Pectin distribution pattern in the apertural intine of Euphorbia peplus L. (Euphorbiaceae) pollen

The apertural inner layer (intine) of Euphorbia L. pollen grains has a characteristic but original structure that has paired thickenings, one on either side of the colpus. To determine the nature and role of this intine layer, pollen grains of Euphorbia peplus L. were germinated in vivo and in vitro. The germination process involves wall changes that facilitate formation of the pollen tube and its subsequent growth. In the thickenings of the intine of E. peplus, the unesterified pectin epitopes are more densely localised in the inner part of the middle intine. No such epitopes are located in the intine portion adjacent to the plasma membrane (cellulosic endintine). Unesterified pectin epitopes are also localised in the outer part of the intine but are restricted to the centre of the aperture, around and in the pore. The de-esterification of pectins is very advanced at the time of dehiscence and pollen germination. The stratification of the aperture intine may take the following pathway at the time of germination: the thin outer zone of the intine in the pore region becomes disorganised and undergoes dissolution with liberation of unesterified and esterified pectins; the middle intine thickenings undergo an important elastic modification, but without liberation of unesterified pectins; the cellulosic inner intine is the progenitor of the pollen tube wall. This special intine of E. peplus is an adaptation to the hydration process preceding germination, increasing intine and pollen grain wall elasticity.