Studies on Sporopollenin Biosynthesis: The Effect of Inhibitors of Carotenoid Biosynthesis on Sporopollenin Accumulation

Abstract

1. The unequivocal chemical composition of sporopollenin is unknown. Ir was often hypothesized that sporopollenin represents a derivative of carotenoids and/or carotenoid esters. Proceeding from this working hypothesis the influence of inhibitors of carotenoid biosynthesis (DPA, nicotine, Sandoz) on the sporopollenin accumulation was studied in order to test whether an intact carotenoid metabolism is involved. 2. Drastic changes in carotenoid spectra as well as in carotenoid patterns in pollen extracts were used as a “marker” for the uptake and transport of the inhibitors into the anther loculus, the site of sporopollenin biosynthesis. Of the three inhibitors of carotenoid metabolism, DPA, nicotine and Sandoz, only Sandoz appeared to influence carotenoid metabolism in the anthers. 3. After application of Sandoz the sporopollenin accumulation in anthers and pollen was only slightly affected. The amount of sporopollenin after treatment is similar to, or possibly even marginally higher than the control, if dry weight is chosen as reference. If the amount is expressed with respect to one pollen grain, the content is very slightly decreased in the case of pollen from treated plants. Therefore, it is concluded that severe interference in carotenoid biosynthesis does not result in a subsequent drastic inhibition of sporopollenin biosynthesis. 4. A significant increase of PAL activity under the influence of Sandoz indicates that effects upon phenylpropanoid metabolism are possible.

Biogenesis and function of the lipidic structures of pollen grains

“…Finally, labelling studies have shown that phenylalanine is efficiently incorporated into sporopollenin, hence demonstrating unequivocally that phenylpropanoids are genuine components of sporopollenin Wiermann 1992, 1993;. The deposition of the exine begins soon after the completion of meiosis II; it continues during the tetrad and ring-vacuolate stages of microspore development and is largely complete by the time of the first pollen mitosis (Prahl et al 1985;Wiermann and Gubatz 1992;Scott 1994). Exine formation starts with the synthesis of a cellulosic matrix, the primexine, which accumulates as a homogeneous layer between the plasma membrane and the callose wall of the microspore (Fitzgerald and Knox 1995).…”

Stamen Structure and Function

“…Early literature frequently cites carotenoids as the main constituents of sporopollenin (Shaw, 1971;reviewed by Scott, 1994). However, the demonstration that a potent inhibitor of carotenoid biosynthesis, norflurazon, failed to prevent sporopollenin biosynthesis in Cucurbita pepo (Prahl et al, 1985) began a reevaluation of sporopollenin composition. Subsequently, a large body of experimental evidence (reviewed by Scott, 1994) established that sporopollenin consists mainly of long-chain fatty acids and a minor component of phenolic compounds, perhaps p-coumaric acid.…”

The remarkable biology of pollen.

“…The extraordinary chemical resistance of sporopollenin was first reported more than 150 years ago (John, 1814), but its molecular structure is not yet understood. Until fairly recently, sporopollenin was thought to be a carotenoid polymer, but more recent inhibitor and physical data indicate that this is not the case (Prahl et al, 1985;Guilford et al, 1988). It is clear that sporopollenin is one of the most resistant biopolymers known; this property is presumably essential for the survival of the pollen grain free from the sporophyte prior to fertilization.…”