Plant growth depends on sugar production and export by photosynthesising source leaves and sugar allocation and import by sink tissues (grains, roots, stems, 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, rice), it is well documented that sugar accumulation in source leaves, due to source-sink imbalance, negatively feedbacks 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, sugarcane). Recent work with the C4 model 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.