An Example of QTL Analyses for Utilization of Wild Species to Increase Salt Tolerance in Tomato

Tomato is one of the most important horticultural crops. In terms of human health, tomato fruit is a major component of daily meals in many countries and constitutes an important source of minerals, vitamins, and antioxidant compounds. However the areas for tomato optimal growing conditions are becoming narrower around the world.

About 20% of irrigated agricultural land and 2% of dry land agriculture are affected by salinity (Yeo et al. 1999). Since salt tolerance, such as tolerance to any abiotic stress, means adaptation, breeding for salt tolerance should take advantage of the evolution of Solanum species occurred through adaptation to marginal environments. In this sense, two tomato wild species have been considered as possible donors of salt tolerance: S. pimpinellifolium L. (Bolarin et al. 1991; Cuartero et al. 1992; Asins et al. 1993; Foolad and Lin 1997) and S. cheesmaniae (L. Riley) Fosberg (Rush and Epstein 1976; Tal and Shannon 1983; Mahmoud et al. 1986; Asins et al. 1993). However, in spite of the great effort devoted to breed for salt tolerance, only a small number of cultivars, partially tolerant to salinity, have been developed (Owen et al. 1994; Al-Doss and Smith 1998; Dierig et al. 2001; Steiner and Banuelos 2003). Two major problems are encountered: the definition, or selection criteria, for "salt tolerance" and the efficient use of the wild germplasm to increase the salt tolerance of the crop.

Efforts on salt tolerance dissection using tomato experimental populations have been carried out taking into account different kinds of traits; however, in the case of crop plants, it is ultimately the yield under specific field conditions that will determine whether or not a gene or combination of genes (or QTLs) is of agronomic importance. In a previous study, we compared QTLs involved in the salt tolerance, in terms of fruit yield, of three F2 families derived from crosses of the tomato cultivated species (Solanum lycopersicum L.) and two wild species S. pimpinellifolium L. and S. cheesmaniae (L. Riley) Fosberg (Monforte et al. 1997a, b). Nevertheless, breeding for salt tolerance requires breeding for a wide adaptation to different salinity levels, because salinity is an abiotic stress factor that varies in time and space. Since the possibilities to study the genetics and physiology of adaptation to different salinity levels in terms of G x E interaction were found to be very limited in F2 populations, we developed and genetically characterized two populations of RILs derived from those F2 populations (Villalta et al. 2005). Besides, a better understanding of the whole plant behavior under changing salinity levels was needed to improve salt tolerance efficiently. This knowledge should involve not only the trait defined as salt tolerance but also other correlated traits because correlated responses may constrain the success of breeding programmes. Salt tolerance in terms of fruit yield was then studied by QTL analysis using two RIL populations (Villalta et al. 2007) and contrary to expected, it was found that the wild allele (i.e. from the wild salt tolerant genotype) was advantageous only at one total fruit yield QTL on chromosome 10 (tw10.1, near the salt specific fn10.1). In fact, they found that the advantageous allele at all fruit weight QTLs came from the cultivated, salt sensitive, species. Moreover, when QTL controlling Na+ and K+ leaf accumulation, as physiological components of salt tolerance, were investigated in these populations, only two sodium QTLs (lnc1.1 and lnc8.1) mapped in genomic regions where fruit yield QTLs were also located. In both cases, the profitable allele corresponded to the salt sensitive, cultivated species. Therefore, other approaches to raising tolerance to salt using wild germplasm needed to be considered.

The grafting technique has been used in agriculture since ancient time to improve horticultural crops. Nonetheless, the mechanism by which the rootstock affects the scion trait remains elusive. In relation to salinity, Estan et al. (2005) had shown that grafting raised fruit yield of a tomato hybrid variety under salinity. This moved us to genetically study the rootstock effect on the fruit yield of a grafted tomato variety under salinity using as rootstock the same two populations of RILs now at F9. Since these populations at F7, had been previously used for QTL analysis of fruit yield of the non-grafted lines allowing the comparison of genetic parameters between grafted and non-grafted plants (Estan et al. 2008). Main results were: (1) there were root-stock lines from the two populations (up to 65% in the P population) that raised the fruit yield of the commercial hybrid under saline conditions; (2) this salt tolerance rootstock effect is a heritable trait (h2 near 0.3), governed by at least eight QTLs; (3) most detected QTLs corresponded to the number of fruits, in agreement with the major relevance of this component among rootstock effects on fruit yield; (4) in general, QTL gene effects were medium-sized, with contributions from 8.5% up to 15.9% at most, and the advantageous allele came from the wild, salt tolerant, species;

(5) only two fruit yield QTLs on chromosomes P9 and C11 might correspond to fruit yield QTLs of the non-grafted lines indicating their root system dependence; and

(6) no common QTL between population was found but a fruit yield QTL on chromosome 3 was acting epistatically in both populations.

The fact that a certain proportion of lines increased the fruit yield of the grafted hybrid variety under salinity was promising but since salinity is variable in time and space it was important to test the lack of negative effects on yield under non saline irrigation. This experiment was carried out with 50 lines from each population and two controls: the non-grafted variety (Boludo, Bol) and the self-grafted variety (Bol/Bol). As shown in Fig. 1.1, most lines are similar than controls in absence of salinity. Therefore, an easy, efficient and profitable utilization of wild germplasm can be carried out through the improvement of rootstocks that confer salt tolerance in terms of fruit yield to the grafted variety instead of introgression their beneficial QTL alleles into the genome of the cultivated tomato.

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