Phosphite accelerates programmed cell death in phosphate-starved oilseed rape (Brassica napus) suspension cell cultures

Phosphite (H2PO3-, Phi) prevents the acclimation of plants and yeast to orthophosphate (Pi, HPO42-) deprivation by specifically obstructing the derepression of genes encoding proteins characteristic of their Pi-starvation response. In this study, we report that prolonged (i.e., 3-4 weeks) culture of Brassica napus L. suspension cells in Pi-deficient (-Pi) media leads to programmed cell death (PCD). However, when the B. napus cells were subcultured into -Pi media containing 2 mM Phi, they initiated PCD within 5 days, with 95% cell death observed by day 9. Dying cells exhibited several morphological and biochemical features characteristic of PCD, including protoplast shrinkage, chromatin condensation, and fragmentation of nuclear DNA. Immunoblotting indicated that B. napus cells undergoing PCD upregulated a 30-kDa cysteine endoprotease that is induced during PCD in the inner integument cells of developing B. napus seeds. It is concluded that PCD in B. napus suspension cells is triggered by extended Pi starvation, and that Phi treatment greatly accelerates this process. Our results also infer that the adaptive value of acclimating at the molecular level to Pi-stress is to extend the viability of -Pi B. napus cell cultures by about 3 weeks.