Learning from halophytes: functional genomics of plant salinity stress tolerance

Sustainability of world agriculture is critically dependent on the development of new generation crops that will be more resilient to drought and capable of coping with increasing concentration of salt in the soil, without yield penalties. The latter can be done by incorporating some specific traits found in halophytes. In this talk, I will focus on two major mechanisms conferring salinity tolerance in halophytes that can be targeted in crop breeding programs. The first one is effective Na⁺ sequestration in either internal (vacuoles) or external (epidermal bladder cells; EBC) structures. The second mechanism is plant’s ability to maintain efficient photosynthetic gas exchange via stomata under hyperosmotic stress conditions. Working along the first lines, we show that internal (vacuolar) Na⁺ sequestration in halophytes is achieved by the orchestrated regulation of at least four major traits: (1) higher expression levels and operation of tonoplast NHX Na⁺/H⁺ exchangers; (2) higher tonoplast H⁺-pump activity; (3) better cytosolic K⁺ retention originating from intrinsically higher plasma membrane H⁺ pumping ability; and (4) efficient control of SV and FV tonoplast channels to prevent futile Na⁺ cycling between the cytosol and vacuole. I then analyse mechanism underlying sequestration of toxic Na⁺ and Cl^- species in external structures such as epidermal bladder cells. I describe the essentiality of the tissue-specific expression of specific transporters and show experimental evidence for the functional expression and operation of HKT, SOS1, and HAK transporters in epidermis-stalk cell- EBC continuum. In the final part of my presentation I discuss peculiarities of stomata operation in halophytes. Once molecular identity of key genes and control modes of the above processes are understood, plant breeders would be able to devise breeding strategies to incorporate desirable features into traditional crops via molecular or traditional breeding tools thus expanding the sustainable limits for productive use of soil and water resources.