Aquaporin expression in response to water-deficit stress in two Malus species: relationship with physiological status and drought tolerance

Aquaporins play an essential role in regulating the water balance in plants. We compared responses between Malus prunifolia (Willd.) Borkh., a reputedly drought-tolerant rootstock, and M. hupehensis Pamp. Reld., which is drought-sensitive. One-year-old seedlings were treated with long-term, moderate water deficit (field capacity of 45-50 %), and were monitored for photosynthetic characteristics, leaf relative water content (LRWC). The relative expressions of mRNA encoding six putative aquaporins also were determined by Q-RT-PCR. Throughout the experimental period, M. prunifolia had higher readings for all four photosynthetic characteristics. Values for LRWC were rapidly reduced under stress, especially for M. hupehensis. Except for TIP2;1, the transcript abundance of most aquaporin genes was elevated after drought stress, reaching a peak at Day 14 before declining in the leaves and roots of both species. For TIP2;1, however, the transcript level was continuously reduced in M. prunifolia leaves throughout the entire course of treatment. Expression of aquaporin genes varied according to the gene, water status, and plant organ being examined. We conclude that Malus aquaporins are responsive to water stress at the transcriptional level, probably playing several roles in conferring drought tolerance. Ours is the first report of this relationship between aquaporins and drought tolerance in Malus species.