Marker-Assisted Introgression of Quantitative Trait Loci (QTLs) for Stay-green in Sorghum [Sorghum bicolor (L). Moench]

Kassahun, B., Bidinger, F. and Hash, C.T., Jimma University, College of Agriculture and Veterinary Medicine, Department of Crop Science, P.O. Box 307, Jimma, Ethiopia. Tel.: +251-47-1110102; Fax: +251-47-1110904; E-mail: kassahunb@gmail.com

Drought is a major limiting factor in agriculture and is considered as the most important cause of yield reduction in crop plants (Boyer, 1982), especially in water-limited areas where most of the world’s poorest farmers live. Development of more drought tolerant crops will enhance food production and the livelihoods of farmers in these areas.

Figure 1.  Stay-green expression in donor parent, B35 (left of photo) neighboring a senescent line (right of photo) during field evaluation under early-onset of stress	Figure 2. Stay-green expression in RSG 04001 (BC2F4), one of the best stay- green introgression lines, during field evaluation under  early- onset of  stress

In sorghum, the best characterized form of drought tolerance during grain-filling stage of crop growth is the “stay-green” trait. It is defined as the ability to resist premature plant senescence, retain green leaf area, fill grain normally, and resist lodging under conditions of post-flowering drought stress (Rosenow, 1987). When water is limited during the grain-filling period, some sorghum genotypes with this trait maintain photosynthetically active leaf area better than other genotypes with out the trait. Stay-green can be viewed as a consequence of the balance between nitrogen (N) demand by the grain and that supplied from the roots during grain filling (Borrell et al., 2000b).

As the stay-green trait in sorghum is expressed only in those environments in which post-anthesis drought is sufficiently severe, neither efficiency nor reliability of selection is high when conventional breeding is used to select for this trait. Marker-assisted selection for stay-green should thus greatly enhance the efficiency of its selection (Borrell et al., 2000c).

In a marker-assisted backcrossing programme, four stable stay-green QTLs (StgB, Stg1, Stg3 and Stg4) were selected to be transferred from the stay-green donor parent (B35) to the senescent recurrent parent (R16). After two additional marker-assisted backcrossing and two selfing generations, four backcross families with single (Stg1, Stg3, Sg4 and StgB) and five backcross families with double putative QTLs (Stg1+Stg3, Stg1+Stg4, Stg3+Stg4, Stg3+StgB and Stg4+StgB) have been selected.

Previously derived, early generation stay-green introgression lines (BC2F2/BC1F3) were evaluated for their agronomic and stay-green related characteristics under well watered and water-stress conditions both in field (in the post rainy season) and greenhouse conditions for two seasons. Most of the introgression lines were better than their senescent parents in terms of stay-green {% green leaf area (GLA)}. Among the introgression lines, RSG 04001 was found to be the most promising both in terms of its grain yield and stay-green related characteristics.  

In addition, the stay-green genotypes had high chlorophyll content and leaf nitrogen concentration compared with the senescent lines indicating a relationship among these traits. The correlations of relative grain yield to relative %GLA in the stress environments was highly significant (R² = 0.47 in 2004-05, and R² = 0.77 in 2005-06) indicating the contribution of stay green to grain yield. The advanced generation (BC3 and BC4) introgression progeny lines are currently being evaluated for both stay-green and grain yield related characters.

Citations

Borrell AK and Hammer GL (2000). Nitrogen dynamics and the physiological basis of stay-green in sorghum. Crop Science 40, 1295-1307.

Borrell AK, Tao YZ and McIntyre CL (2000). In ‘Molecular Approaches for the Genetic Improvement of Cereals for Stable Production in Water-Limited Environments’. (Ed. J.-M. Ribaut and D. Poland). pp. 142-146. (Mexico D.F.: CIMMYT).

Boyer J.S. 1982. Plant productivity and environment. Science 218: 443-448.