Enhancing salt tolerance in crops through molecular breeding: a new strategy

Abstract

Salinity limits crop production in many rainfed ecosystems and is an increasing problem in irrigated areas. Developing and developed countries alike need more efficient methods of enhancing salt tolerance in crops. The traditional breeding approach consists of (i) screening germplasm collections for donors of salt tolerance, (ii) crossing a donor with an elite line and advancing the F₁ hybrid to about the F₇ or F₈ generation while selecting for elite traits, and concurrently (iii) selecting for salt tolerance starting at about the F₄ generation. The low efficiency of this pedigree approach is due principally to the difficulty of recovering elite genotypes with salt tolerance traits, the genetic complexity of salt tolerance, and the strength of genotype × environment interactions (GEI). Advances in genomics are making possible an alternative approach in which a pre-breeding phase is used to pyramid several known genes and finely mapped major QTLs for complementary aspects of salt tolerance. The known genes can include transgenes modified by deliberate mutation or promoter switching. DNA-based selection protocols that are used to pyramid these genes are again employed during the breeding phase to transfer the entire set of genes for salt tolerance into any elite line by backcrossing. Breeding efficiency is increased because (i) elite traits are more easily recovered by backcrossing than by the pedigree approach, (ii) the genetic complexity of salt tolerance is reduced to a small set of well-defined genes and loci of large effect, (iii) the confounding effects of GEI are eliminated from the breeding phase, and (iv) the investment in gene discovery and QTL mapping can be recouped by transferring the same set of genes to a range of recipient lines that require salt tolerance. Efficiency will increase over time as more genes are added to the gene set and as QTL are replaced by the corresponding gene. Here we summarize the scientific advances underlying this new strategy, which should be applicable to other complex traits such as drought tolerance, durable resistance to pests and diseases, yield potential and product quality.