1.A.130.  The Mildew-resistance Locus O (MLO) Family

Precise signalling between pollen tubes and synergid cells in the ovule initiates fertilization in flowering plants (Johnson et al. 2019). Contact of the pollen tube with the ovule triggers calcium spiking in the synergids(Ngo et al. 2014) that induces pollen tube rupture and sperm release. This process, termed pollen tube reception, entails the action of three synergid-expressed proteins in Arabidopsis: FERONIA (FER), a receptor-like kinase; LORELEI (LRE), a glycosylphosphatidylinositol-anchored protein; and NORTIA (NTA), a transmembrane protein (Escobar-Restrepo et al. 2007; Liu et al. 2016). Genetic analyses have placed these three proteins in the same pathway. Gao et al. 2022 identified two pollen-tube-derived small peptides that belong to the rapid alkalinization factor (RALF) family (Blackburn et al. 2020) as ligands for the FER-LRE co-receptor, which in turn recruits NTA to the plasma membrane. NTA functions as a calmodulin-gated calcium channel required for calcium spiking in the synergid. Gao et al. 2022 also reconstituted the biochemical pathway by which FER-LRE perceives pollen-tube-derived peptides to activate the NTA calcium channel and initiate calcium spiking, a second messenger for pollen tube reception. The FER-LRE-NTA trio therefore forms a previously unanticipated receptor-channel complex in the female cell to recognize male signals and trigger the fertilization process (Gao et al. 2022). Several members of the MLO family have been shown to transport Ca²⁺. These include: AtMLO2, AtMLO3, AtMLO4, AtMLO10, AtMLO12, HvMLO, PpMLO2 and PpMLO3.

Mildew resistance Locus O (MLO) proteins are polytopic integral membrane proteins that are largely plant-specific although distant homologs are found in the Sar kingdom ((see TC subfamily 1.A.130.2 and primarily concerned with plant-powdery mildew interactions. These proteins were previously in TC family 9.B.197 before the function as Ca²⁺ channels was determined. They have been reported to have a conserved topology with seven transmembrane domains and an intrinsically unstructured C-terminus. They also have a calmodulin-binding motif. MLO proteins diverged into a family with several clades whose members are associated with different physiological processes. A dataset of MLO amino acyl sequences, comprising nearly all major land plant lineages, has been compiled (Kusch et al. 2016). Seven phylogenetic clades and several MLO peptide motifs that are either conserved in all MLO proteins or confined to one or several clades were identified. Thus, clade-specific diversification of MLO functions is associated with particular sequence motifs. In baker's yeast, some of these motifs are functionally linked to transmembrane transport of organic molecules and ions.  MLO-like proteins with highly diverse membrane topologies are present in green and red algae (Rhodophyta), Amoebozoa and Chromalveolata, as well as plants. Putative fusion events between MLO proteins and different kinds of proteins suggest specific MLO functions (Kusch et al. 2016).

MLO5 and MLO9 selectively recruit the Ca²⁺ channel CNGC18-containing vesicles to the plasma membrane through the R-SNARE proteins, VAMP721 and VAMP722 in trans mode. Meng et al. 2020 identified members of the conserved 7 TMS MLO family (expressed in the pollen tube) as tethering factors for Ca²⁺ channels, revealing a mechanism of molecular integration of extracellular ovular cues and selective exocytosis. This work sheds light on the general regulation of MLO proteins in cell responses to environmental stimuli (Meng et al. 2020 identified members of the conserved 7 TMS MLO family (expressed in the pollen tube) as tethering factors for Ca²⁺ channels, revealing a mechanism of molecular integration of extracellular ovular cues and selective exocytosis. This work sheds light on the general regulation of MLO proteins in cell responses to environmental stimuli (Meng et al. 2020).

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