Stay Greenness and its Genetic Improvement in Crop Plants-A Review

The stay-green trait is regarded as the best characterized characteristic conferring drought adaptation in several crops. Plants control the abundance of pigments within their cells depending on the developmental stage of particular tissues or organs and in response to environmental cues. Chlorophyll is degraded in plants as a result of natural or induced senescence. Early explanations focused on a role of stay-green as maintaining photosynthetic activity. It has also been hypothesized that the stay-green trait relates to the plant nitrogen balance and in particular to the capacity to absorb nitrogen during the post-anthesis period. It is only relatively recently that water availability during the post-anthesis period, that is, when the stay-green phenotype expresses itself, has been proposed as a possible cause for the stay-green phenotype. Stay-green mutants are delayed in leaf senescence and have been identified from different plant species, including many crops. Many mutants have been identified that are unable to degrade chl during leaf senescence. Recently, the genetic defect of some of these mutants was shown to be due to mutations in a gene called STAY-GREEN (SGR). Originally, absence of SGR was considered to inhibit chl breakdown. Quantitative trait loci (QTLs) for stay-green have been identified using several populations. Several of these QTLs are currently being used for introgression in a number of genetic backgrounds. Current progress in marker technology is contributing to enhancing the marker coverage of QTL intervals and this would improve breeding efficiency.