Pollen–Pistil Interaction
Pollination does not guarantee the transfer of the right type of pollen to the stigma. Often, pollen of the wrong type, either from another species or from the same plant in case of self-incompatibility, may also land on the stigma. The pistil has the ability to recognise whether the pollen is of the right type (compatible) or of the wrong type (incompatible). If the pollen is of the right type, the pistil accepts it and promotes the post-pollination events that lead to fertilisation. If the pollen is of the wrong type, the pistil rejects it by preventing pollen germination on the stigma or pollen tube growth through the style. The ability of the pistil to recognise pollen followed by its acceptance or rejection is the result of a continuous dialogue between the pollen grain and the pistil. This dialogue is mediated by chemical components of the pollen interacting with those of the pistil. It is only in recent years that botanists have been able to identify some of the pollen and pistil components and their interactions that lead to recognition, followed by acceptance or rejection.
Following compatible pollination, the pollen grain germinates on the stigma and produces a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. The pollen tube grows through the tissues of the stigma and style and reaches the ovary. In some plants, pollen grains are shed at the two-celled condition, consisting of a vegetative cell and a generative cell. In such plants, the generative cell divides during the growth of the pollen tube in the stigma and forms the two male gametes. In plants which shed pollen in the three-celled condition, the pollen tube carries the two male gametes from the beginning. The pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus. Recent studies have shown that the filiform apparatus present at the micropylar part of the synergids guides the entry of the pollen tube.
All these events—from pollen deposition on the stigma until the pollen tube enters the ovule—are together referred to as pollen–pistil interaction. As pointed out earlier, pollen–pistil interaction is a dynamic process involving pollen recognition followed by promotion or inhibition of pollen growth. The knowledge gained in this area helps plant breeders manipulate pollen–pistil interaction, even in incompatible pollinations, to obtain desired hybrids.
Pollen Germination
Pollen germination can be studied by dusting pollen from flowers such as pea, chickpea, Crotalaria, balsam and Vinca on a glass slide containing a drop of sugar solution of about 10 per cent. After about 15–30 minutes, the slide can be observed under a low-power microscope. Pollen tubes emerging from the pollen grains can be observed.
Artificial Hybridisation
A breeder is interested in crossing different species and often genera to combine desirable characters and produce commercially superior varieties. Artificial hybridisation is one of the major approaches of crop improvement programmes. In such crossing experiments, it is important to ensure that only the desired pollen grains are used for pollination and that the stigma is protected from contamination by unwanted pollen. This is achieved by emasculation and bagging techniques. If the female parent bears bisexual flowers, removal of anthers from the flower bud before the anther dehisces is necessary. This is done using a pair of forceps and the process is called emasculation.
Emasculated flowers are then covered with a bag of suitable size, generally made of butter paper, to prevent contamination of the stigma with unwanted pollen. This process is called bagging. When the stigma of the bagged flower becomes receptive, mature pollen grains collected from the anthers of the male parent are dusted on the stigma. The flowers are then rebagged and the fruits are allowed to develop. If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower is rebagged.
Double Fertilisation
After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus, thus completing syngamy. This results in the formation of a diploid cell, the zygote. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce the triploid primary endosperm nucleus (PEN).
Since this involves the fusion of three haploid nuclei, it is termed triple fusion. Thus, two types of fusions, syngamy and triple fusion, take place in an embryo sac. The phenomenon is therefore termed double fertilisation, an event unique to flowering plants. The central cell, after triple fusion, becomes the primary endosperm cell (PEC). The primary endosperm cell develops into the endosperm, while the zygote develops into an embryo.