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In this article we will discuss about the role of proteins in pollen grains.
T. Singer and Petrovskya – Baranova (1961) were the first to show the existence of proteins with enzymatic properties in certain layers of the pollen walls of Paeonia and Amaryllis. Subsequently, Makinen and McDonald (1968) demonstrated the pollen enzymes and isoenzymes in flowering plants.
The details of the pollen wall proteins and their significance are highlighted through the works of J. Heslop- Harrison and others.
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Certain proportion of the total protein of the pollen grain occurred in the wall in three sites:
i. Many proteins are situa ted most prominently in the cellulosic intine (endintine) specially at the apertural part in the form of radially arranged tubules or tangentially oriented leaflets.
ii. Exine proteins are present in the cavities between the baculae of the tectate grains or in the surface depressions of the intectate or subtectate grains.
iii. Some proteins are located in the superficial materials (e.g. pollen kitts) der. ed from tapetum.
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The major portions of these proteins are enzymes, mostly hydrolytic enzymes and the wall proteins are quickly lost when the pollen grain is moistened. All the six classes of enzymes namely, dehydrogenases, oxidases, transferases, hydrolases, lyases and ligases are reported to be present and remain active in pollen grains of higher plasts (Table 3.6).
Esterases are the predominant enzymes of the exine layer, while acid phosphatases are the chief enzymes of the intine. Therefore, in general esterases and acid phosphatases can be used as marker proteins for exine and intine layers respectively.
Some exceptions are there, for example Helianthus pollen contains esterases and acid phosphatases in both exine and intine. In the mature, dormant pollen the hydrolase activity is found to be associated with the wall, with very little in the protoplast of the vegetative cell.
In some species with exine absent or highly reduced, the wall protein comprising of acid phosphatase activity are exclusively confined to intine. These proteins are surrounded by a layer of mucilage with glycoprotein fractions originate from tapetum which help to prevent the release of wall protein into water.
In inaperturate pollen of Crocus vernus, with a thick intine and relatively thin exine, the Site of esterase and acid phosphatase activity is found in the central zone of the intine over the whole wall. A very low activity of such enzymes are also observed within the plasmalemma and in the superficial pollen-kitt.
In monoporate grains (e.g., grass pollen) hydrolase activity is confined to the intine underlying the pore. In pantoporate grains (like Malvaceae, Convol- vulaceae) such activity is mainly confined in the intine at each pore, though a slight activity is also observed in interporal intine. Sometimes, intense esterase activity is observed in pollenkitt of such grains (e.g., Cobaea).
In monosulcate (Liliaceae, Amaryllidaceae) or triporate (Urticaceae, Ulmaceae, Juglandaceae) or tricolpate (Lamiaceae, Brassicaceae) pollen grains, intine is the site of hydrolase activity, but much less activity is noticed in such grains having thin intine. In all cases such activity is concentrated in the intine of apertural zone.
Among gymnosperm (Pinus, Abies, Cryptomeria) acid phosphatase and RNase activity is confined to intine especially in the apertural region. Among pteridophytes only Equisetum palustre shows enzymatic activity in the entire intine. But spores of several ferns and six species of Bryophytes examined showed no detectable wall enzyme.
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It is evident from the isoenzymic analysis that zymograms of pollen are rarely identical to those of tapetal and pistillate tissue. Pollen grains exhibit few of the isoenzymes that are common to seed or vegetative tissue, but the zymograms of dehydrogenase, peroxidase and esterases have revealed bands apparently unique to pollen grain supporting unique physiological properties of pollen.
Pollen grains are normally devoid of catechol oxidases (Polyphenol oxidase) and the enzymes associated with plastid and plant pigments (Though many pollen grains are pigmented).
Other enzymes namely, maltase, lipase, [3- glucoronidase, arylsulphatase, p-diphenyl oxidase (laccase), several pyrophosphorylases and zymase have not yet been reported from pollen. The activity of some enzymes like alkaline phosphatase, ATPase, DNA synthetase and β-1,3-gluconase are reported to be very low or absent in pollen grains.
Pollen enzymes are freely and rapidly diffusible from pollen grains. Enzymatic activity for many enzymes like amylase, P-fructofurano- sidase, phosphorylase, transaminase is increased during germination.
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It is noticed that most of the pollen enzymes are synthesized subsequently to meiosis, under control of the haploid genome. Pollen slowly looses certain types (peroxidases, phosphatases) of enzymatic activity during storage but these cannot account for the rapid loss of viability.
i. Formation of intine proteins:
After the release of monad grain from tetrad, the plasmalemma of pollen cytoplasm puts out radially oriented tubules with their protein addition into the developing intine (Fig.3.1). Subsequently these tubules disconnect from plasmalemma and are sealed off from the cell surface with the deposition of a further layer of intine free from the tubules.
In inaperturate grains proteins are dispersed throughout the intine, while in aperturate grains intine proteins are mostly condensed in the apertural region. In some taxa (e.g. Cosmos bipinnatus) in place of tubular invagination, leaflets of plasmalemma with their protein inclusions get incorporated into the intine as a series of tangential lamellae after parting from the cytoplasm. Intine proteins are the product of pollen cytoplasm, thus gametophytic in origin.
ii. Formation of exine proteins:
Exine proteins originate from the sporophytic tapetum tissue. During the meiotic division of pollen mother cell, proteins in single membrane-bound vesicles derived from rough ER and lipids resultant from plastids get accumulated in tapetal cells (Fig.3.1). At the end of pollen development these proteins and lipids are free into thecal cavity after breaking down of tapetal cells.
These proteins are deposited in the surface depression of exine in intectate or sub-tectate grains, but in tectate grains the protein fraction passes through the micropores of the tectum and finally deposited in the spaces between the baculae. In all the pollen types lipids remain in the surface of the exine. Exine proteins are the product of tapetal cells, hence they are sporophytic in origin.
