Scope and Progress of Rice Research Harnessing Cold Tolerance

Rice crops are substantially affected by low-temperature or cold stress across the world ranging from floodplains to high altitudes. Rice is affected at various growth stages starting from germination to the reproductive stages depending on the intensity of low-temperature and duration of exposure. The primary symptoms of cold injury at the vegetative stage are, low germination, stunted growth, yellowing, seedling death and delayed heading, and during the reproductive stage these manifest as reduced stem elongation, incomplete panicle exertion, panicle degeneration, and spikelet sterility. All these ultimately result in a low yield of rice. There are different crop management practices to reduce cold injury but the genetic improvement of rice for cold tolerance is the most sustainable one. However, genetic improvement is largely limited by the polygenic control of cold-tolerant genes/quantitative trait loci (QTLs), which are mostly confined in japonica background, although indica rice contributes more than 80% of the global rice production. The advent of molecular marker technology has shown much promise in exploiting genetic variations of japonica germplasm for cold tolerance and using it in the breeding program for both indica and japonica rice. So far more than 270 QTLs have been mapped and reported for the seedling and reproductive stages but due to their polygenic and epistatic nature, no noteworthy progress has been reported so far. Genomic selection, high throughput SNP genotyping and repeatable phenotyping, and the use of functional markers in forward breeding can enhance favorable alleles in the breeding population and, thereby, rice segregants can be developed with repeatable variance for cold tolerance in order to withstand and yield higher in cold prone environments.