POD1 Regulates Pollen Tube Guidance in Response to Micropylar Female Signaling and Acts in Early Embryo Patterning inArabidopsis

The pollen tube germinates from pollen and, during its migration, it perceives and responds to guidance cues from maternal tissue and from the female gametophyte. The putative female cues have recently been identified, but how the pollen tube responds to these signals remains to be unveiled. In a genetic screen for male determinants of the pollen tube response, we identified the pollen defective in guidance1 (pod1) mutant, in which the pollen tubes fail to target the female gametophyte. POD1 encodes a conserved protein of unknown function and is essential for positioning and orienting the cell division plane during early embryo development. Here, we demonstrate that POD1 is an endoplasmic reticulum (ER) luminal protein involved in ER protein retention. Further analysis shows that POD1 interacts with the Ca(2+) binding ER chaperone CALRETICULIN3 (CRT3), a protein in charge of folding of membrane receptors. We propose that POD1 modulates the activity of CRT3 or other ER resident factors to control the folding of proteins, such as membrane proteins in the ER. By this mechanism, POD1 may regulate the pollen tube response to signals from the female tissues during pollen tube guidance and early embryo patterning in Arabidopsis thaliana.