Possible role played by R1 protein in starch accumulation in bean (Phaseolus vulgaris) seedlings under phosphate deficiency.

The effect of phosphate (Pi) deficiency on starch accumulation was studied in bean (Phaseolus vulgaris). After 3 weeks of Pi deprivation total Pi concentration in root and shoot was reduced by 68% and 42%, respectively; however, only shoot growth was affected. In leaves, Pi deprivation induced glucose, fructose and starch accumulation. Pi deficiency did not affect starch synthesis, but it reduced its mobilization during the dark period. At the same time, starch produced by Pi deficient plants have fewer Pi bound and was also less susceptible to beta-amylase hydrolysis. R1 protein is the protein responsible of phosphorylating C3 and C6 glucosyl residues of the polyglucan, increasing the hydration capacity and the interaction with amylolytic enzymes. Pi deprivation did not change the amount of R1 protein detected in total extracts but decreased its association with starch granules.