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Phloem-related genes are downregulated during radial growth of smxl4 smxl5 double mutant stems

ABSTRACT: As a prerequisite for constant growth, plants can produce vascular tissues at different sites in their postembryonic body. In particular, the formation of vascular tissues during longitudinal and radial expansion of growth axes differs fundamentally with respect to its anatomical configuration. This raises the question to which level regulatory mechanisms of vascular tissue formation are shared throughout plant development. Here, we show that, similar as primary phloem formation during longitudinal growth, the cambium-based formation of secondary phloem depends on the function of SMXL genes. Using promoter reporter lines, we observe that SMXL4 and SMXL5 activities are associated with different stages of secondary phloem formation in Arabidopsis stems and that specific loss of SMXL5 function results in the absence of secondary phloem. Interestingly, the additional disruption of SMXL4 activity increases cell proliferation rates in the cambium region without forming secondary phloem. Based on genome-wide transcriptional profiling and expression analyses of phloem-related markers we conclude that early steps of phloem formation are impaired in smxl4;smxl5 double mutants and that additional cambium-derived cells fail in establishing any phloem-related feature. Our results show that molecular mechanisms determining primary and secondary phloem share important features but differ slightly with SMXL5 playing a more dominant role in the formation of secondary phloem.

ORGANISM(S): Arabidopsis thaliana

PROVIDER: GSE129190 | GEO | 2019/04/04

REPOSITORIES: GEO

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Project description:Vascular cambium is a secondary meristem which produces xylem (wood) inwards and phloem (bark) outwards. The activity of cambium leads to expansion in the diameters in plants, that is, secondary growth, and thus contributes to biomass increase. The regulation of cambium development is at multilevel including phytohormones and peptide-receptor kinase (CLE41/44-PXY) signalling pathways. However, only limited progress has been made on the transcriptional regulation level. To construct the transcriptional network that regulates cambium development, we performed a genome wide transcript profiling in sorted procambial and cambial cells in Arabidopsis roots. More than 500 genes were identified as cambium abundant genes via comparison against transcriptomes of other Arabidopsis root cell types. We then investigated the roles of almost all the cambial transcription factors (TFs) and some of their homologous genes during secondary growth. Many of the candidate TFs were highly expressed in cambium based on the promoter GUS fusion and in situ hybridization. An unbiased transcriptional regulatory network was constructed using transcript profiling data collected from inducible overexpression lines for selected candidate TFs and a few major nodes were identified in the network including WOX4 and KNAT1. Moreover, the severity of mutant phenotypes was predicted based on the network. Next, we used the predication as a guide and generated over 70 double mutants within the same or among different TF families to explore the genetic interactions of candidate genes. Phenotype characterization on the secondary growth of the mutants and the overexpression lines identified promoters and inhibitors of cambium activity. The phenotypic data also suggested the redundancy within and among TF families because the phenotypes could be enhanced when additional TFs were mutated. We also found that combinations of certain overexpression lines and mutants led to pronounced increase of cell proliferation or xylem differentiation and in the extreme case the formation of ectopic cambium. We herein propose that cambium development is orchestrated by a wide network at the transcriptional level, where each TF has a different contribution to cell proliferation and differentiation.

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Phloem-related genes are downregulated during radial growth of smxl4 smxl5 double mutant stems

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