Options for Developing Salt Tolerant Crops

Soil salinization is a major factor limiting agricultural productivity, reducing crop yields on 25-30% of irrigated land in the US and about 40% of irrigated land worldwide.   An inevitable consequence of irrigation, salinity is consequently a problem in containerized and controlled environment production and management.  It is often a problem on marginal lands worldwide, particularly in arid regions where salinization is essentially irreversible because fresh water for leaching is not available.  However, many plants are salt tolerant.  Some partition or accumulate potentially toxic levels of salt to older tissues that are shed, some have active exclusion structures, e.g., salt glands, and some at the cellular level specifically exclude salts from the cytosol.  Although salt tolerance is seen as a complex trait and breeding improvement has been slow, recent experiments suggest potential for creating new salt tolerant crops.  These involve various transgenes that utilize transport mechanisms that sequester salt in the vacuole or exclude it from the cytosol in the apoplast, or alternatively use osmoprotectants, e.g., proline, ectoine, betaines, polyols, and trehalose, that nullify toxic effects of salts, by raising osmotic pressure in the cytoplasm, by stabilizing proteins and membranes when salt levels or temperatures are unfavorable, or by effects on stress related biosynthetic and signaling pathways.  There are also regulatory genes involved in cold and drought tolerance that may play a role.  There are now a number of first generation single gene examples of these, but they are not always without problems.  Salt Overly-Sensitive 1 (SOS1) encodes a plasma membrane Na+/H+ antiporter from Arabidopsis that is a salt excluder.  Similarly, an Arabidopsis AtNHX1 antiporter on the tonoplast sequesters salt in the vacuole.  Both of these require proton motive forces generated by membrane located ATPases or pyrophosphatases, and both can result in substantial salt tolerance in transgenic plants.  Alternatively, a C-repeat binding factor/drought responsive element binding factor (CBF3/DREB1a) encodes a transcription factor for abiotic stress gene regulation, and while this has some positive effects on salt and drought tolerance there can be negative effects on growth.   Several transgenes result in accumulation of osmoprotectants.  One is the mannose-6-phosphate reductase gene (M6PR) that codes for an enzyme resulting in mannitol biosynthesis, and plants transgenic for M6PR that ordinarily lack mannitol become substantially salt tolerant.  However, not all osmoprotectant transgenes result in equivalent levels of protection, and some have been deleterious.   We will review these and other options and compare their potential.