Sink regulation of photosynthesis in sugarcane

Abstract

The C4 plant, sugarcane (Saccharum spp. hybrids), accumulates sucrose to high concentrations and, as a result, has been the focus of extensive research into the biochemistry and physiology of sucrose accumulation. Despite this, the relationship between source leaf photosynthetic rates and sucrose accumulation in the culm has not been well documented. The observations that photosynthetic activity declines during culm maturation in commercial cultivars and that high-sucrose accumulating ancestral genoptypes photosynthesize at rates two-thirds of those of low-sucrose ancestral Saccharum species indicate that source-sink communication may play a pivotal role in determining sucrose yield. The relationship between source and sink tissues in sugarcane was investigated using a supply-demand paradigm, an approach novel in the study of the crop. The demand for photosynthate from the primary culm growth sink was shown to be closely linked to photosynthetic rates, sucrose export and the eventual physiological decline of source leaves. Results from initial field experiments revealed that leaf assimilation rates were negatively correlated with leaf hexose concentrations, but not those of sucrose. Further manipulation of leaf sugar status, through sugar-feeding and cold-girdling techniques, demonstrated the regulatory role of leaf sugar concentrations on photosynthetic activity, thus revealing sucrose, and particularly hexose, as key signal molecules in the modulation of the amount of photosynthate available for export to the sink. Gene expression profiling, by means of array technologies, indicated that changes in leaf sugar status and photosynthetic rates result in concurrent modifications in the expression of several genes involved in fundamental metabolic pathways, including photosynthesis, carbohydrate metabolism, stress response and sugar-signaling. Notable amongst these, was the identification of a potential trehalose 6-phosphate (T6P) sugar-signaling mechanism, thus implicating the trehalose pathway as a central regulatory system in the communication of sink carbon requirements to the source leaf. This study demonstrated that maturation of the culm results in a decreased demand for sucrose, which invokes a sugar-mediated feedback signal to decrease leaf photosynthetic supply processes. However, sugarcane leaves appear to retain the capacity to increase the supply of assimilate to culm tissues under conditions of increased assimilate demand. Uncoupling of the signaling pathways that mediate negative feedback between source and sink tissues may result in improved leaf assimilation rates and, consequently, lead to increased sugarcane sucrose yields.