Molecular Breeding for Stay‐Green: Progress and Challenges in Sorghum

Abstract

The stay-green trait is regarded as the best characterized characteristic conferring drought adaptation in several crops including sorghum. Quantitative trait loci (QTLs) for stay-green have been identified using several bi-parental populations. Several of these QTLs are currently being used for introgression in a number of genetic backgrounds. Part of the challenge in the introgression of these QTLs lays in the limited polymorphism between donor and recurrent parents. As a consequence, certain QTL can't always be distinguished, such as Stg3 and StgB which are on the same chromosome, SBI-02. Current progress in marker technology is contributing to enhancing the marker coverage of QTL intervals and this would improve breeding efficiency. Despite the knowledge of genomic regions conferring the stay-green trait, it is surprising that knowledge of the physiological mechanisms explaining staygreen are still relatively unknown. 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 staygreen phenotype expresses itself, has been proposed as a possible cause for the stay-green phenotype. However, the reasons that water is left for absorption are still unexplained and could be accounted for by either a deeper soil extraction depth or water saving traits operating at early stages. As the mechanisms responsible for stay-green become more evident and as DNA-sequencing technologies offer denser genome coverage, the likelihood is that the future of manipulating the stay-green trait will be about manipulating its physiological components.

Introgression of staygreen QLT's for concomitant improvement of food and fodder traits in Sorghum bicolor

“…Introgression of so-called staygreen genes may offer opportunities to improve stover fodder quality while maintaining, and even improving, agronomic yield traits, especially under the water -limiting conditions as encountered in Rabi sorghum cropping (Van Oosterom et al, 1996). Several quantitative trait loci (QTLs) have been identified, using different breeding populations and staygreen QTL donors, and different types of drought stress (Vadez et al, 2013a;Rama Reddy et al, 2014). Given the potential benefit of the staygreen trait, genotypes displaying this trait have been used to identify the genomic regions responsible for this phenotype.…”

A comparative assessment of the performance of a stay-green sorghum (Sorghum bicolor (L) Moench) introgression line developed by marker-assisted selection and its parental lines

“…Improved photosynthesis leads to better drought tolerance (Vadez et al, 2013). Photosynthesis depends on the functioning of light-harvesting and electron transport systems of the chloroplast, which is indicated by the photochemical efficiency of ΦPSII.…”

Sorghum

“…Hash et al (2003) selected six stay-green QTL, including Stg1, Stg2, Stg3, and Stg4 (Subudhi et al, 2000;Sanchez et al, 2002;Harris et al, 2007) as well as StgA and StgB for transfer into a number of genetically diverse, tropically adapted elite sorghum varieties, which already have a range of drought tolerance. They have generated single-QTL introgression lines that can now be used individually as donor parents of specific stay-green QTL; these QTL have been shown to have the largest favorable phenotypic effects across several genetic backgrounds (Vadez et al, 2013). Markerassisted selection in both Striga resistance and drought tolerance demonstrated the power of the technique if robust markers are identified close to the underlying QTL.…”