Search

The succulent xerophyte Zygophyllum xanthoxylum (Bunge) Engl. can absorb Na+ from the soil as an osmoticum in order to resist osmotic stress. The tonoplast Na+/H+ antiporter ZxNHX1 is essential for maintaining the salt-accumulation characteristics of Z. xanthoxylum by compartmentalizing Na+ into vacuoles. Previous results revealed that the silencing of ZxNHX1 greatly decreased Na+ accumulation in Z. xanthoxylum under 50 mM NaCl due to the weakened compartmentalisation; in addition, K+ concentration also significantly reduced in ZxNHX1-RNAi lines. Yet, whether the reduction of K+ concentration was directly triggered by the silencing of ZxNHX1 remains unclear. In this study, the growth parameters and expression levels of ZxSOS1, ZxHKT1;1, ZxAKT1 and ZxSKOR were measured in wild-type and ZxNHX1-RNAi lines under control or –0.5 MPa osmotic stress. The results showed that the silencing of ZxNHX1 inhibited the plant growth, decreased Na+ concentration in leaves, reduced the transcript abundance of ZxSOS1 and dramatically increased that of ZxHKT1;1 in roots of Z. xanthoxylum under osmotic stress; whereas tissue K+ concentrations and the expression level of ZxSKOR displayed no significant variations, and the expression of ZxAKT1 were significantly reduced in ZxNHX1-RNAi lines under osmotic stress, compared with the wild type. These results suggest that in Z. xanthoxylum, ZxNHX1 can maintain the normal growth by compartmentalizing Na+ into vacuoles, and regulate the spatial distribution of Na+ indirectly by affecting the expressions of ZxSOS1 and ZxHKT1;1. Moreover, the silencing of ZxNHX1 is not the main reason that led to the reduction of K+ concentration in ZxNHX1-RNAi lines under 50 mM NaCl, and ZxNHX1 might be indirectly involved in regulating K+ homeostasis.

Lotus corniculatus L. is an important legume for forage, but is sensitive to salinity and drought. To develop salt- and drought-resistant L. corniculatus, ZxNHX and ZxVP1-1 genes encoding tonoplast Na+/H+ antiporter and H+-pyrophosphatase (H+-PPase) from a succulent xerophyte Zygophyllum xanthoxylum L., which is well adapted to arid environments through accumulating Na+ in its leaves, were transferred into this forage. We obtained the transgenic lines co-expressing ZxNHX and ZxVP1-1 genes (VX) as well as expressing ZxVP1-1 gene alone (VP). Compared with wild-type, both VX and VP transgenic lines grew better at 200 mM NaCl, and also exhibited higher tolerance and faster recovery from water-deficit stress: these performances were associated with more Na+, K+ and Ca2+ accumulation in their leaves and roots, which caused lower leaf solute potential and thus retained more water. Moreover, the transgenic lines maintained lower relative membrane permeability and higher net photosynthesis rate under salt or water-deficit stress. These results indicate that expression of tonoplast Na+/H+ antiporter and H+-PPase genes from xerophyte enhanced salt and drought tolerance of L. corniculatus. Furthermore, compared with VP, VX showed higher shoot biomass, more cations accumulation, higher water retention, lesser cell membrane damage and higher photosynthesis capacity under salt or water-deficit condition, suggesting that co-expression of ZxVP1-1 and ZxNHX confers even greater performance to transgenic L. corniculatus than expression of the single ZxVP1-1.

To investigate seasonal and annual influences of environmental conditions on leaf net CO2 uptake (A), three codominant species from the north-western Sonoran Desert differing in photosynthetic pathway and leaf phenology were examined: the C3 deciduous sub-shrub Encelia farinosa, the C4 deciduous bunchgrass Pleuraphis rigida, and the CAM evergreen leaf succulent Agave deserti. To allow interspecific comparisons and to predict field responses from 1974 through 1995, an environmental productivity index (EPI) model previously developed for CAM plants was used, which scaled the responses of A to water, temperature, and photosynthetic photon flux (PPF) over 24-h periods to individual dimensionless values. The net CO2 uptake predicted using the EPI approach agreed well with field measurements. Agave deserti was the most drought-tolerant and E. farinosa was the least; the optimum day/night air temperatures and the PPF requirement for A were highest for P. rigida and lowest for A. deserti. For 1974 through 1995, daily EPI averaged over a year was highest for E. farinosa, indicating that it operates closest to its photosynthetic optimum. However, the predicted A was highest for P. rigida. Variations in A were annually bimodal, with the greatest differences among the three species in wet years. Afor all three species increased linearly as annual rainfall increased. Leaf area per plant for E. farinosa was highest in the winter and early spring and did not respond appreciably to summer rainfall; leaf area for P. rigida was also highest in the winter. For the evergreen A. deserti, which based on ground cover is the dominant species at the field site, new leaves unfolded in response to both winter and summer rainfall but most photosynthetic area was contributed by older leaves, leading to the highest annual plant net CO2 uptake.