The immune MEKK1-MKK1/2-MPK4 signaling cascade prevents invasive pollen tube growth in young seeds

Pollen tubes (PT) deliver the immotile sperm cells to the female gametes thereby enabling double fertilization. In Arabidopsis, several factors have been reported that restrict the entrance of a single PT in the female gametophyte. These factors are located in or are associated with the female gametophyte. However, sporophytic-originating signaling cascades, that govern PT reception and entrance in the female gametophyte, remained largely elusive. Disruption of the MEKK1-MKK1/2-MPK4 cascade by pathogenic effectors activates the resistance protein SUMM2-mediated immunity. The knockout of SUMM2 in the mekk1 and mkk1/mkk2 mutant reduces their strong autoimmunity and enables flowering and pollen tube attraction. Here, we present evidence that the plant immune-associated MEKK1-MKK1/2-MPK4 signaling cascade prevents supernumerary PT invasion in the young seed. We found invasive PT growth after fertilization in mekk1/summ2 and mkk1/2/summ2 accompanied by a reduced seed formation. Deep inspection of the mpk4 single and mpk4/summ2 double mutant further revealed a SUMM2-independent invasive PT growth phenotype. The allele-transmission analysis of mpk4 indicates a sporophytic origin of this observed phenomenon. This outcome highlights a function of the defense-associated MEKK1-MKK1/2-MPK4 signaling cascade in the prevention of invasive PT. Our results demonstrate that the sporophytic control of PT reception and entrance is adapted from defense mechanisms.