BOLOGNA, ITALY
University of Bologna

Mapping QTLs for leaf abscisic acid concentration in a maize population
--Landi, P; Sanguineti, MC; Salvi, S; Tuberosa, R; Casarini, E; Conti, S

Several studies have indicated that abscisic acid (ABA) has a pivotal role in the adaptive response of plants to drought-stress. In the present work, we have analyzed a maize population in order to identify QTLs affecting leaf-ABA (L-ABA) concentration and their overlap with QTLs for physiological traits involved in the response to drought. The population considered in this study included 80 random F4 families derived from the cross Os420 x IABO78. Previous experiments (Conti et al., Euphytica 78:81, 1994) have shown that under drought-stressed conditions Os420, as compared to IABO78, has a 2-3 fold higher L-ABA concentration. In 1994 and 1995, the F4 families were tested in replicated field trials conducted under conditions of partial water deficit. The following traits were investigated: L-ABA concentration measured, according to Hanway's classification, at stage 3 (L-ABA-3) and stage 4 (L-ABA-4), corresponding to rapid stem elongation and anthesis, respectively; drought stress index (DSI) visually scored; stomatal conductance (SC) at stage 4; leaf temperature (LT) at stage 4 in 1994 (mean of two measurements) and between stages 3 and 4 in 1995 (mean of seven measurements).

Southern analysis was carried out using 109 RFLP markers obtained with probes kindly provided by T. A. Musket (University of Missouri). The genetic map was computed using the program JOINMAP (Stam, Plant J. 5:503, 1993) and a LOD score of 2.5. QTL analysis was performed with the composite interval mapping procedure adopted in the program PLABQTL (Utz and Melchinger, JQTL 2:1, 1996).

The heritability values for L-ABA estimated on a family mean basis varied from 0.60 to 0.88. The genetic map spanned 1578 cM, which corresponds approximately to 75% of the length of the maize map. QTL analysis showed that the number of significant (LOD > 2.0) QTLs for L-ABA concentration, at each sampling, varied from two to eight (Table 1). In total, 16 QTLs were revealed in at least one sampling. The alleles increasing L-ABA were mainly contributed by Os420. The QTL with the strongest effect on L-ABA concentration was found on chr. 2, in the region between markers csu133 and csu4. In both years, this QTL was detected only at the second sampling, despite the fact that the mean value of L-ABA concentration in 1994 was higher at the first sampling (587 vs. 366 ng ABA/g d.w.). Therefore the action of this QTL seems less influenced than others by the level of water stress experienced by the plant and its growth-stage

The supporting intervals of the QTLs for L-ABA concentration did not overlap with the known location of ABA mutants, such as the viviparous mutants. It seems more likely that the effects of these QTLs on L-ABA relate to those of genes controlling morpho-physiological traits (e.g. root morphology, leaf area, etc.) which affect the water balance of the plant. The effects of genes modulating the intensity of the signal transduction associated with turgor loss, a major determinant for the concentration of ABA, could also be involved. Interestingly, the map position of the QTL on chr. 2 coincided with those of one QTL for L-ABA concentration and one QTL for root-pulling strength which have been found in a different maize population (Lebreton et al., J. Exp. Bot. 46:853, 1995).

Significant QTLs were also identified for the other investigated traits. Several of these QTLs overlapped with QTLs for L-ABA. As a general trend, an increase in ABA was associated to a higher DSI, lower SC, and higher LT. Only in a few cases, the reverse trend was noticed. Although it is not possible to ascertain if these associated effects are due to linkage and/or pleiotropy, our result indicate that in the population herein investigated an increase in L-ABA seems to have mainly represented an indicator of the degree of water-stress experienced by the plant at the time of sample collection, rather than having a causal role in the improvement of the level of drought-tolerance in the field.

Table 1. Additive effect (ng ABA g-1 d.w.) and LOD score at the QTLs for leaf ABA (L-ABA) concentration investigated at two samplings (stage 3 and stage 4) in 1994 and 1995.
 
1994 1995
L-ABA-3
L-ABA-4
L-ABA-3
L-ABA-4
Chrom.
Effect (1)
LOD
Effect (1)
LOD
Effect (1)
LOD
Effect (1)
LOD
1 47 2.97 - - - - - -
- - 49 6.62 - - - -
2 - - - - - - 34 2.54
- - 41 5.56 - - 72 8.88
- - 33 5.56 34 7.17 32 3.46
3 - - - - 13 3.64 - -
- - 32 3.11 - - - -
-28 2.70 - - - - - -
4 - - 16 2.65 18 2.47 32 2.87
5 - - - - 18 2.83 - -
6 - - - - - - 25 2.01
- - - - - - -25 2.22
7 - - - - -21 2.80 - -
- - - - 25 4.85 - -
- - -14 2.57 -21 2.89 - -
9 - - -28 4.23 -32 5.01 - -
QTL (no.)
2
7
8
6
R2 (%)
18
60
51
65
(1) Computed as (Os420 - IABO78)/2. 


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