Strategies to Improve Salt Tolerance by Modulating Ion Homeostasis

Discoveries about the identity and function of salt stress signaling components and the transport proteins that mediate Na⁺ homeostasis make it possible to propose strategies for the biotechnological improvement of crops with high probability to increase yield stability in saline environments encountered in cultivated agriculture. Strategies include regulating salt stress signal pathway(s) to be constitutively active or more responsive to stress or modulating effector activity or efficacy. Constitutive activation of the sos pathway is achievable by modifying the sos2 kinase through deletion of its autoinhibitory domain or site-specific modifications to the catalytic region, or by ectopic inducible coexpression of sos3/sos2 (Gaxiola et al., 2001; Quintero et al., 2002; Talke et al., 2003). Presumably, the sos signal pathway modulation can be engineered to enhance the salt adaptation capacity of plants. Another approach is the coordinate control of net Na⁺ flux across the plasma membrane and vacuolar compartmentalization. Other yet unidentified salt adaptation determinants that are outputs of the sos pathway could also be positively affected (Zhu, 2003).

Obviously, overexpression of the putative sodium/proton antiporter AtNHX1 also enhances plant salt tolerance, possibly by increasing vacuolar Na⁺ compartmentalization that minimizes the toxic accumulation of the ion in the cytosol and facilitates growth in the saline environment (Apse et al., 1999; Zhang and Blumwald, 2001; Zhang et al., 2001). The authors report that in the NHX overexpressing tomato plants, Na⁺ is not accumulating in all organs, which might indicate that the altered Na⁺ flux could initiate yet other mechanisms, possibly influencing other ion transporters. Furthermore, overexpression of sos1 increases salt tolerance of Arabidopsis (Shi et al., 2003). These results indicate that regulating net Na⁺ influx across the plasma membrane together with enhancing the capacity for vacuolar compartmentalization should substantially facilitate Na⁺ homeostasis and salt tolerance. With current understanding, this is achievable by modulating the expression or activity of sos1 (Na⁺ efflux) and/or HKT1 (Na⁺ influx) at the plasma membrane, as well as modulating the activity of the vacuolar Na⁺ /H⁺ antiporter and/or H ⁺ pump (Rubio et al., 1995; Vitart et al., 2001; Zhu, 2003).

Future molecular genetic resources for bioengineering of salt tolerance include alleles that encode transport determinants with greater capacity to mediate Na⁺ homeostasis. Halophytes are a potential germplasm resource. Alternatively, new alleles may be generated by directed molecular evolution. For example, mutant variant forms of HKT1 transport more K⁺ at the expense of Na⁺ and render greater salt tolerance (Cheeseman, 1988; Rubio et al., 1999) Promoters that direct the tissue- and/or inducer-specific regulation of target genes can condition the expression of the signal intermediates and the effectors. Thus regulation of the numerous salt tolerance determinants can be coordinated for an effective plant response but much of the cost associated with salt tolerance in nature might be minimized because some essential evolutionary necessities can be compensated for by agricultural practices. Again, halophytes may be natural resources or synthetic promoters will be constructed. Apart from the sos cascade, it is presumed that yet other salt adaptation signal regulatory pathways exist that await discovery and dissection. Conceivably of equally critical importance are growth and development pathways that perceive and interact with salinity perception and response, and are modulated by salt to affect yield stability. In this respect, the redistribution of sodium along the xylem stream from root to reproductive organs could become an avenue for additional intervention. The capacity to short-circuit effects of increasing Na⁺ on growth and development will require a clear understanding about how and why environmental perturbation has such a negative impact on processes required for crop production.