Asymmetric Evolution of Protein Domains in the Leucine-Rich Repeat Receptor-Like Kinase Family of Plant Signaling Proteins

The coding sequences of developmental genes are expected to be deeply conserved, with cis-regulatory change driving the modulation of gene function. In contrast, proteins with roles in defense are expected to evolve rapidly, in molecular arms races with pathogens. However, some gene families include both developmental and defense genes. In these families, does the tempo and mode of evolution differ between genes with divergent functions, despite shared ancestry and structure? The leucine-rich repeat receptor-like kinase (LRR-RLKs) protein family includes members with roles in plant development and defense, thus providing an ideal system for answering this question. LRR-RLKs are receptors that traverse plasma membranes. LRR domains bind extracellular ligands; RLK domains initiate intracellular signaling cascades in response to ligand binding. In LRR-RLKs with roles in defense, LRR domains evolve faster than RLK domains. To determine whether this asymmetry extends to LRR-RLKs that function primarily in development, we assessed evolutionary rates and tested for selection acting on 11 subfamilies of LRR-RLKs, using deeply sampled protein trees. To assess functional evolution, we performed heterologous complementation assays in Arabidopsis thaliana (Arabidopsis). We found that the LRR domains of all tested LRR-RLK proteins evolved faster than their cognate RLK domains. All tested subfamilies of LRR-RLKs had strikingly similar patterns of molecular evolution, despite divergent functions. Heterologous transformation experiments revealed that multiple mechanisms likely contribute to the evolution of LRR-RLK function, including escape from adaptive conflict. Our results indicate specific and distinct evolutionary pressures acting on LRR versus RLK domains, despite diverse organismal roles for LRR-RLK proteins.

CLE-CLAVATA1 Signaling Pathway Modulates Lateral Root Development under Sulfur Deficiency

Plant root system architecture changes drastically in response to availability of macronutrients in the soil environment. Despite the importance of root sulfur (S) uptake in plant growth and reproduction, molecular mechanisms underlying root development in response to S availability have not been fully characterized. We report here on the signaling module composed of the CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (CLE) peptide and CLAVATA1 (CLV1) leucine-rich repeat receptor kinase, which regulate lateral root (LR) development in Arabidopsis thaliana upon changes in S availability. The wild-type seedlings exposed to prolonged S deficiency showed a phenotype with low LR density, which was restored upon sulfate supply. In contrast, the clv1 mutant showed a higher daily increase rate of LR density relative to the wild-type under prolonged S deficiency, which was diminished to the wild-type level upon sulfate supply, suggesting that CLV1 directs a signal to inhibit LR development under S-deficient conditions. CLE2 and CLE3 transcript levels decreased under S deficiency and through CLV1-mediated feedback regulations, suggesting the levels of CLE peptide signals are adjusted during the course of LR development. This study demonstrates a fine-tuned mechanism for LR development coordinately regulated by CLE-CLV1 signaling and in response to changes in S availability.

Synergistic action of GCN5 and CLAVATA1 in the regulation of gynoecium development in Arabidopsis thaliana

In Arabidopsis thaliana the CLAVATA1 (CLV1) receptor and GENERAL CONTROL NON DEREPRESSIBLE 5 (GCN5) histone acetyltransferase both regulate inflorescence meristem size and affect the expression of the meristem-promoting transcription factor WUSCHEL (WUS). Single and multiple mutants of GCN5 and CLAVATA members, were analysed for their gynoecium development, using morphological, physiological, genetic and molecular approaches. The clv1-1gcn5-1 double mutants exhibited novel phenotypes including elongated gynoecia with reduced valves and enlarged stigma and style, indicating a synergistic action of CLAVATA signaling and GCN5 action in the development of the gynoecium. Reporter line and gene expression analysis showed that clv1-1gcn5-1 plants have altered auxin and cytokinin response, distribution and ectopic overexpression of WUS. WUS expression was found in the style of wild-type gynoecia stage 10-13, suggesting a possible novel role for WUS in the development of the style. CLV1 and GCN5 are regulators of apical-basal and mediolateral polarity of the Arabidopsis gynoecium. They affect gynoecium morphogenesis through the negative regulation of auxin biosynthesis and promotion of polar auxin transport. They also promote cytokinin signaling in the carpel margin meristem and negatively regulate it at the stigma. Finally, they synergistically suppress WUS at the centre of the gynoecium.