Phenylpropanoid Derivatives Are Essential Components of Sporopollenin in Vascular Plants.

The outer wall of pollen and spores, namely the exine, is composed of sporopollenin, which is highly resistant to chemical reagents and enzymes. In this study, we demonstrated that phenylpropanoid pathway derivatives are essential components of sporopollenin in seed plants. Spectral analyses showed that the autofluorescence of Lilium and Arabidopsis sporopollenin is similar to that of lignin. Thioacidolysis and NMR analyses of pollen from Lilium and Cryptomeria further revealed 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. The phenylpropanoid pathway is expressed in the tapetum in Arabidopsis , consistent with the fact that the sporopollenin precursor originates from the tapetum. Further germination and comet assays showed that this pathway plays an important role in protection of pollen against UV radiation. In the pteridophyte plant species Ophioglossum vulgatum and Lycopodium clavata , phenylpropanoid derivatives including p -BA and p -CA were also detected, but G units were not. Taken 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 the diversity of sporopollenin among land plants. The spectral and biochemical characteristics of sporopollenin are distinct among seed plants, pteridophytes, and bryophytes. This study discovers that the phenylpropanoid pathway is required for the synthesis of both lignin and sporopollenin in vascular plants. Moreover, this study shows that this pathway plays an important role in pollen protection against UV radiation. [ABSTRACT FROM AUTHOR]