Gibberellin promotes cambium reestablishment during secondary vascular tissue regeneration after girdling in an auxin-dependent manner in Populus

Secondary vascular tissue(SVT)development and regeneration are regulated by phytohormones.In this study,we used an in vitro SVT regeneration system to demonstrate that gibberellin(GA)treatment significantly promotes auxin-induced cambium reestablishment.Altering GA content by overexpressing or knocking down ent-kaurene synthase(KS)affected secondary growth and SVT regeneration in poplar.The poplar DELLA gene GIBBERELLIC ACID INSENSITIVE(PtoGAI)is expressed in a specific pattern during secondary growth and cambium regeneration after girdling.Overexpression of PtoGAI disrupted poplar growth and inhibited cambium regeneration,and the inhibition of cambium regeneration could be partially restored by GA application.Further analysis of the PtaDR5:GUS transgenic plants,the localization of PIN-FORMED 1(PIN1)and the expression of auxin-related genes found that an additional GA treatment could enhance the auxin response as well as the expression of PIN1,which mediates auxin transport during SVT regeneration.Taken together,these findings suggest that GA promotes cambium regeneration by stimulating auxin signal transduction.

High-resolution anatomical and spatial transcriptome analyses reveal two types of meristematic cell pools within the secondary vascular tissue of poplar stem

The secondary vascular tissue emanating from meristems is central to understanding how vascular plants such as forest trees evolve,grow,and regulate secondary radial growth.However,the overall molecular characterization of meristem origins and developmental trajectories from primary to secondary vascular tissues in woody tree stems is technically challenging.In this study,we combined high-resolution anatomic analysis with a spatial transcriptome(ST)technique to define features of meristematic cells in a developmental gradient from primary to secondary vascular tissues in poplar stems.The tissue-specific gene expression of meristems and derived vascular tissue types were accordingly mapped to specific anatomical domains.Pseudotime analyses were used to track the origins and changes of meristems throughout the development from primary to secondary vascular tissues.Surprisingly,two types of meristematic-like cell pools within secondary vascular tissues were inferred based on high-resolution microscopy combined with ST,and the results were confirmed by in situ hybridization of,transgenic trees,and single-cell sequencing.The rectangle shape procambium-like(PCL)cells develop from procambium meristematic cells and are located within the phloem domain to produce phloem cells,whereas fusiform shape cambium zone(CZ)meristematic cells develop from fusiform metacambium meristematic cells and are located inside the CZ to produce xylem cells.The gene expression atlas and transcriptional networks spanning the primary transition to secondary vascular tissues generated in this work provide new resources for studying the regulation of meristem activities and the evolution of vascular plants.A web server(https://pgx.zju.edu.cn/stRNAPal/)was also established to facilitate the use of ST RNA-seq data.

The PtoTCP20-miR396d-PtoGRF15 module regulates secondary vascular development in Populus

Secondary vascular development is a key biological characteristic of woody plants and the basis of wood formation.Our understanding of gene expression regulation and dynamic changes in microRNAs(miRNAs)during secondary vascular development is still limited.Here we present an integrated analysis of the miRNA and mRNA transcriptome of six phase-specific tissues-the shoot apex,procambium,primary vascular tissue,cambium,secondary phloem,and secondary xylem-in Populus tomentosa.Several novel regulatory modules,including the PtoTCP20-miR396d-PtoGRF15 module,were identified during secondary vascular development in Populus.A series of biochemical and molecular experiments confirmed that PtoTCP20 activated transcription of the miR396d precursor gene and that miR396d targeted PtoGRF15 to downregulate its expression.Plants overexpressing miR396d(35S:miR396d)showed enhanced secondary growth and increased xylem production.Conversely,during the transition from primary to secondary vascular development,plants with downregulated PtoTCP20expression(PtoTCP20-SRDX),downregulated miR396 expression(35S:STTM396),and PtoGRF15 overexpression(35S:PtoGRF15)showed delayed secondary growth.Novel regulatory modules were identified by integrated analysis of the miRNA and mRNA transcriptome,and the regulatory role of the PtoTCP20-miR396d-PtoGRF15 signaling cascade in secondary vascular development was validated in Populus,providing information to support improvements in forest cultivation and wood properties.