Pollen Tube Pathway in Chalazogamous Pistacia vera L.

Abstract

Chalazogamy, the penetration of the pollen tube into the ovule via the chalaza instead of following the normal path through the micropyle, has been described in a number of unrelated species. The causes determining this behavior are unknown. We examined the pollen tube path from pollination to fertilization in Pistacia vera L., a chalazogamous species. While pollen germination and pollen tube growth in the style follow a pattern very similar to that observed in most plant species, pollen tube penetration into the ovule occurs via the funiculus. This is mediated through the formation of a ponticulus, a funicular protuberance that physically connects ovule and style. Once in the ponticulus, the pollen tube reaches the core of the funiculus, and there it takes a 90⚬ turn in direction to the nucellus, where it enters via the chalaza and grows bordering the embryo sac to penetrate through a synergid. While no specialized cells can be observed in the ponticulus, the central core of the funiculus, where the pollen tubes grow, very much resembles a transmitting tissue with cells rich in starch reserves. At anthesis the megagametophyte is immature: on pollen tube arrival, a mature embryo sac is present. Chalazogamy occurs in a consistent way in pistachio, leading to a successful fertilization. This event is associated with an immature stage at anthesis of both the obturator and the ovule with underdeveloped integuments that only partially surround the nucellus. Pollen tube penetration is supported by the ponticulus, a structure that fulfills the role of an obturator, and by the funicular transmitting tissue that conducts the pollen tube to the nucellus, as does the micropyle. The fostering of these functions by other structures giving an alternative pathway may have been responsible for making fertilization possible in this species.