Rice cultivars with differing salt tolerance contain similar cation channels in their root cells

Salinity poses a major threat for agriculture worldwide. Rice is one of the major crops where most of the high-yielding cultivars are highly sensitive to salinity. Several studies on the genetic variability across rice cultivars suggest that the activity and composition of root plasma membrane transporters could underlie the observed cultivar-specific salinity tolerance in rice. In the current study, it was found that the salt-tolerant cultivar Pokkali maintains a higher K(+)/Na(+) ratio compared with the salt-sensitive IR20 in roots as well as in shoots. Using Na(+) reporter dyes, IR20 root protoplasts showed a much faster Na(+) accumulation than Pokkali protoplasts. Membrane potential measurements showed that root cells exposed to Na(+) in IR20 depolarized considerably further than those of Pokkali. These results suggest that IR20 has a larger plasma membrane Na(+) conductance. To assess whether this could be due to different ion channel properties, root protoplasts from both Pokkali and IR20 rice cultivars were patch-clamped. Voltage-dependent K(+) inward rectifiers, K(+) outward rectifiers, and voltage-independent, non-selective channels with unitary conductances of around 35, 40, and 10 pS, respectively, were identified. Only the non-selective channel showed significant Na(+) permeability. Intriguingly, in both cultivars, the activity of the K(+) inward rectifier was drastically down-regulated after plant growth in salt but gating, conductance, and activity of all channel types were very similar for the two cultivars.