Plant Salt Tolerance

The yield of durum wheat on saline soil is reduced more than bread wheat, mainly because of a higher rate of Na accumulation in leaves which reduces photosynthetic capacity, causes premature leaf senescence, and limits the supply of photosynthate to the developing grain. A search for genetic variation in the ability to exclude Na from leaves within a wide range of tetraploids from the Australian Winter Cereals Collection revealed a monococcum-derived durum breeding line containing two major genes for Na exclusion. These were named Nax1 and Nax2, and were mapped to chromosomes 2A and 5A respectively. The genes lower the Na concentration in the leaf blade by removing Na from the transpiration stream as it flows from roots to shoots. This results in a higher K/Na ratio in leaves. Nax1 (cloned as the Na transporter HKT1;4-A2) is expressed in both roots and shoots. Nax2 (cloned as the Na transporter HKT1;5-A) is expressed only in roots, and only in the cells surrounding the xylem vessels. It is closely related in sequence and in function to the Kna1 gene HKT1;5-D. The Nax genes have been found only in Triticum monococcum and boeoticum, not urartu, so they are not present in durum or bread wheat. When crossed into Australian durum wheat and bread wheat cultivars, they greatly reduced the Na accumulation in leaves. Field trials of durum wheat showed that introduction of Nax2 increased yield in saline soil by up to 25%.