Fig. 8: Proposed model for phenylpropanoid regulation in potato.

Sucrose has both a regulatory role and a metabolic role by supplying hexoses that can be channeled to phenylpropanoid biosynthesis. In the left green panel, sucrose increases MYB expression, which in turn increases phenylpropanoid metabolism. StAN1 concurrently increases expression of sucrolytic genes that generate hexoses to feed increased phenylpropanoid metabolism. In the right red panel, phenylpropanoid synthesis decreases either due to reduced StAN1 expression resulting from falling sucrose levels caused by StAN1 induced sucrolysis, or from increased expression of MiR858, a negative regulator. Sucrose concentrations and MYB expression form a self-regulatory cycle, where sucrose promotes MYB expression but MYB expression promotes sucrolysis. The circular arrows connecting sucrose in the two panels, indicate that the concentration of sucrose cycles and influences phenylpropanoid metabolism. Green indicates conditions that increase phenylpropanoid metabolism and red conditions that do not. Solid arrows indicate direct effects on phenylpropanoid regulation and dashed arrows indirect