Phenylpropanoid Derivatives Are Essential Components of Sporopollenin in Vascular Plants

Abstract The outer wall of pollen and spores, namely the exine, is composed of sporopollenin which is highly resistant to chemical reagents and enzymes. Here, we demonstrate that phenylpropanoid pathway derivatives are essential components of sporopollenin in seed plants. Spectral analyses show that the autofluorescence of Lilium and Arabidopsis sporopollenin is similar to that of lignin. Thioacidolysis and NMR analyses of pollen from Lilium and Cryptomeria show that the sporopollenin of seed plants contains phenylpropanoid derivatives, including p-hydroxybenzoate (p-BA), p-coumarate (p-CA), ferulate (FA) and lignin guaiacyl (G) units. Signals from the phenylpropanoid pathway genes fused to VENUS fluorescent proteins demonstrate that the phenylpropanoid pathway is expressed in the tapetum in Arabidopsis, which is consistent with the fact that the sporopollenin precursor originates from the tapetum. Further germination and comet assays show that this pathway plays an important role in pollen protection against UV radiation. In the pteridophyte plants species Ophioglossum and Lycopodium, phenylpropanoid derivatives including p-BA and p-CA were also detected, but G units were not. Together, our results indicate that phenylpropanoid derivatives are essential for sporopollenin synthesis in vascular plants. In addition, sporopollenin autofluorescence spectra of bryophytes such as Physcomitrella and Haplocladium exhibit distinct characteristics compared with those of vascular plants, indicating diversity of sporopollenin among land plants.