Proceedings of the XLVII Italian Society of Agricultural Genetics - SIGA Annual Congress

Verona, Italy - 24/27 September, 2003

ISBN 88-900622-4-X

Poster Abstract - 4.09

CONSTITUTION OF MAIZE SEGMENTAL BDLs FOR THE FINE MAPPING OF root-ABA1

S. SALVI, M. BELLOTTI, P. LANDI, S. GIULIANI, M.C. SANGUINETI, S. STEFANELLI, S. CONTI, R. TUBEROSA

Department of Agroenviromental Sciences and Technology, University of Bologna, Via Fanin 44, 40137 Bologna, Italy

abscisic acid, maize, positional cloning, QTL, root traits

Root-ABA1 is a quantitative trait locus (QTL) affecting root traits, leaf ABA concentration and grain yield in maize. Root-ABA1 had been previously identified in a mapping population derived from the cross Os420 x IABO78 (Tuberosa et al. 1998, T.A.G. 97:744-755) as a major QTL influencing leaf ABA concentration (L-ABA). The QTL region for high L-ABA (“+” allele) is contributed by Os420 and the QTL region for low L-ABA (“-” allele) by IABO78. Root-ABA1 was mapped on maize chromosome 2 near csu133 on bin 2.04.

Following five backcrosses, backcross derived lines (BDLs) homozygous for the (+/+) or the (-/-) QTL alleles were investigated for L-ABA concentration in a field trial at different water regimes (Landi et al. 2002, M.N.L. 76:7-8). Highly significant differences in L-ABA were detected under both water regimes. Furthermore, the (+/+) and (-/-) hybrids between the BDLs were evaluated for L-ABA, grain yield and other agronomic traits in 2001 and 2002. In all years, highly significant differences were detected for L-ABA. Interestingly, in 2001 the high L-ABA hybrids showed a significantly lower root lodging (44.6%) compared to the low L-ABA hybrids (66.1%). However, the grain yield of the high L-ABA hybrids (41.6 q/ha) was significantly lower than that of the low L-ABA hybrids (54.7 q/ha). Kernel number/plant was the yield component most affected. The results of a pot experiment have shown a significant effect of the QTL on root length, but not on root biomass. Based on these results, we hypothesize that the primary action of the QTL is on root structure which, in turn, may affect directly root lodging and, indirectly via an effect on water status, ABA concentration, ear sterility and yield.

BDLs with contrasting allelic composition at root-ABA1 have also been sampled for RNA extraction at different levels of water stress. Microarray analysis with an oligonucleotide rice array (Affimetrix GeneChip^TM) is in progress in collaboration with Syngenta (Giuliani et al., unpublished results).

The positional cloning of root-ABA1 will be attempted starting from a large mapping population produced by crossing Os420 and the Os420 (-/-) BDL carrying the root-ABA1 region (ca. 30-cM long) from IABO78. To improve the resolution around root-ABA1 several SSRs markers (from MaizeDB, http://www.agron.missouri.edu) were tested. Ten SSRs were mapped in the region of interest. In 2002, ca. 1,000 BC₆F₂ plants were grown. Two SSRs loci (bnlg381 and umc2030) flanking (30-cM) root-ABA1 were used to genotype each plant at the region harbouring root-ABA1. All F₂ plants showing recombination events (ca. 200) in the 30-cM interval were selfed and were grown as recombinant BC₆F₃ families in 2003. After molecular markers analysis, all F₃ plants homozygous for a recombination event between the two markers flanking root-ABA1 were selfed. Eventually, all segmental BDLs homozygous for different crossover events at root-ABA1 will be tested for fine mapping and to ascertain to what extent the effects on L-ABA, root traits and yield might be due to one or more linked QTLs.

Based on the results of the fine mapping of segmental BDLs, the microarray analysis, and by exploiting the maize-rice synteny maps we plan to identify a maize physical chromosome region (e.g. BAC contig) carrying the gene/s responsible for root-ABA1. Interestingly, the results of a preliminary survey indicated that the rice region syntenic to maize bin 2.04 harbors root QTLs in several rice populations (Tuberosa et al., unpublished results).