Differential expression of ion transporters and aquaporins in leaves may contribute to different salt tolerance in Malus species.

Maintaining ion and water homeostasis in plants is an important defense strategy against salinity stress. Divergence in ion homeostasis between the salt-tolerant Malus hupehensis Rehd. and salt-sensitive Malus prunifolia 'yingyehaitang' was studied to understand their mechanisms for tolerance. Compared with the control on Day 15, plants of those two genotypes under high-salinity treatment had less K(+) in the leaves, stems, and roots. Contents were higher in the roots but lower in the leaves of M. hupehensis while levels in the stems were similar to those from M. prunifolia. For both genotypes, the sodium content increased after salinity treatment in all tissue types. However, the leaves from M. hupehensis had less Na(+) and maintained a lower Na(+)/K(+) ratio. To understand the basis for these differences, we studied the ion transporters and regulation of aquaporin transcripts in the leaves. Transcript levels for both MdHKT1 and MdSOS1 were higher in M. hupehensis, implying that this species had better capacity to exclude sodium so that less Na(+) occurred in the leaves but more in the stems. M. hupehensis also had a greater amount of MdNHX1 transcripts, which could have assisted in sequestering excess Na(+) into the vacuoles and sustaining a better cellular environment. A relatively higher level of aquaporin transcript was also found in M. hupehensis, suggesting that those plants were more capable of maintaining a better leaf water status and diluting excess ions effectively under high-salinity conditions. Therefore, these tested transporters may play important roles in determining how salinity tolerance is conferred in Malus species.