through partial shading resulted in increased photosynthetic rates that correlated with a reduction in hexose levels in the leaves. In an extension of that study, we have examined source regulation in cold-girdled and detached leaves (second and third fully-expanded) of pot grown Saccharum spp. hybrid cv. N19 (N19) with the aim of elucidating the mechanisms that determine carbon partitioning in sugarcane. Coldgirdled leaves (at 5C) showed increased sucrose and hexose levels and a decline in photosynthetic rates over the duration of the 5 d treatment. Excised leaves, preincubated in darkness for 3 h, had increased photosynthetic rates on transfer back to light, relative to control plants maintained in the light. Tissue sucrose accumulation was reduced by darkness, but accumulated again upon transfer to the light. However, after the dark period, hexose levels remained significantly lower for the remainder of the incubation time; possibly indicating that photosynthesis was up-regulated by lack of hexose accumulation. When the excised leaves were fed or pre-fed sucrose via the transpiration stream, dark-treated leaves exhibited reduced photosynthetic rates, which were associated with increased sucrose and hexose concentrations within the leaf tissue. The observed down-regulation of photosynthesis by sugar accumulation has provided a starting point for future identification of gene transcripts that have putative roles in mediating the source-sink relationship. Introduction Although sugarcane is the primary source of the world’s sugar, little is known about the relationship between leaf (source) and culm (sink) tissues, even though the carbon requirements of sinks have been shown to effectively govern the overall photosynthetic rates in several species (Paul and Foyer, 200l). The molecular control mechanisms involved are still not fully understood, although work on other crops has indicated that they are linked strongly to the local status of the primary transport sugar, sucrose, and its constituent hexoses (glucose and fructose) (Rolland et al., 2002; Gibson, 2005). To further develop our current understanding of the influence of these sugars