Mapping QTLs for root system architecture of maize (Zea mays L.) in the field at different developmental stages

Root system architecture (RSA) is seldom considered as a selection criterion to improve yield in maize breeding, mainly because of the practical difficulties with their evaluation under field conditions. In the present study, phenotypic profiling of 187 advanced-backcross BC(4)F(3) maize lines (Ye478 × Wu312) was conducted at different developmental stages under field conditions at two locations (Dongbeiwang in 2007 and Shangzhuang in 2008) for five quantitative root traits. The aims were to (1) understand the genetic basis of root growth in the field; (2) investigate the contribution of root traits to grain yield (GY); and (3) detect QTLs controlling root traits at the seedling (I), silking (II) and maturation (III) stages. Axial root (AR)-related traits showed higher heritability than lateral root (LR)-related traits, which indicated stronger environmental effects on LR growth. Among the three developmental stages, root establishment at stage I showed the closest relationship with GY (r = 0.33-0.43, P < 0.001). Thirty QTLs for RSA were detected in the BC(4)F(3) population and only 13.3 % of the QTLs were detected at stage III. Most important QTLs for root traits were located on chromosome 6 near the locus umc1257 (bin 6.02-6.04) at stage I, and chromosome 10 near the locus umc2003 (bin 10.04) for number of AR across all three developmental stages. The regions of chromosome 7 near the locus bnlg339 (bin 7.03) and chromosome 1 near the locus bnlg1556 (bin 1.07) harbored QTLs for both GY- and LR-related traits at stages I and II, respectively. These results help to understand the genetic basis of root development under field conditions and their contribution to grain yield.