The Key Regulators and Metabolic Intermediates of Lignin Response to Low Temperatures Revealed by Transcript and Targeted Metabolic Profiling Analysis in Poplar

Cold stress restricts the growth and development of plants. Lignin plays an important role in stress resistance. However, there are few studies on lignin mechanisms regulation under low-temperature stress. In this study, the contents of both acid-soluble and -insoluble lignin were reduced after a short period of cold treatment. A targeted metabolomics analysis showed that the contents of caffeate, ferulic acid, coniferaldehyde, and p-coumaraldehyde decreased after cold stress, while the contents of L-phenylalanine and sinapaldehyde increased. The C3′H (p-coumaroylshikimate 3-hydroxylase) and COMT (caffeic acid O-methyltransferase) families, which catalyze the formation of caffeate and coniferaldehyde, were analyzed. Among them, the transcriptional levels of PtrC3′H1, PtrCOMTL4, and PtrCOMT1 were positively correlated with the decreased lignin after cold stimulation. The lignin-related transcription factor PtrMYBs, such as PtrMYB021, PtrMYB074, PtrMYB125/85, and PtrMYB103/46, were clearly induced by cold stress. The reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) further verified that the level of transcription of key genes was consistent with the transcriptomic data. The identified key genes and metabolic intermediates in lignin synthesis provide a foundation for the functional characterization of the molecular mechanism of lignin biosynthesis under cold stress, which should help to efficiently utilize lignin in forest resources.