Sugar sensing in C₄ source leaves: a gap that needs to be filled

Plant growth depends on sugar production and export by photosynthesizing source leaves and sugar allocation and import by sink tissues (grains, roots, stems, and young leaves). Photosynthesis and sink demand are tightly coordinated through metabolic (substrate, allosteric) feedback and signalling (sugar, hormones) mechanisms. Sugar signalling integrates sugar production with plant development and environmental cues. In C₃ plants (e.g. wheat and rice), it is well documented that sugar accumulation in source leaves, due to source–sink imbalance, negatively feeds back on photosynthesis and plant productivity. However, we have a limited understanding about the molecular mechanisms underlying those feedback regulations, especially in C₄ plants (e.g. maize, sorghum, and sugarcane). Recent work with the C₄ model plant Setaria viridis suggested that C₄ leaves have different sugar sensing thresholds and behaviours relative to C₃ counterparts. Addressing this research priority is critical because improving crop yield requires a better understanding of how plants coordinate source activity with sink demand. Here we review the literature, present a model of action for sugar sensing in C₄ source leaves, and suggest ways forward.