Sharon A. Kessler
Department of Microbiology and Plant Biology, The University of Oklahoma, Norman, OK 73019
Synergid cells have a unique role in plant development, their sole purpose is to attract and receive pollen tubes so that double fertilization can occur to produce viable seeds. In Arabidopsis thaliana nortia (nta) and feronia (fer) mutants, cell-to-cell communication at early stages of pollination is normal, but upon reaching the synergid, the pollen tubes continue to grow instead of bursting to release the sperm, leading to infertility. NTA encodes a member of the MILDEW RESISTANCE O (MLO) family and is localized to the endomembrane system of synergids before pollen tube arrival and redistributes to the filiform apparatus during pollen tube reception, while FER encodes a receptor-like kinase that is necessary for NTA recruitment to the filiform apparatus. The other 14 Arabidopsis MLO proteins share the same domain structure of NTA, with 7 transmembrane spans and a calmodulin-binding domain (CaMBD) in the C-terminal tail. The NTA CaMBD is necessary for its function in pollen tube reception; truncated NTA and NTA with a critical amino acid change in the CaMBD both fail to rescue the nta pollen tube reception phenotype. The ability of other MLO proteins to rescue the nta pollen reception phenotype when expressed in synergids seems to be linked to subcellular localization and distribution rather than amino acid conservation, indicating that the transport of MLO-containing vesicles is important for synergid communication with the pollen tube during pollen tube reception.