Progress in Breeding Wheat with Tolerance to Low Temperature in Different Phenological Developmental Stages

Abstract

Low-temperature (LT) tolerance is a complex quantitative character that is expressed in anticipation of and during exposure of plants to temperatures that approach freezing. This environmentally induced character is determined by a highly integrated system of structural and developmental genes that are regulated by environmentally responsive, complex pathways. Genetic analyses at the whole plant level have shown there is developmental regulation of LT-tolerance gene response and transition from the vegetative to the reproductive growth stage is a critical switch that initiates the down regulation of LT-tolerance genes. Consequently, full expression of cold- hardiness genes only occurs in the vegetative stage and plants in the reproductive phase have a limited ability to cold acclimate. Our ability to manipulate the differences in genetic and environmental response has allowed for separation of the genetic factors that determine the rate acclimation from those responsible for the duration of LT acclimation. The developmental genes (vernalization, photoperiod, etc) determine the duration of expression of LT-tolerance conferring genes while the rate of LT acclimation is determined by genotype dependent expression levels of these genes. An understanding of these relationships has permitted the successful transfer of the superior LT-tolerance genes from a winter wheat cultivar into spring wheat (Triticum aestivum L.). In the last decade, a virtual flood of genetic and genomic information has arisen from investigations using model plant systems and tools with an unprecedented level of sophistication for analyses. The superior LT-tolerance genes have been tagged using molecular markers that allow plant breeders to select hardy genotypes without having to wait for a test frost in the field. A greater appreciation of the interactions between growth stage and LT-tolerance gene expression has provided us with the ability to design strategies to minimize the risk of LT damage in different stages of phenological development. However, even with the opportunities offered by advances in technology, we have been unable to produce super-hardy cultivars. For example, while the structural genes within the Triticeae have a high degree of homology and the regulation of LT tolerance is operational across genomes, we have not been able to successfully exploit the superior LT tolerance of rye (Secale cereale L.) for improvement of related cereal species. Progress in this area will have to wait for a clearer understanding of LT signal transduction and the genetic cascade controlling LT-gene expression