The circadian clock of Populus affects physiological, transcriptional and metabolomic responses to osmotic and ionic components of salt stress

The circadian oscillator is an innate timing mechanism present in most organisms, including plants. In this study, Populus tremula × P. tremuloides (Populus) trees with reduced expression of circadian clock components were exposed to gradually increases in the osmotic and ionic components of salt stress. Reduced levels of the morning components PttLATE ELONGATED HYPOCOTYL 1 and 2 (PttLHY1,2) or of the evening components PttPSEUDO-RESPONSE REGULATOR 7a and b (PttPRR7a,b) and PttGIGANTEA1,2 (PttGI1,2) affected growth adaptation under stress conditions. PttLHY1,2 regulated growth under NaCl treatment via the control of PttCyclin D3 expression. PttPRR7a,b and PttGI1,2 were instrumental in maintaining growth in roots by enabling effective adaptation of the metabolome. Major changes in the root metabolome under prolonged stress included alterations in carbohydrate, amino acids, and fatty acids. This study places the circadian clock at the centre of adaptation to adverse conditions in trees and will help the development of stress-resistant trees.