Receptor-like kinases (RLKs) play key roles in regulating various physiological aspects in plant growth and development. In Arabidopsis thaliana, there are at least 223 leucine-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. Here we report that two independent triple mutants of these RLKs, named ROOT ELONGATION RECEPTOR KINASES (REKs), exhibit increased cell numbers in the root apical meristem and enhanced cell size in the elongation and maturation zones. The promoter activities of a number of Quiescent Center marker genes are significantly up-regulated in the triple mutant. However, the promoter activities of several marker genes known to control root stem cell niche activities are not altered. RNA-seq analysis revealed that a number of cell wall remodeling genes are significantly up-regulated in the triple mutant. Our results suggest that these REKs play key roles in regulating root development likely via negatively regulating the expression of a number of key cell wall remodeling genes.

To identify novel regulators of stem cell renewal, we mined an existing but little explored cell type-specific 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 clades II and V or those in clades I and III. 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.

The mechanisms that balance plant growth and stress responses are poorly understood, but they appear to involve abscisic acid (ABA) signaling mediated by protein kinases. Here, to explore these mechanisms, we examined the responses of Arabidopsis thaliana protein kinase mutants to ABA treatment. We found that mutants of BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) were hypersensitive to the effects of ABA on both seed germination and primary root growth. The kinase OPEN STOMATA 1 (OST1) was more highly activated by ABA in bak1 mutant than the wild type. BAK1 was not activated by ABA treatment in the dominant negative mutant abi1-1 or the pyr1 pyl4 pyl5 pyl8 quadruple mutant, but it was more highly activated by this treatment in the abi1-2 abi2-2 hab1-1 loss-of-function triple mutant than the wild type. BAK1 phosphorylates OST1 T146 and inhibits its activity. Genetic analyses suggested that BAK1 acts at or upstream of core components in the ABA signaling pathway, including PYLs, PP2Cs, and SnRK2s, during seed germination and primary root growth. Although the upstream brassinosteroid (BR) signaling components BAK1 and BR INSENSITIVE 1 (BRI1) positively regulate ABA-induced stomatal closure, mutations affecting downstream components of BR signaling, including BRASSINOSTEROID-SIGNALING KINASEs (BSKs) and BRASSINOSTEROID-INSENSITIVE 2 (BIN2), did not affect ABA-mediated stomatal movement. Thus, our study uncovered an important role of BAK1 in negatively regulating ABA signaling during seed germination and primary root growth, but positively modulating ABA-induced stomatal closure, thus optimizing the plant growth under drought stress.

Auxin and auxin-mediated signaling pathways are known to regulate lateral root development. Although exocytic vesicle trafficking plays an important role in recycling the PIN-FORMED (PIN) auxin efflux carriers and in polar auxin transport during lateral root formation, the mechanistic details of these processes are not well understood. Here, we demonstrate that BYPASS1-LIKE (B1L) regulates lateral root initiation via exocytic vesicular trafficking-mediated PIN recycling in Arabidopsis thaliana. b1l mutants contained significantly more lateral roots than the wild type, primarily due to increased lateral root primordium initiation. Furthermore, the auxin signal was stronger in stage I lateral root primordia of b1l than in those of the wild type. Treatment with exogenous auxin and an auxin transport inhibitor indicated that the lateral root phenotype of b1l could be attributed to higher auxin levels and that B1L regulates auxin efflux. Indeed, compared to the wild type, C-terminally green fluorescent protein-tagged PIN1 and PIN3 accumulated at higher levels in b1l lateral root primordia. B1L interacted with the exocyst, and b1l showed defective PIN exocytosis. These observations indicate that B1L interacts with the exocyst to regulate PIN-mediated polar auxin transport and lateral root initiation in Arabidopsis.

Root cap not only protects root meristem, but also detects and transduces the signals of environmental changes to affect root development. The symplastic communication is an important way for plants to transduce signals to coordinate the development and physiology in response to the changing enviroments. However, it is unclear how the symplastic communication between root cap cells affects root growth. Here we exploit an inducible system to specifically block the symplastic communication in the root cap. Transient blockage of plasmodesmata (PD) in differentiated collumella cells severely impairs the root development in Arabidopsis, in particular in the stem cell niche and the proximal meristem. The neighboring stem cell niche is the region that is most sensitive to the disrupted symplastic communication and responds rapidly via the alteration of auxin distribution. In the later stage, the cell division in proximal meristem is inhibited, presumably due to the reduced auxin level in the root cap. Our results reveal the essential role of the differentiated collumella cells in the root cap mediated signaling system that directs root development.

