Response to waterlogging and drought of timothy and related Phleum species: phenotype and transcriptome diversity

Timothy (Phleum pratense) is a cool-season perennial forage grass commonly used in temperate regions for silage, hay and grazing. Climate change is causing abiotic stresses on agriculture and there is an increasing need for tolerant cultivars. To expand the gene pool for breeding stress-tolerant cultivars, the diversity in natural populations can be exploited. One goal of this study was to understand the diversity of plant traits among a set of Phleum accessions in response to waterlogging and drought stress. A second goal was to identify accessions with putative beneficial traits based on their responses in growth, development, anatomy, and transcriptome. Diversity in biomass and development was evaluated in the field and in a greenhouse for a large collection of 244 wild and domesticated accessions of timothy and two related species, turf timothy (P. nodosum) and alpine timothy (P. alpinum), from different locations and habitats in Europe. Large diversity was found between species and accessions. The groups of wild accessions and cultivars of P. pratense differed in development, but not in biomass. The same pattern was found for P. nodosum. A subset of 19 wild accessions and cultivars from the large collection, were studied in a greenhouse for physiological responses to 21 days of waterlogging, and to 28 days of drought followed by a recovery period. The Phleum species and accessions differed in their response to waterlogging and drought and in their ability to recover. Under waterlogging, some accessions had beneficial adaptations in root growth and aerenchyma formation in roots that were positive for shoot biomass production. In drought, shoot growth was severely affected, but P. nodosum had a greater ability to produce tillers than P. pratense. Based on indexes for drought stress tolerance and resilience, some wild accessions appeared to be more tolerant and to have better recovery. Interestingly, one wild P. pratense accession appeared to have tolerance to both waterlogging and drought. Eight accessions were further investigated for transcriptional responses to waterlogging and drought. Clear differences in the expression pattern due to stress were observed, and abundant tissue-specific (leaf or root) expression to stress was found. Differentially expressed genes (DEGs) associated with stress responses, transport, hormone biosynthesis were found in response to waterlogging and drought. In all three species, DEGs were mainly accession specific. Interestingly, in P. pratense, a wild accession and an old cultivar shared 56 DEGs under drought and showed a similar phenotypic response. The knowledge gained on the diversity of trait responses and transcriptome, together with the identified accessions, are valuable resources for further pre-breeding and development of stress-resilient cultivars.