Professor and Chair of the Department University of California, Davis
Populus nigra is a fast-growing, perennial tree with significant potential for exploitation as a bioenergy feedstock. However, high productivity is often coupled with large water requirements and the species can be sensitive to drought. An important step in breeding resilient trees for future climates is to identify a drought tolerant ideotype, the heritability of traits that contribute to a successful ideotype and how this may be exploited in future. Through complementary association genetics, transcriptomics and detailed phenotyping approaches in the laboratory and field, we have identified the morphological, physiological and genomic components of yield resilience in P. nigra. Using a diverse germplasm collection with genotypes selected from extremely dry to wet habitats across Europe, we have explored the importance of trees adapted to droughted environments and compared their responses to drought with those from wetter, cooler climates that nevertheless show an extreme plastic acclimatory response to drought. At a physiological level, we found that genotypes with high hydraulic capacity, and large leaves made up of many cells to be best suited to the multiple drought and non-stressed environments across Europe. Thus we conclude (i) that yield penalty may be acceptable for moderate drought environments across Europe, as a consequence (ii) understanding the genetic basis of vigour, captured in the Yield Stability Index (YSI) may be of greater value than studying the Drought Resistance Index (DRI). If we accept that yield is a key target for our understanding then development of large leaves with many cells provides an important target for selection and breeding. Extending these ideas further, we took more than 500 genotypes from a natural, P. nigra mapping population, drawn from across the species’ western European range. They were subjected to drought under short rotation coppice (SRC) in Savigliano, Italy and phenotyped for biomass yield and leaf size traits under irrigated and moderate drought treatments. Sequence capture genotyping-by-sequencing (GBS) targeting the gene space yielded 132 000 single nucleotide polymorphisms (SNPs) representing 29 400 genes. Genome wide association genetics identified candidate genes for biomass, leaf size and drought tolerance (DRI, YSI) while genetic outlier analyses identified additional signatures of adaptive selection to important precipitation and temperature variables. RNA-seq was then employed to elucidate the transcriptomic response of three diverse genotypes to drought stress with over 2 700 differentially expressed genes found to be involved in the drought response. When gene-phenotype networks were re-constructed for leaves of distinct developmental stages using the top 100 most differentially expressed genes, five key candidates were identified through gene ontology as being central to the drought response. These microtubule-related genes act as gene hubs in young leaves, playing a central role in leaf development and resilience to drought. Subsequently, as leaves mature, these genes are reduced in their connectivity and are situated in the peripheral parts of the networks. Genes of interest overlapping between the GWAS, outlier and RNA-seq analyses were identified and these are strong candidates for application in advanced molecular breeding or genome editing for the sustainable intensification of this important lignocellulosic crop. Moreover, although water use efficiency and saccharification potential are less heritable breeding targets, we identified genotypes which combine yield, water use efficiency and saccharification potential and these are an important resource for future breeding programmes.