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

Abstract

Abscission of the stigmas of pearl millet (Pennisetunl typhoides (Burman) Stapf et Hubbard) after pollination results from the activation of a specialized zone of tissue near the base. This abscission zone comprises a well-defined stretch of the basal column made up of thin-walled cells lacking a cuticularized epidermis. The penetration of the first few pollen tubes following a compatible pollination results in loss of turgor throughout the tissue, followed by loss of adhesion between the cells and ultimately disruption of the whole zone. Pectolytic enzymes are released concomitantly, but the cellulosic skeletons of the cells persist. Numerous calcium oxalate druses, identified cytochemically and by energy-dispersive X-ray analysis, are present in the cells of the abscission zone; these undergo dissolution after activation of the zone, while simultaneously the druse population in the stigma stump and the upper ovary wall increases. The barrier formed by the degenerating abscission zone tissues excludes pollen tubes arriving after the initial activation and prevents the penetration of pathogens such as ergot and so has a prophylactic role. We suggest that the collapsed abscission zone forms not only a physical barrier but also a chemical one. Dissolution products of the numerous calcium oxalate druses during the breakdown of the tissue are likely to raise the calcium ion concentrations beyond the optimum for pollen-tube growth, thereby preventing further penetration. Inhibited tubes swell at the tips and may show growth disorientation before arrest, responses similar to those seen in pearl millet tubes in culture when exposed to higher than normal calcium ion concentration.Resum6 : L'abscission des stigmates du millet perlt (Penniseturn typhoides (Burman) Stapf et Hubbard), aprks la pollinisation, rtsulte de I'activation d'une zone sptcialiste du tissus situC a sa base. Cette zone d'abscission comporte une portion bien dCfinie de la colonne basale, constitute de cellules i paroi mince dCpourvues d'Cpiderme cuticularist.La penetration du premier grain de pollen, i la suite d'une pollinisation compatible, conduit i une perte de turgescence dans l'ensernble du tissus, suivi par une perte de cohtsion entre les cellules et tventuellement d'une dCsorganisation de I'ensemble de la zone. Des enzymes pectolytiques sont reldchtes i ce moment mais le squelette cellulosique des cellules persiste. De nombreux cristaux d'oxalate de calcium, identifiCs cytochimiquement et par I'tnergie dispersive des rayons X, sont prtsents dans les cellules de la zone d'abscission; ceux-ci subissent une dissolution aprks l'activation de la zone, alors que simultanCment la population de cristaux dans la base du stigmate et dans la partie suptrieure de l'ovaire augmente. La barrikre formte par les tissus de la zone d'abscission en dCgtntrescence exclut les tubes polliniques qui arrivent aprks l'activation initiale et prtvient la ptnttration par des organismes pathogknes tels que I'ergot, comportant ainsi des proprittts prophylactiques. Les auteur...

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

“…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 passage of the pollen tube ends after penetration of the micropyle and direct growth through the intercellular spaces of the subadjacent nucellus.…”

Aspects of the Cell Biology of Pollination and Wide Hybridization

“…This intracellular movement persists after dispersal and throughout the period of viability, with both the large inclusions such as the vegetative nucleus and sperms and the smaller bodies such as the P-particles moving within the cell (Heslop-Harrison et al 1997). Such grass pollen is short lived, and viability reduces greatly within an hour of anthesis.…”

“…Figs 11.1 and 11.2 illustrate one of the mechanisms which limits both the number of pollen tubes entering the ovule and prevents later entry of fungal hyphae. After pollination, an abscission zone comprising well-defined cells with thin walls near the base of the style, is activated by the growth of pollen tubes (Heslop-Harrison and Heslop-Harrison 1997). As the first tubes pass through the zone, the cells of the zone start to collapse and functionally occlude the transmitting tract within an hour.…”

Maize Tapetum Xylanase Is Synthesized as a Precursor, Processed and Activated by a Serine Protease, and Deposited on the Pollen

“…A major step in sexual reproduction in plants is the interaction between the male gamete-containing pollen and the pollen-receiving stigma in flowers (1)(2)(3)(4). This interaction manifests at the contact between the pollen coat and the surface structures of the stigma.…”