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

Giardini Naxos, Italy - 18/21 September, 2002

ISBN 88-900622-3-1

Oral Communication Abstract - S4b

CHARACTERIZATION OF NEAR-ISOGENIC LINES DIFFERING FOR A QTL REGION AFFECTING LEAF ABA CONCENTRATION IN MAIZE

SALVI S., GIULIANI S., STEFANELLI S., SANGUINETI M.C.

Department of Agroenviromental Science and Technology, University of Bologna, Via Filippo Re 6-8, 40126 Bologna, Italy

maize, NILs, ABA, QTL, microarray

Abscisic acid (ABA) plays a pivotal role in modulating the adaptive response of plants to abiotic stresses, particularly drought. Several QTLs controlling leaf ABA concentration (L-ABA) have been identified in drought-stressed maize grown in the field (Tuberosa et al. 1998, T.A.G. 97: 744-755) in Os420 x IABO78. The same authors hypothesized that a number of such QTLs could be related to genes influencing the water status of the plant. A major QTL for L-ABA mapping on chromosome 2 (bin 2.04) near the RFLP locus csu133 accounted for 32% of the total variation and showed a stronger and more stable effect near flowering. The same QTL region has also been identified to affect L-ABA variation in F2 x Polj17 and A632 x B73. Interestingly, in F2 x Polj17 this QTL also influenced root-pulling strength, a trait associated with root size and architecture. Starting from F₄ (Os420 x IABO78) plants heterozygous at a ca. 10 cM-long segment flanking csu133, the QTL region for high ABA (“+” allele from Os420) was backcrossed into IABO78 by means of marker-assisted selection; viceversa, the QTL region for low L-ABA (“-” allele from IABO78) was backcrossed into Os420. After five backcrosses, plants heterozygous (+/-) at the RFLP loci flanking the QTL were selfed to finally produce near isogenic lines (NILs; BC₅F₃ families) homozygous for the parental QTL alleles (+/+ and -/-). In 2001, two pairs of NILs derived from Os420 and two pairs of NILs derived from IABO78 were tested in replicated field trials under well watered (WW) and water stressed (WS) conditions. The mean values for L-ABA of the two pairs of NILs derived from Os420 were higher than those of the two pairs of NILs derived from IABO78. Within each family, the (+/+) NIL showed a higher mean value than the corresponding (-/-) NIL. On a family mean basis, the mean additive effect was equal to 14.4 and 9.2% of the mean for the Os420 and IABO78 families, respectively. It should be noted that the two IABO78 families, when compared to the Os420 families, showed significantly lower L-ABA values. Plants of the IABO78 and Os420 NILs were also grown in pots. Four water-regime treatments were considered: fully irrigated and three levels of progressively increasing drought stress causing a decrease in leaf relative water content (RWC) of ca. 5, 10 and 20% as compared to the RWC of the fully irrigated treatment. Stomatal conductance, leaf water potential, leaf ABA concentration, root length, root weight and other phenotypic characters were measured. A highly significant difference for L-ABA was detected between Os420(+/+) and Os420(-/-); interestingly, the IABO78 NILs, showed a significant difference in root length. The NILs have also been sampled for RNA extraction at different levels of water stress. Microarray analysis with an oligonucleotide rice chip (Affimetix GeneChip^TM ) is in progress. When the effects of the QTL region were tested in a (Os420 x IABO78) hybrid background, significant differences between the (+/+) and (-/-) hybrids were detected for leaf ABA concentration, root lodging and grain yield, thus confirming the importance of the region near csu133 in affecting agronomically valuable traits. The results herein reported suggest that the differences detected for the investigated traits between the NIL pairs are likely due to a QTL affecting root architecture and size. In order to proceed with the positional cloning of the QTL, an F₂ population was developed from the cross between the recurrent parent Os420 (+/+) and a NIL (-/-). Ca. F₂ 1,500 plants are being characterized for the markers flanking the QTL. Plants showing a recombination event will be selfed.