In order to produce seeds, flowering plants must successfully unite immotile sperm with female gametes. To accomplish this, sperm are delivered inside the pollen grain which germinates a tube into floral tissue and shuttles its cargo toward the female gametes. One mechanism by which plants prevent interspecific pollen from delivering foreign sperm is rejection of foreign pollen tube suitors by synergid cells, which act as gate-keepers for access to female gametes. To investigate which female genes are responsible for scrutinizing the genetic identity of pollen tubes, we took advantage of the observation that Ca ++ oscillations in the synergid cells are a marker for successful same-species interactions. We find that interspecific pollen tubes elicit perturbed Ca ++ oscillations in Arabidopsis thaliana synergids, suggesting that genetic identity is communicated after the pollen tube signals its arrival. To investigate which male genes define the genetic fingerprint of the pollen tube, we performed crosses using mutant pollen lacking three MYB transcription factors. We find that myb mutant pollen tubes elicit perturbed Ca ++ oscillations in synergids similar to those induced by interspecific pollen, suggesting that the MYBs define the pollen tube’s genetic identity. To identify candidate genes from a large group MYB-dependent genes, we use both traditional and CRISPR-mediated mutagenesis approaches. We find that re-expression of a single MYB-dependent gene in the myb background is sufficient to partially restore fertility to the myb mutant. Our future goals are to define the molecules that are sufficient to approve or reject interspecific pollen tubes by the synergid cells, and investigate the effect of MYB-target genes on synergid cell responses.
Coauthors: Mark Johnson, Associate Professor – Brown University