Receptor-like protein kinase-mediated signaling in controlling root meristem homeostasis

Generation of the root greatly benefits higher plants living on land.Continuous root growth and development are achieved by the root apical meristem,which acts as a reservoir of stem cells.The stem cells,on the one hand,constantly renew themselves through cell division.On the other hand,they differentiate into functional cells to form diverse tissues of the root.The balance between the maintenance and consumption of the root apical meristem is governed by cell-to-cell communications.Receptor-like protein kinases(RLKs),a group of signaling molecules localized on the cell surface,have been implicated in sensing multiple endogenous and environmental signals for plant development and stress adaptation.Over the past two decades,various RLKs and their ligands have been revealed to participate in regulating root meristem homeostasis.In this review,we focus on the recent studies about RLK-mediated signaling in regulating the maintenance and consumption of the root apical meristem.

Plant stem cells and their regulations in shoot apical meristems

Stem cells in plants,established during embry-ogenesis,are located in the centers of the shoot apical meristem(SAM)and the root apical meristem(RAM).Stem cells in SAM have a capacity to renew themselves and to produce new organs and tissues indefinitely.Although fully differentiated organs such as leaves do not contain stem cells,cells in such organs do have the capacity to re-establish new stem cells,especially under the induction of phytohormones in vitro.Cytokinin and auxin are critical in creating position signals in the SAM to maintain the stem cell organizing center and to position the new organ primordia,respectively.This review addresses the distinct features of plant stem cells and focuses on how stem cell renewal and differentiation are regulated in SAMs.

Notes from the Underground:Receptor-Like Kinases in Arabidopsis Root Development

During plant development, the frequency and context of cell division must be controlled, and cells must differentiate properly to perform their mature functions. In addition, stem cell niches need to be maintained as a reservoir for new cells. All of these processes require intercellular signaling, whether it is a cell relaying its position to other cells, or more mature cells signaling to the stem cell niche to regulate the rate of growth. Receptor-like kinases have emerged as a major component in these diverse roles, especially within the Arabidopsis root. In this review, the functions of receptor-like kinase signaling in regulating Arabidopsis root development will be examined in theareas of root apical meristem maintenance, regulation of epidermal cell fate, lateral root development and vascular differentiation.

TGA factors promote plant root growth by modulating redox homeostasis or response

To identify novel regulators of stem cell renewal,we mined an existing but little explored cell typespecific transcriptome dataset for the Arabidopsis root.A member of the TGA family of transcription factors,TGA8,was found to be specifically expressed in the quiescent center(QC).Mutation in TGA8 caused a subtle root growth phenotype,suggesting functional redundancy with other TGA members.Using a promoter::HGFP transgenic approach,we showed that all TGA factors were expressed in the root,albeit at different levels and with distinct spatial patterns.Mutant analyses revealed that all TGA factors examined contribute to root growth by promoting stem cell renewal,meristem activity,and cell elongation.Combining transcriptome analyses,histochemical assays,and physiological tests,we demonstrated that functional redundancy exists among members of cladesⅡandⅤor those in cladesⅠandⅢ.These two groups of TGA factors act differently,however,as their mutants responded to oxidative stress differently and quantitative reverse transcription polymerase chain reaction assays showed they regulate different sets of genes that are involved in redox homeostasis.Our study has thus uncovered a previously unrecognized broad role and a mechanistic explanation for TGA factors in root growth and development.