Transcriptomic and epigenomic remodeling occurs during vascular cambium periodicity in Populus tomentosa

Trees in temperate regions exhibit evident seasonal patterns,which play vital roles in their growth and development.The activity of cambial stem cells is the basis for regulating the quantity and quality of wood,which has received considerable attention.However,the underlying mechanisms of these processes have not been fully elucidated.Here we performed a comprehensive analysis of morphological observations,transcriptome profiles,the DNA methylome,and miRNAs of the cambium in Populus tomentosa during the transition from dormancy to activation.Anatomical analysis showed that the active cambial zone exhibited a significant increase in the width and number of cell layers compared with those of the dormant and reactivating cambium.Furthermore,we found that differentially expressed genes associated with vascular development were mainly involved in plant hormone signal transduction,cell division and expansion,and cell wall biosynthesis.In addition,we identified 235 known miRNAs and 125 novel miRNAs.Differentially expressed miRNAs and target genes showed stronger negative correlations than other miRNA/target pairs.Moreover,global methylation and transcription analysis revealed that CG gene body methylation was positively correlated with gene expression,whereas CHG exhibited the opposite trend in the downstream region.Most importantly,we observed that the number of CHH differentially methylated region(DMR)changes was the greatest during cambium periodicity.Intriguingly,the genes with hypomethylated CHH DMRs in the promoter were involved in plant hormone signal transduction,phenylpropanoid biosynthesis,and plant–pathogen interactions during vascular cambium development.These findings improve our systems-level understanding of the epigenomic diversity that exists in the annual growth cycle of trees.

Apoplastic barriers of Populus×canescens roots in reaction to different cultivation conditions and abiotic stress treatments

Populus is an important tree genus frequently cultivated for economical purposes.However,the high sensitivity of poplars towards water deficit,drought,and salt accumulation significantly affects plant productivity and limits biomass yield.Various cultivation and abiotic stress conditions have been described to significantly induce the formation of apoplastic barriers(Casparian bands and suberin lamellae)in roots of different monocotyledonous crop species.Thus,this study aimed to investigate to which degree the roots of the dicotyledonous gray poplar(Populus×canescens)react to a set of selected cultivation conditions(hydroponics,aeroponics,or soil)and abiotic stress treatments(abscisic acid,oxygen deficiency)because a differing stress response could potentially help in explaining the observed higher stress susceptibility.The apoplastic barriers of poplar roots cultivated in different environments were analyzed by means of histochemistry and gas chromatography and compared to the available literature on monocotyledonous crop species.Overall,dicotyledonous poplar roots showed only a remarkably low induction or enhancement of apoplastic barriers in response to the different cultivation conditions and abiotic stress treatments.The genetic optimization(e.g.,overexpression of biosynthesis key genes)of the apoplastic barrier development in poplar roots might result in more stress-tolerant cultivars in the future.