From source to sink: An integrated metabolic network for enhancing yield and resilience in cassava and sweet potato

Cassava (Manihot esculenta Crantz) and sweet potato (Ipomoea batatas [L.] Lam) exemplify an extreme yet underexploited paradigm of carbon allocation, in which yield is constrained less by photosynthetic capacity than by source-sink integration. In these crops, productivity emerges from the dynamic coordination of energy metabolism, transient carbon buffering, long-distance transport, and sink developmental plasticity, rather than carbon fixation alone. Persistent yield gaps reflect systemic inefficiencies in carbon partitioning, particularly under environmental stress, where assimilates are diverted toward maintenance respiration and lignification at the expense of storage. While this diversion constrains maximum yield, it represents a highly adaptive physiological response that ensures plant survival and baseline yield stability under adverse conditions. Recent advances have begun to reveal conserved regulatory hubs underlying this coordination, notably the T6P-SnRK1-TOR axis, which integrates carbon status with growth by linking diel starch turnover in source leaves to sucrose export and sink activation. At the sink, competitive carbon allocation between starch biosynthesis and phenylpropanoid-driven lignification acts as a metabolic switch dictating storage root yield, modulated by transcription factor networks (e.g., MYB, NAC, and bHLH). We argue that future gains in root crop productivity require a conceptual shift from enhancing carbon capture to engineering carbon allocation, emphasizing stabilized carbon flux, optimized transport, and a rebalanced starch-lignin trade-off. This integrated strategy provides a tractable and largely untapped route to developing high-yielding, climate-resilient root crops adapted to increasingly variable environments.