Distinct mechanisms orchestrate the contra-polarity of IRK and KOIN, two LRR-receptor-kinases controlling root cell division

In plants, cell polarity plays key roles in coordinating developmental processes. Despite the characterization of several polarly localized plasma membrane proteins, the mechanisms connecting protein dynamics with cellular functions often remain unclear. Here, we introduce a polarized receptor, KOIN, that restricts cell divisions in the Arabidopsis root meristem. In the endodermis, KOIN polarity is opposite to IRK, a receptor that represses endodermal cell divisions. Their contra-polar localization facilitates dissection of polarity mechanisms and the links between polarity and function. We find that IRK and KOIN are recognized, sorted, and secreted through distinct pathways. IRK extracellular domains determine its polarity and partially rescue the mutant phenotype, whereas KOIN’s extracellular domains are insufficient for polar sorting and function. Endodermal expression of an IRK/KOIN chimera generates non-cell-autonomous misregulation of root cell divisions that impacts patterning. Altogether, we reveal two contrasting mechanisms determining these receptors’ polarity and link their polarity to cell divisions in root tissue patterning.

Activation loop phosphorylaton of a non-RD receptor kinase initiates plant innate immune signaling

Receptor kinases (RKs) are fundamental for extracellular sensing and regulate development and stress responses across kingdoms. In plants, leucine-rich repeat receptor kinases (LRR-RKs) are primarily peptide receptors that regulate responses to myriad internal and external stimuli. Phosphorylation of LRR-RK cytoplasmic domains is among the earliest responses following ligand perception, and reciprocal transphosphorylation between a receptor and its coreceptor is thought to activate the receptor complex. Originally proposed based on characterization of the brassinosteroid receptor, the prevalence of complex activation via reciprocal transphosphorylation across the plant RK family has not been tested. Using the LRR-RK ELONGATION FACTOR TU RECEPTOR (EFR) as a model, we set out to understand the steps critical for activating RK complexes. While the EFR cytoplasmic domain is an active protein kinase in vitro and is phosphorylated in a ligand-dependent manner in vivo, catalytically deficient EFR variants are functional in antibacterial immunity. These results reveal a noncatalytic role for EFR in triggering immune signaling and indicate that reciprocal transphoshorylation is not a ubiquitous requirement for LRR-RK complex activation. Rather, our analysis of EFR along with a detailed survey of the literature suggests a distinction between LRR-RKs with RD- versus non-RD protein kinase domains. Based on newly identified phosphorylation sites that regulate the activation state of the EFR complex in vivo, we propose that LRR-RK complexes containing a non-RD protein kinase may be regulated by phosphorylation-dependent conformational changes of the ligand-binding receptor, which could initiate signaling either allosterically or through driving the dissociation of negative regulators of the complex.

Tubular microdomains of Rab7-endosomes retrieve TrkA, a mechanism disrupted in Charcot-Marie-Tooth 2B

Axonal survival and growth requires signalling from tropomyosin receptor kinases (Trks). To transmit their signals, receptor-ligand complexes are endocytosed and retrogradely trafficked to the soma where downstream signalling occurs. Vesicles transporting neurotrophic receptors to the soma are reported to be Rab7-positive late endosomes/multi vesicular bodies where receptors localize within so-called intraluminal vesicles. Therefore, one challenging question is how downstream signalling is possible given the insulating properties of intraluminal vesicles. In this study, we report that Rab7-endosomes/multi vesicular bodies retrieve TrkA through tubular microdomains. Interestingly, this phenotype is absent for the EGF-receptor. Further, we found that EndophilinA1, EndophilinA2 and EndophilinA3 together with WASH1 are involved in the tubulation process. In Charcot-Marie-Tooth 2B, a neuropathy of the peripheral nervous system, this tubulating mechanism is disrupted. In addition, the ability to tubulate correlates with the phosphorylation levels of TrkA as well as with neurite length in neuronal cultures from dorsal root ganglia. In all, we report a new retrieval mechanism of late Rab7-endosomes, which enables TrkA signalling and sheds new light onto how neurotrophic signalling is disrupted in CMT2B.

S-acylation is a positive regulator of FLS2-mediated plant immunity

Plant receptor kinases are key transducers of extracellular stimuli and are regulated by numerous post-translational modifications. S-acylation involves the addition of long-chain fatty acids to cysteine residues within proteins, altering their biophysical properties. Here we identify S-acylation at a conserved cysteine of the receptor kinase FLS2 as crucial for function during plant immunity. We observe rapid S-acylation of FLS2 upon perception of its flg22 ligand in a BAK1 co-receptor dependent manner. Notably, S-acylation is essential for several aspects of FLS2-mediated early and late signalling, including anti-bacterial immunity. Biochemical analysis suggests that FLS2 S-acylation assists the stabilisation of activated receptor kinase protein complexes at the plasma membrane to increase signalling efficiency.

Early signalling processes in roots play a crucial role in the differential salt tolerance in contrasting Chenopodium quinoa accessions

Significant variation in epidermal bladder cell (EBC) density and salt tolerance (ST) exists amongst quinoa accessions, suggesting that salt sequestration in EBCs is not the only mechanism conferring ST in this halophyte. In order to reveal other traits that may operate in tandem with salt sequestration in EBCs and whether these additional tolerance mechanisms acted mainly at the root or shoot level, two quinoa (Chenopodium quinoa) accessions with contrasting ST and EBC densities (Q30, low ST with high EBC density versus Q68, with high ST and low EBC density) were studied. The results indicate that responses in roots, rather than in shoots, contributed to the greater ST in the accession with low EBC density. In particular, the tolerant accession had improved root plasma membrane integrity and K+ retention in the mature root zone in response to salt. Furthermore, superior ST in the tolerant Q68 was associated with faster and root-specific H2O2 accumulation and reactive oxygen species-induced K+ and Ca2+ fluxes in the root apex within 30 min after NaCl application. This was found to be associated with the constitutive up-regulation of the membrane-localized receptor kinases regulatory protein FERONIA in the tolerant accession. Taken together, this study shows that differential root signalling events upon salt exposure are essential for the halophytic quinoa; the failure to do this limits quinoa adaptation to salinity, independently of salt sequestration in EBCs.

Genome-wide Identification and Characterization of G-type lectin in Fragaria Vesca

Background: Lectins make up a large and diverse group of proteins in plants. G-type lectins are important type of lectins involved in plant development and defense process. However, studies about G-type lectins are limited to lectin receptor kinases.Results: In this study, genome-wide identification was carried out on G-type lectin gene family in Fragaria vesca. A total of 133 genes were found belonging to this family and they were classified into four groups: G-type lectin receptor kinases, G-type lectin kinases, G-type lectin receptor proteins and G-type lectin proteins, according to their domain organizations. Their chromosome localization, phylogenetic and evolutionary relationship were also analyzed. The results showed that tandem and dispersed duplication occurred frequently, which led to the expansion of G-type lectin gene family in F. vesca and may have increased the types of domain arrangement. The expression profile of G-type lectin genes at different developmental stages of F. vesca and under various biotic/abiotic stresses was inferred from the available databases. G-type lectin genes are actively expressed during F. vesca development and respond to multiple biotic/abiotic stresses. Additionally, to comprehend the functions of G-type lectins, we predicted strawberry genes that may co-express with these G-type lectin genes. Conclusions: G-type lectin gene family is a large gene family in F. vesca. Domain organization and expression analysis imply their functions under biotic/abiotic stresses.