Sink regulation of photosynthesis

The concept that photosynthetic flux is influenced by the accumulation of p hoto-assimilate persisted for 100 years before receiving any strong experim ental support. Precise analysis of the mechanisms of photosynthetic respons es to sink activity required the development of a battery of appropriate mo lecular techniques and has benefited from contemporary interest in the effe cts of elevated CO2 on photosynthesis. Photosynthesis is one of the most hi ghly integrated and regulated metabolic processes to maximize the use of av ailable light, to minimize the damaging effects of excess light and to opti mize the use of limiting carbon and nitrogen resources. Hypotheses of feedb ack. regulation must take account of this integration. In the short term, d eparture from homeostasis can lead to redox signals, which cause rapid chan ges in the transcription of genes encoding photosystems I and II. End-produ ct synthesis can exert short-term metabolic feedback control through Pi rec ycling. Beyond this, carbohydrate accumulation in leaves when there is an i mbalance between source and sink at the whole plant level can lead to decre ased expression of photosynthetic genes and accelerated leaf senescence. In a high CO2 world this may become a more prevalent feature of photosyntheti c regulation. However, sink regulation of photosynthesis is highly dependen t on the physiology of the rest of the plant. This physiological state regu lates photosynthesis through signal transduction pathways that co-ordinate the plant carbon: nitrogen balance, which match photosynthetic capacity to growth and storage capacity and underpin and can override the direct short- term controls of photosynthesis by light and CO2. Photosynthate supply and phytohormones, particularly cytokinins, interact with nitrogen supply to co ntrol the expression of photosynthesis genes, the development of leaves and the whole plant nitrogen distribution, which provides the dominant basis f or sink regulation of photosynthesis.