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Platanus acerifolia (Aiton) Willdenow is a plane tree, widely grown as an ornamental tree in many cities of the United States and Western Europe, which has become an important source of airborne allergens in our cities. The aim of the present study is to immunolocalize the major allergens in the pollen grain and to examine their potential function in the fertilization process. Observations were made in mature and hydrated, activated pollen of P. acerifolia for 5, 15, 30 min and 2 h in the germination medium. Specimens were fixed using freezing protocols for transmission electron microscopy (TEM). For immunogold labelling, cryosections and resin-embedded ultrathin sections were incubated using rabbit antisera against the purified pollen allergens Pla a 1 and Pla a 2. Elution of P. acerifolia allergens took place after 5 min of pollen incubation in buffered medium. Intense labelling of Pla a 1 and Pla a 2 was detected after pollen exudates were released. In pollen grains, Pla a 1 was predominantly localized in concentric cisternae of the endoplasmic reticulum (ER), situated between the vegetative nucleus and the generative cell, and was released from pollen grains 5 min after hydration; cytoplasmic localization decreased 15 min after hydration. In pollen grains, glycoprotein Pla a 2 was abundant in association with Golgi cisternae and vesicles situated in the apertural periphery of the mature pollen grains. Pla a 2 proteins were also detected in ER and in the generative cell wall. Immunolabelling of Pla a 2 decreased 5 min after pollen hydration but was again intense after 15-30 min in germination medium, presumably as a consequence of renewed expression and glycosylation of this protein. Pla a 1 belongs to a new class of allergens related to proteinaceous invertase and pectin methyl esterase inhibitors (PII, PMEI) which could be involved in membrane protection and pectin de-esterification control during pollen hydration. Pla a 2 has an exopolygalacturonase (PG) enzymatic activity consistent with pollen-stigma adhesion mechanisms or compatibility systems. Moreover, the expression of Pla a 2 found 15-30 min after hydration might contribute to pollentube growth and the modification of transmitting tissue cell walls. The abundant production and elution of Pla a 1 and Pla a 2 proteins may alter the environment in which pollen tube elongation occurs, thus promoting a potential crosstalk between the pollen and the gynoecium.

The composition of the inner layer (intine) of mature, activated, and germinated Zygophyllum fabago L. (Zygophyllaceae) pollen grains was studied. Cytochemical techniques showed neutral and acidic polysaccharides to be the major component of the thin and unlayered intine. The intine lacks lipids, with only scattered lipid globules being observed near the plasma membrane. Immunocytochemical localization of esterified and unesterified pectins in the intine was performed to determine the behaviour (permeability and elasticity) of germinal apertures. The high density of unesterified pectins in the intine of Z. fabago may be related to harmomegathic changes, which increase the elasticity of the intine during hydration and germination processes. A new layer was deposited in germinated pollen grains, recognized by 1,3-β-glucan (callose) antibodies; this layer plays a role in keeping the grains swollen during the germination process and probably forms a selective barrier to control the movement of substances through the pollen walls. Indeed, the composition of the Z. fabago intine was related to both the hydration process preceding germination and the passage of allergenic proteins through it.Key words: callose, germination, intine, pectins, pollen grains, Zygophyllum fabago.