Arabidopsis ROOT ELONGATION RECEPTOR KINASES Negatively Regulate Root Growth Via Altering Cell Wall Remodeling Gene Expression

Receptor-like kinases(RLKs)play key roles in regulating various physiological aspects in plant growth and development.In Arabidopsis thaliana,there are about 223leucine-rich repeat(LRR)RLKs.The functions of the majority of RLKs in the LRR XI subfamily were previously revealed.Only three RLKs were not characterized.The purpose of my dissertation was to analyzed the biological functions of these three RLKs.We obtained two independent sets of T-DNA insertion lines for these three RLKs from the Arabidopsis Biological Resource Center(ABRC).These three RLKs were named ROOT ELONGATION RECEPTOR KINASES(REKs)based on their biological functions.We generated two independent triple mutants for these three REKs,rek1‐1 rek2‐1rek3‐1(tri‐1)and rek1‐2 rek2‐2 rek3‐2(tri‐2).We discovered through genetic,molecular biology,and physiological experiments that the primary roots of the triple mutants are significantly longer than those of wild-type.Additional analyses found the triple mutants exhibit increased cell numbers in the root apical meristem and enhanced cell size in the elongation and maturation zones.Conversely,transgenic plants overexpressing REK2 showed significantly shortened roots relative to wild-type.The data indicated that REKs negatively regulate cell division in the meristematic zone and cell expansion in the elongation and maturation zones of Arabidopsis roots.To explore possible molecular mechanisms controlling the root length of tri-1,we tested the promoter activities of a number of QC marker genes in wild type and the triple mutants.They were found to be significantly up-regulated in the triple mutant,the range of expression was expanded.There are three known main signaling pathways that regulate the maintenance of the root apical stem cell nich,including Auxin-RGFs-RGIs-PLTs,SHR/SCR-WOX5,and CLE40-ACR4-WOX5 signaling pathways.Transgenic plants harboring various reporter genes of these three signaling pathways were crossed with the triple mutant and the REK2-overexpressed transgenic line.Homozygous lines were obtained.Confocal and GUS staining analyses showed that the promoters of these reporter genes were not significantly altered in the triple mutant or ox REK2 backgrounds.The tri-1mutant ox REK2 transgenic plants were subsequently treated with various exogenous hormones and stresses.The root growth changes of the tri-1 mutant were similar to those of wild-type.These results suggest that REKs regulate root growth and development likely via an unknow signaling pathway.We therefore extracted total RNAs from the roots of wild-type and tri-1 mutant and performed RNA-seq analyses.Our results revealed that six cell wall remodeling genes are significantly up-regulated in the triple mutant.The proteins encoded by these genes regulate cell wall remodeling and modifications and they are involved in cell wall development.Cell wall remodeling plays an important role in cell elongation.Our results suggest that the three REKs play key roles in regulating root development likely via negatively regulating the expression of a number of key cell wall remodeling genes.