The ponticulus: a structure bridging pollen tube access to the ovule in Pistacia vera

Abstract

The path of the pollen tube has been examined in pistachio (Pistacia vera), a chalazogamous species where the pollen tube penetrate the ovule via the chalaza. Special attention was paid to the way the pollen tube gains access to the ovule. A single anatropous ovule with a big funiculus occupies the entire ovary cavity. At anthesis, a physical gap exists between the ovule and the base of the style. However, upon pollen tube arrival a protuberance, the ponticulus, develops in the uppermost area of the funiculus between the style and ovule. This structure appears to facilitate access to the ovule by the pollen tube. The pollen tube penetrates the ovule via this ponticulus. Upon penetration, callose develops in the ponticulus cells surrounding the pollen tube. After pollen tube passage, the upper layer of the ponticulus lignifies and isolates the ovule from the style. This separation is further enlarged 2 weeks later when the ovary starts to develop without expansion of the ovule and a large gap develops separating the ovule from the style. Except for the induction of callose formation by the pollen tube in the funiculus, this process is independent of pollination and appears to be developmentally regulated since it occurs in the same way and at the same time in pollinated and unpollinated flowers. The ponticulus, although by a different mechanism, appears to be playing the role of an obturator regulating access of the pollen tube to the ovule. Furthermore, this access is restricted to a particular time during the development of the ovule.