Dr Barend HJ de Graaf
Pollen Pistil Communication and Membrane Trafficking in Plants
The angiosperms (flowering plants) include the most diverse and largest phylum in the plant kingdom with more than 250.000 different species. During evolution, over about 130 million years, the angiosperms developed an enormous variety of flowers. Each species generated a flowering program with its own specific flower morphology, sexual reproduction mechanisms, and protection against pathogens. For a large number of flowering plants, the reproduction process depends on specific interactions with certain members of the animal kingdom, such as pollinators to actuate pollen and predators for seed dispersal. These sometimes highly specialized plant-animal relations with respect to the pollination process and seed dispersal have to be co-evolved (Darwin, 1862).
Approximately half of the angiosperm families include species exhibiting one of several forms of self incompatibility whereas the other half is self-compatible. The terms self-compatible (SC) and self-incompatible (SI) represent the property of flowers to respectively accept or specifically reject self pollen, i.e., pollen from plants with the same genotype. Moreover, a large number of both, SC and SI, plant species invest much energy in the development of colourful and odorous flowers with nectar to attract and reward pollinators. SI and the formation of conspicuous flowers are typical examples of means to prevent inbreeding and promote outbreeding. In addition, SI may have an additional function: the protection of SI-species against pollen of closely related SC-species. Beside SI, both SI and SC-species must have additional 'recognition' and 'rejection' mechanisms playing a role during interspecific and intergeneric crossings. Ultimately in all angiosperms, independent of the presence of active pollination and fertilization barriers, flowering must result in successful compatible interactions to guarantee the next generation of seeds.
Adhesion of pollen to the stigma is considered as one of the first interactions in pollen-pistil recognition. Directly after pollen adhesion, pollen and pistil factors can be exchanged and may trigger responses in both pollen and/or pistils. These responses can have a promoting or inhibitory effect towards pollen tube germination and pistil sustainability and, in turn, create an optimal environment for species specific pollen-pistil interactions. How pollen and pollen tubes communicate with pistil tissues and/or vice versa during the progamic phase of compatible pollen-pistil interactions is largely unknown.
Pollen tube growth is achieved by tip growth. Pollen tube extension is a highly polarized process and restricted to the tube tip where secretory vesicles fuse with the tip membrane (A). Part of this membrane is retrieved by clathrin mediated endocytosis which occurs just behind the growing tip (B). Currently, the role of endocytosis in pollen tube growth, mainly based on in vitro pollen tube tip growth studies, is believed to be the retrieval of excess of pollen tube membrane and the recycling of various protein fractions from the growing tip to maintain pollen tube architecture and the progression of tip growth.
Proper regulated membrane trafficking between the pollen tube membrane and the different pollen tube compartments, such as the endoplasmic reticulum (ER), golgi, endosomes and vacuole(s), is crucially important for the rapid tip growth but also for regulating the direction of pollen tube growth.
I am particularly interested in the role of pollen tube membrane trafficking, i.e., endocytosis, in the communication between pollen and pistils after compatible (own) and incompatible (foreign) pollinations.
A) Secretory vesicles originate from the trans golgi network (TGN) and are transported over the actin cytoskeleton into the growing tip where they fuse with the tip membrane. Besides pollen tube wall proteins, these transport vesicles contain mainly methyl-esterified pectins as part of the primary tube tip wall.
B) Just behind the growing tip a zone rich in clathrin coated pits is involved in the endocytic activity of growing pollen tubes which is necessary for the retrieval of excess of tube tip membrane and the recycling of pollen tube (membrane) proteins from the growing tip.