Is ABA involved in tolerance responses to salinity by affecting cytoplasm ion homeostasis in rice cell lines?

The ability of plant cells to maintain cytoplasm ion homeostasis under saline stress is among the main mechanisms involved in salt tolerance. To cope with excess Na+, cells extrude it from the cytoplasm, which requires expenditure of metabolic energy, provided by H+ gradients generated by membrane-bound H+-pumps. ABA is well-known to be involved in physiological processes elicited or enhanced by stresses causing cell dehydration. In this work we studied the possible implication of this plant hormone in the control of salt-induced cellular mechanisms conducting to Na+ extrusion from the cytoplasm. We used rice (Oryza sativa L.) cell lines selected for their different tolerance to salinity to measure the response to ABA of H+-pumps and Na+/H+-antiporters associated to the plasma membrane and the tonoplast. Our results show that ABA generally enhances H+-pumping under salt stress but not under control conditions. This effect occurs to a higher extent across the tonoplast in the more tolerant lines (L-T). Na+/H+ antiport activity is practically undetectable in calli under control conditions, pre-treated or not with ABA, but shows a strong activation under salinity across the tonoplast, particularly in L-T lines (cv Bahia) and also across de plasma membrane in cv Bomba. In these lines, prior treatments with ABA tend to reduce the NaCl enhanced activity of both antiporters. Overall, under saline conditions ABA seems to affect synergistically H+ pumping and antagonistically Na+ extrusion. A complex network of positive and negative regulatory signals seems involved in restoring ion cell homeostasis under salt stress.

► In control conditions ABA does not affect H+-pumping or Na+/H+-antiport activity.
► Under salinity, ABA effect on H+-pumping, when detectable, is always positive.
► Na+/H+ antiport activity is down-regulated or not affected by ABA under salinity.
► Fine regulation of ion homeostasis might involve promotion/inhibition effects of ABA.
► Preferential Na+ extrusion to the apoplast or to the vacuole depends on genotype.