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SPOROPOLLENIN The makeup of a gamete space capsule Sporopollenin, which encapsulates gametes in spore and pollen grains, is probably the most chemically inert biopolymer. This inertness is essential for gamete protection, but also hinders the elucidation of sporopollenin molecular structure. Now, the macromolecular network forming sporopollenin is described in unprecedented detail. Paula Guzmán-Delgado and Maciej A. Zwieniecki

he establishment of plants on land, aliphatic molecules), each flanked at one wall a certain degree of dynamism, and about 450 million years ago, is one end by an α-pyrone moiety and crosslinked thus the ability to adapt to (and reflect) Tof the most significant episodes in at the other end through an ester group. different conditions. Second, determining life evolution, with far-reaching impacts The PVA units are crosslinked by 7-O-p- the molecular structure of sporopollenin that shaped the global ecosystem. The coumaroylated aliphatic units composed will contribute to our understanding of the transition from aquatic or semi-aquatic of 16 carbon atoms through an m-dioxane biosynthetic machinery that allowed plant environments — where plant ancestors moiety that features an acetal. The PVA-like terrestrialization1,9. Finally, unravelling (green algae) grew — into permanent units are probably linked through ether the structure of perhaps the most resistive terrestrial conditions presented a great bonds with glycerol-like moieties, and minor biopolymer is likely to help the design of challenge for plants, resulting in the amounts of naringenin. Concerning the biomimetic products with such long-lasting evolution of concomitant adaptive changes. relative abundance of sporopollenin chemical desired properties. Again, the heterogeneous Sporopollenin, the major constituent of the constituents, the p-coumaroylated aliphatic nature of sporopollenin and associated outer cell wall (exine) of spores and their units and PVA-like units are the main molecules confers exines a wide range of homologue, pollen, presumably played products of sporopollenin ‘soluble’ and non- physico-chemical properties. Exines can a key role in facilitating this transition degradative fractions, respectively, and the serve as microparticles and microcapsules by furnishing an optimal encapsulation proportion of carbon is higher in aliphatic for a number of applications, from metal of plant gametes that enabled dispersal domains than in aromatic ones. While this is remediation to oral vaccination3. Thus, the and sexual reproduction. Sporopollenin the average structure of pine sporopollenin development of a standardized, consistent is extremely resistant to physical and based on the 13C nuclear magnetic resonance methodology that allows the identification chemical degradation, probably more so spectra, its native structure is likely to be of sporopollenin molecular structure is a than any other plant product1. This high more heterogeneous, especially in terms of breakthrough in diverse research fields. resistance has also likely contributed to inter-molecular crosslinking. Additional studies using sporopollenin from the preservation of spores and pollen in The presence of two major types of different plant species and material should the fossil record, providing extensive and linkages in pine sporopollenin (ester and be performed to test the replicability of the continuous records over geological time, acetal) is likely to confer sporopollenin with method by Li et al.6. ❐ including evidence of the first land plants2. a higher resistance to decay as opposed to However, such recalcitrance also results in other plant polymers that may contain a Paula Guzmán-Delgado* and difficulties in accurately determining its single major linkage type. Moreover, the Maciej A. Zwieniecki chemical composition and ultrastructure, identification of α-pyrone, p-coumarate and Department of Plant Sciences, University of which renders sporopollenin the least- naringenin moieties as integral constituents California Davis, Davis, CA, USA. known plant polymer3. In this regard, of sporopollenin, supports sporopollenin’s *e-mail: [email protected] several studies suggest a polymeric structure essential function in protecting gametes for sporopollenin, potentially having both against ultraviolet radiation, and provides Published online: 17 December 2018 aliphatic and aromatic components, but the insight into biosynthetic mechanisms. https://doi.org/10.1038/s41477-018-0342-3 specific nature of its molecular constituents The molecular structure of sporopollenin and inter-molecular linkages are poorly has been a matter of interest for a long References 4,5 7 6 1. Niklas, K. J., Cobb, E. D. & Matas, A. J. J. Exp. Bot. 19, understood . Now, in a paper published in time , and the investigation by Li et al. 5261–5269 (2017). 6 Nature Plants, Li et al. shed new light on the may have profound implications for both 2. Wellman, C. H. & Strother, P. K. Palaeontology 58, 601–27 (2015). fine structure of sporopollenin. the fundamental and applied viewpoints. 3. Mackenzie, G., Boa, A. N., Diego-Taboada, A., Atkin, S. L. & Li et al.6 developed a methodology that First, given the extensive fossil record Sathyapalan, T. Front. Mater. 2, 66 (2015). 4. Domínguez, E., Mercado, J. A., Quesada, M. A. & Heredia, A. combines optimized traditional procedures of spores and pollen, as well as their Sex. Plant Reprod. 12, 171–178 (1999). for biopolymer analysis with innovative ubiquitous presence in current land plants, 5. Jardine, P. E., Abernethy, F. A., Lomax, B. H., Gosling, W. D. & techniques aimed at unravelling the the analysis of sporopollenin from different Fraser, W. T. Rev. Palaeobot. Palyno. 238, 1–6 (2017). 6. Li, F. S., Phyo, P., Jacobowitz, J., Hong, M. & Weng, J. K. Nat. sporopollenin molecular structure. The sources may offer essential information Plants. https://doi.org/10.1038/s41477-018-0330-7 (2019). authors were able to deduce the structure to understand Earth’s history (since the 7. Shaw, G. in Sporopollenin (eds Brooks, J. et al.) 305–350 of pitch pine pollen sporopollenin with an first algae bearing sporopollenin8), and (Academic Press, London, 1971). 8. Atkinson, J. A., Gunning, B. E. & John, P. C. Planta 107, 1–32 (1972). extraordinary level of detail. Sporopollenin to predict future scenarios. In this sense, 9. Daku, R. M. et al. PLoS ONE 11, e0146817 (2016). was inferred to be mainly constituted of two the heterogeneity of sporopollenin can be fatty-acid-derived polyvinyl alcohol (PVA)- interpreted as a fundamental feature that Competing interests like units (relatively long polyhydroxylated may provide both the polymer and the cell The authors declare no competing interests.

8 Nature Plants | VOL 5 | JANUARY 2019 | 8 | www.nature.com/natureplants