Regulation of arbuscule formation by carbon in the plant

Arbuscules are proposed to be the key site of interchange of carbon between root cells and the hyphae of arbuscular mycorrhizal (AM) fungi. This paper addresses how carbon availability is a driving force in regulating locatio n and function of arbuscules in cortical cells. We discuss physical and bio logical limitations on arbuscule position. Altered expression, specifically in the arbusculated cell, of genes that govern sucrose hydrolysis may crea te a sink for sucrose in these cells. We propose a role for vacuolar invert ase and cytoplasmic sucrose synthase in catalyzing the intracellular hydrol ysis of sucrose, thus maintaining a gradient for symplastic influx of sucro se into the arbusculated cell and establishing a gradient for hexose efflux to the apoplast for fungal utilization. AM fungi may regulate hydrolysis o f sucrose by stimulating the expression and activities of plant invertases by the production of plant hormones as well as through acidification of the arbuscular interface. We speculate that altered plant defense gene express ion in arbusculated cells is consistent with regulation by sugar sensing me chanisms.