Oscillations in cytosolic free calcium determine the polarity of tip‐growing root hairs. The Ca²⁺ channel cyclic nucleotide gated channel 14 (CNGC14) contributes to the dynamic changes in Ca²⁺ concentration gradient at the root hair tip. However, the mechanisms that regulate CNGC14 are unknown. In this study, we detected a direct interaction between calmodulin 7 (CaM7) and CNGC14 through yeast two‐hybrid and bimolecular fluorescence complementation assays. We demonstrated that the third EF‐hand domain of CaM7 specifically interacts with the cytosolic C‐terminal domain of CNGC14. A two‐electrode voltage clamp assay showed that CaM7 completely inhibits CNGC14‐mediated Ca²⁺ influx, suggesting that CaM7 negatively regulates CNGC14‐mediated calcium signaling. Furthermore, CaM7 overexpressing lines phenocopy the short root hair phenotype of a cngc14 mutant and this phenotype is insensitive to changes in external Ca²⁺ concentrations. We, thus, identified CaM7‐CNGC14 as a novel interacting module that regulates polar growth in root hairs by controlling the tip‐focused Ca²⁺ signal.

Root stem cell niche (SCN) consists of a quiescent center (QC) and surrounding stem cells. Disrupted symplastic communication leads to loss of stemness in the whole SCN. Several SCN regulators were reported to move between cells for SCN maintenance. However, single mutant of these regulators is insufficient to abolish QC stemness despite the high differentiation rate in surrounding stem cells. To dissect the mechanism behind such distinct stemness in SCN, we combined the mis‐expression strategy with pWOX5:icals3m system in which QC is symplastically isolated. We found the starch accumulation in QC could be synergistically repressed by WUSCHEL‐RELATED HOMEOBOX 5 (WOX5), SHORT‐ROOT (SHR), SCARCROW (SCR), and PLETHORA (PLT). Like PLTs, other core regulators also exhibited dimorphic functions by inhibiting differentiation at a higher dose while promoting cell division at a low protein level. Being located in the center of the intersected expression zones, QC cells receive the highest level of core regulators, forming the most robust stemness within SCN. WUSCHEL‐RELATED HOMEOBOX 5 was sufficient to activate PLT1/2 expression, contributing to the QC‐enriched PLTs. Our results provide experimental evidence supporting the long‐standing hypothesis that the combination of spatial expression, synergistic function and dosage effect of core regulators result in spatially distinct stemness in SCN.

Low molecular weight secreted peptides have recently been shown to affect multiple aspects of plant growth, development, and defense responses. Here, we performed stepwise BLAST filtering to identify unannotated peptides from the Arabidopsis thaliana protein database and uncovered a novel secreted peptide family, secreted transmembrane peptides (STMPs). These low molecular weight peptides, which consist of an N‐terminal signal peptide and a transmembrane domain, were primarily localized to extracellular compartments but were also detected in the endomembrane system of the secretory pathway, including the endoplasmic reticulum and Golgi. Comprehensive bioinformatics analysis identified 10 STMP family members that are specific to the Brassicaceae family. Brassicaceae plants showed dramatically inhibited root growth upon exposure to chemically synthesized STMP1 and STMP2. Arabidopsis overexpressing STMP1, 2, 4, 6, or 10 exhibited severely arrested growth, suggesting that STMPs are involved in regulating plant growth and development. In addition, in vitro bioassays demonstrated that STMP1, STMP2, and STMP10 have antibacterial effects against Pseudomonas syringae pv. tomato DC3000, Ralstonia solanacearum, Bacillus subtilis, and Agrobacterium tumefaciens, demonstrating that STMPs are antimicrobial peptides. These findings suggest that STMP family members play important roles in various developmental events and pathogen defense responses in Brassicaceae plants.