270px, SEM image of pollen grains Sporopollenin is a biological

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

found as a major component of the tough outer (exine) walls of

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

s and

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

grains. It is chemically very stable (one of the most inert among biopolymers) and is usually well preserved in

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

s and

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s. The exine layer is often intricately sculptured in species-specific patterns, allowing material recovered from (for example) lake sediments to provide useful information to palynologists about plant and fungal populations in the past. Sporopollenin has found uses in the field of

paleoclimatology Paleoclimatology ( British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of ...

as well. Sporopollenin is also found in the cell walls of several taxa of green alga, including '' Phycopeltis'' (an ulvophycean) and '' Chlorella''.

Spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

s are dispersed by many different environmental factors, such as wind, water or animals. In suitable conditions, the sporopollenin-rich walls of pollen grains and spores can persist in the fossil record for hundreds of millions of years, since sporopollenin is resistant to chemical degradation by organic and inorganic chemicals.

Chemical composition

The chemical composition of sporopollenin has long been elusive due to its unusual chemical stability, insolubility and resistance to degradation by

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s and strong chemical reagents. It was once thought to consist of polymerised carotenoids but the application of more detailed analytical methods since the 1980s has shown that this is not correct. Analyses have revealed a complex

biopolymer Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, ...

, containing mainly long-chain fatty acids,

phenylpropanoid The phenylpropanoids are a diverse family of organic compounds that are biosynthesized by plants from the amino acids phenylalanine and tyrosine in the shikimic acid pathway. Their name is derived from the six-carbon, aromatic phenyl group and ...

phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...

ics and traces of carotenoids in a random co-polymer. It is likely that sporopollenin derives from several precursors that are chemically cross-linked to form a rigid structure. There is also good evidence that the chemical composition of sporopollenin is not the same in all plants, indicating it is a class of compounds rather than having one constant structure. In 2019, thioacidolysis degradation and solid-state NMR was used to determine the molecular structure of pitch pine sporopollenin, finding it primarily composed of polyvinyl alcohol units alongside other aliphatic monomers, all crosslinked through a series of

acetal In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments n ...

linkages. Its complex and heterogeneous chemical structure give some protection from the biodegradative enzymes of bacteria, fungi and animals. Some aromatic structures based on ''p''-coumarate and naringenin were also identified within the sporopollenin polymer. These can absorb ultraviolet light and thus prevent it penetrating further into the spore. This has relevance to the role of pollen and spores in transporting and dispersing the gametes of plants. The DNA of the gametes is readily damaged by the ultraviolet component of daylight. Sporopollenin thus provides some protection from this damage as well as a physically robust container. Analysis of sporopollenin from the clubmoss '' Lycopodium'' in the late 1980s have shown distinct structural differences from that of flowering plants. In 2020, more detailed analysis of sporopollenin from '' Lycopodium clavatum'' provided more structural information. It showed a complete lack of aromatic structures and the presence of a macrocyclic backbone of polyhydroxylated tetraketide-like monomers with pseudo-aromatic

2-pyrone 2-Pyrone (α-pyrone or pyran-2-one) is an Saturated and unsaturated compounds, unsaturated cyclic chemical compound with the molecule, molecular formula C5H4O2. It is isomeric with 4-pyrone. 2-Pyrone is used in organic synthesis as a building blo ...

rings. These were crosslinked to a poly(hydroxy acid) chain by ether linkages to form the polymer.

Biosynthesis

Electron microscopy shows that the tapetal cells that surround the developing pollen grain in the

anther The stamen (: stamina or stamens) is a part consisting of the male reproductive organs of a flower. Collectively, the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filament ...

have a highly active secretory system containing lipophilic globules. These globules are believed to contain sporopollenin precursors. Tracer experiments have shown that

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of ...

is a major precursor, but other carbon sources also contribute. The biosynthetic pathway for phenylpropanoid is very active in tapetal cells, supporting the idea that its products are needed for sporopollenin synthesis. Chemical inhibitors of pollen development and many male sterile mutants have effects on the secretion of these globules by the tapetal cells.

Chemical composition

Biosynthesis

See also

References

Further reading