Date : Thursday, 13 December 2007
Time : 3.30pm - 4.15pm
Venue : Conference Rm, Wagga Wagga Agricultural Institute
Presenter : Professor Roberto Tuberosa, Department of Agroenvironmental Sciences & Technology, University of Bologna, Italy
Improving drought tolerance of cereals has become a top priority in view of the fast increasing demand in food, feed and, more recently, biofuel. This multidisciplinary challenge is even more daunting in view of (i) the effects of global climate change on rainfall distribution and (ii) the decreased availability and escalating costs of irrigation water (Tuberosa et al. 2007). As compared to conventional approaches, genomics-based approaches allow us to elucidate more effectively the genetic basis of the quantitatively inherited traits that influence crop performance under drought conditions (Tuberosa and Salvi 2006, 2007 ). Therefore, quantitative trait loci (QTL) discovery is an unavoidable crossroad for enhancing crops yield and yield sustainability. In our department, both forward and reverse-genetics approaches are used to identify and clone QTLs that influence tolerance to water-limited conditions in cereals.
In durum wheat, an EU-funded project, Improving Durum Wheat for Water-Use Efficiency, based on biparental linkage mapping (population of 250 recombinant inbred lines; Maccaferri et al. 2007) and association mapping (mini-core collection of 190 elite lines; Maccaferri et al. 2006) has identified several QTLs able to influence the adaptive response to water-limited conditions and grain yield. In particular, two major epistatic QTLs on chromosomes 2BS and 3BL consistently influenced grain yield across 7 and 8, respectively, of 16 environments characterized by a broad range of water regimes and, consequently, yield potential (Maccaferri et al. 2007a). At present, we are investigating root traits to verify to what extent variability in root architecture might account for variability in yield due to these two QTLs. Association mapping revealed the presence of a number of chromosome regions able to influence root architecture in durum wheat (Maccaferri et al. 2007b).
In barley, we have identified QTLs for grain yield under water-limited conditions using an AB-QTL approach applied to a H. vulgare x H. spontaneum cross (Talamè et al. 2004). At several QTLs, the agronomically favourable allele was contributed by H. spontaneum . Additionally, we have used microarray profiling to investigate how the dynamics of the dehydration treatment affects gene expression (Talamè et al. 2007): the correlations of the fold-change in gene expression under different dynamics were significant but low ( r from 0.32 to 0.41). As a reverse-genetics tool suitable for elucidating the function of candidate genes via TILLING, we have assembled a sodium-azide mutagenized collection of ca. 5,000 M3 families (Talamè et al., unpublished). The mutation frequency (5-9 mutants/gene) observed at the nine genes that have so far been tilled indicates the suitability of this population for gene discovery purposes.
Once major QTLs are identified, their cloning allows us to better understand the genetic basis of quantitative variation and provides a more direct way for mining (e.g. EcoTILLING) and manipulating (e.g. genetic engineering) valuable alleles (Salvi and Tuberosa 2005 ) . In maize, we have identified a major QTL ( root-ABA1 ) that influences root architecture (i.e. a trait that confers tolerance to drought through "avoidance"), abscisic acid concentration ( ABA ) and grain yield under different water regimes (Landi et al. 2007). Fine mapping of root-ABA1 is in progress as a prerequisite to its positional cloning. Additionally, through collaboration with Pioneer-DuPont, we have cloned Vgt1 , a major QTL for flowering time (i.e. a trait that confers tolerance to drought through "escape") in maize (Salvi et al. 2007).
Although QTL cloning is still in its infancy, further refinement of genomics tools and platforms as well as the increasing availability of sequence information will make QTL cloning more of a routine procedure. We foresee that while QTL analysis and cloning addressing naturally occurring genetic variation will shed light on mechanisms of plant adaptation to drought, more emphasis on approaches relying on candidate gene identification, "omics" platforms and reverse genetics (eg. TILLING, VIGS, etc.) will accelerate the pace of discovery of the genes underlining QTLs for drought tolerance. This information will be pivotal for more effective implementation of marker-assisted selection and genetic engineering for the release of drought-resilient cultivars.