Arabidopsis annexin 5 is involved in maintenance of pollen membrane integrity and permeability.

In Arabidopsis, a dry stigma surface enables a gradual hydration of pollen grains by a controlled release of water. Occasionally the grains may be exposed to extreme precipitations that cause rapid water influx, swelling and eventually lead to pollen membrane (PM) rupture. In metazoans, calcium- and phospholipids-binding proteins, referred to as annexins participate in repair of the plasma membrane damages. It remains unclear, however, how this process is conducted in plants. Here, we examined whether the plant annexin 5 (ANN5), the most abundant member of the annexin family in pollen, is involved in the restoration of PM integrity. We analyzed a cellular dynamics of ANN5 in the pollen grains undergoing hydration under favorable or stress conditions. We observed a transient ANN5 association to PM during the in vitro hydration that did not occur in the pollen grains being hydrated on the stigma. To simulate a rainfall, we performed spraying of the pollinated stigma with deionized water that induced ANN5 accumulation at PM. Interestingly, calcium or magnesium application differentially affected PM properties causing rupture or shrinkage of PM, respectively. Both treatments, however, induced ANN5 recruitment to PM. Our data suggest a model, in which ANN5 is involved in the maintenance of membrane integrity in pollen grains exposed to osmotic or ionic imbalances.