The Pollen-Stigma Interaction in the Grasses. 7. Pollen-Tube Guidance and the Regulation of Tube Number in Zea Mays L.

Abstract

SUMMARY The stigma of Zea mays can support the hydration and germination of considerable numbers of pollen grains, yet, in general, only one tube enters the micropyle to effect fertilisation. The numbers are reduced at various points in the pollen tube pathway, (1) by competition on the receptive trichomes and in the transmitting tracts, (2) by elimination of late‐entering tubes at the stigma abscission zone, (3) at a constricted zone of the transmitting tracts in the upper ovary wall, and (4) in the vicinity of the micropyle. The control imposed at the abscission zone is higly effective. The entry of the first few tubes into the ovary wall induces a loss of turgidity of the cells of the zone and a disruption of the vasculation, and thereafter further tubes are irrevocably blocked in the main body of the stigma. The device maximises the period available for pollen capture and yet provides for the strict regulation of the number of tubes ultimately entering the ovary once pollination has taken place. Pollen tubes enter the ovary cavity by breaking through the inner epidermis of the ovary wall, which, like the contiguous inner integument, bears a thin discontinuous cuticle. Further passage is between the inner epidermis and the integument. Growth is irregular, but not wholly random, the tubes orienting themselves in the general direction of the micropyle following lines demarcated by the elongated cells of the epidermis and the integument. Almost throughout the pollen tube pathway, the cells in the vicinity show characteristics suggestive of a secretory function. The significance of this for pollen tube nutrition and guidance is briefly discussed.

The structure and prophylactic role of the angiosperm embryo sac and its associated tissues:Zea mays as a model

“…At no point do they transgress the limits set by these walls (Y. Heslop-Harrison et al 1985a), indicating that the pollen tube tip does not secrete cellulase. Following the passage of a limited number of pollen tubes through the basal region of the silk, an abscission zone develops within hours, preventing the ingress of pathogens through the tissues of the silk Heslop-Harrison 1984, Y. Heslop-Harrison et al 1985a; similar events occur in other cereals; Willingale andMantle 1985, Y. HeslopHarrison andHeslop-Harrison 1997).…”

The pollen tube activated abscission zone in the stigma of pearl millet: structural and physiological aspects

“…The-present results show-that in pearl millet, as in maize (Heslop-Harrison et al 1985), the AZ of the stigma acts as one effective filter in achieving this, ensuring that only a few tubes actually enter the ovary whatever the original amount of pollination. A second potential function of the AZ is as a barrier preventing miscegenation, a function dramatically illustrated in pearl millet by the almost total blockage of sorghum tubes entering the zone, even when they have successfully traversed the distal parts of the stigma (HeslopHarrison 1982).…”

“…As in maize (Heslop-Harrison and Heslop-Harrison 1984;Heslop-Harrison et al 1985), postpollination abscission of the stigma of pearl millet is brought about by the disruption of an activatable zone near the base. The cells of this abscission zone (AZ) lose turgidity when pollen tubes enter, resulting in collapse of the tissues and occlusion of the transmitting tissues, so preventing the passage of further tubes and ultimately forming a seal at the top of the ovary.…”

From pollination to fertilization in fruit trees

“…This mechanism, apart from having a role in pollen tube growth control, may play a significant part in preventing infection. A different mechanism relating to the control of pollen tube entrance into the ovary has been described in Zea mays [21] in which the first pollen tubes entering the ovary induce a loss of turgidity in the cells at the base of the style which prevents other pollen tubes from entering this region.…”