Arabidopsis leucine-rich repeat malectin receptor-like kinases regulate pollen-stigma interactions.

Flowering plants contain tightly controlled pollen-pistil interactions required for promoting intraspecific fertilization and preventing interspecific hybridizations. In Arabidopsis (Arabidopsis thaliana), several receptor kinases (RKs) are known to regulate the later stages of intraspecific pollen tube growth and ovular reception in the pistil, but less is known about RK regulation of the earlier stages. The Arabidopsis RECEPTOR-LIKE KINASE IN FLOWERS1 (RKF1)/RKF1-LIKE (RKFL) 1-3 cluster of 4 leucine-rich repeat malectin (LRR-MAL) RKs was previously found to function in the stigma to promote intraspecific pollen hydration. In this study, we tested additional combinations of up to 7 Arabidopsis LRR-MAL RK knockout mutants, including RKF1, RKFL1-3, LysM RLK1-INTERACTING KINASE1, REMORIN-INTERACTING RECEPTOR1, and NEMATODE-INDUCED LRR-RLK2. These LRR-MAL RKs were discovered to function in the female stigma to support intraspecific Arabidopsis pollen tube growth and to establish a prezygotic interspecific barrier against Capsella rubella pollen. Thus, this study uncovered additional biological functions for this poorly understood group of RKs in regulating the early stages of Arabidopsis sexual reproduction.

Finding new Arabidopsis receptor kinases that regulate compatible pollen-pistil interactions.

Successful fertilization of a flowering plant requires tightly controlled cell-to-cell communication between the male pollen grain and the female pistil. Throughout Arabidopsis pollen-pistil interactions, ligand-receptor kinase signaling is utilized to mediate various checkpoints to promote compatible interactions. In Arabidopsis, the later stages of pollen tube growth, ovular guidance and reception in the pistil have been intensively studied, and thus the receptor kinases and the respective ligands in these stages are quite well understood. However, the components of the earlier stages, responsible for recognizing compatible pollen grains and pollen tubes in the upper reproductive tract are less clear. Recently, predicted receptor kinases have been implicated in the initial stages of regulating pollen hydration and supporting pollen tube growth through the upper regions of the reproductive tract in the pistil. The discovery of these additional signaling proteins at the earlier stages of pollen-pistil interactions has further elucidated the mechanisms that Arabidopsis employs to support compatible pollen. Despite these advances, many questions remain regarding their specific functions. Here, we review the roles of the different receptor kinases, integrate their proposed functions into a model covering all stages of pollen-pistil interactions, and discuss what remains elusive with regard to their functions, respective binding partners and signaling pathways.