If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: sporopollenin monomer biosynthesis
|Superclasses:||Biosynthesis → Cell Structures Biosynthesis → Plant Cell Structures|
Some taxa known to possess this pathway include : Arabidopsis thaliana col
Expected Taxonomic Range: Embryophyta
The pollen grain wall in most plant species has the same basic structure, although the size, shape and external morphology are species-specific. It consists of an outer and inner layer, called the exine and intine, respectively. The exine consists of an outer sculptured portion, the sexine, and a simpler inner layer, the nexine. The sculptured sexine is made up of radially directed rods, the bacula, which have enlarged heads that fuse to form a patterned wall, and the tectum, which is the outermost edge of the exine. The exine wall has important roles in protecting the pollen from various environmental stresses and bacterial attacks when it moves from the anther to the stigma, and in the species-specific adhesion of pollen grains to the female stigma cells [Ariizumi03].
The interstices of the exine are coated with a sticky hydrophobic lipidic and proteinaceous layer known as the tryphine or pollen coat. A mojor component of the tryphine is sporopollenin - a complex polymer that confers physical strength, chemical inertness and elasticity to the exine. The chemical composition of sporopollenin remains poorly characterized due to its extreme resistance to chemical and biological degradations. However limited data indicates the presence of aliphatic polyhydroxy compounds and phenolic hydroxyl groups [Ahlers03]. Several genes and their encoded enzymes involved in sporopollenin biosynthesis have been identified in the recent years [Grienenberger10, Dobritsa09, Kim10a, Morant07, deAzevedo09]. The genetic and biochemical data further suggest that sporopollenin components include hydroxylated fatty acids and their derived tetraketide α-pyrones. The sporopollenin components are presumably coupled with ester and ether linkages. Nonetheless, the exact chemical nature of the sporopollenin polymer and its components still remains to be elucidated. For example, although in vitro enzyme characterization supports the presence of tetraketide α-pyrones, their occurrence has not been reported in Arabidopsis.
About This Pathway
The pathway starts with fatty acids synrthesized in the plastid - laurate, palmitate, stearate and oleate. These fatty acids are exported from the plastid and imported into the endoplasmic reticulum (ER) in their CoA-activated forms. Once they enter the ER, they are believed to be hydrolyzed to free fatty acids that are hydroxylated by P450 fatty acid monooxygenases. Two routes have been described. Laurate (C12:0) is hydroxylated at position 7 by a dedicated monooxygenase, encoded by the CYP703A2 gene [Morant07]. The other three fatty acids are hydroxylated at the ω position by long-chain fatty acid ω-hydroxylase, encoded by CYP704B1 [Dobritsa09]. Once hydroxylated, the fatty acids are again esterified to coenzyme A in a reaction catalyzed by the fatty acyl-CoA synthetase encoded by ACOS5 [deAzevedo09] and exported from the ER to the cytosol. .
The processes in the cytosol are not well understood, but a few enzymes are known to be involved. The alcohol-forming fatty acyl-CoA reductase encoded by the FAR2 gene is absoltely required, and is assumed to convert the hydroxylated fatty acyl-CoAs to fatty alcohols [Aarts97]. 7-hydroxylauroyl-CoA is also the substrate for the tetraketide α-pyrone synthases encoded by PKSA and PKSB, which convert it to the tetraketide α-pyrone 2-(8-hydroxy-2-oxotridecyl)-6-oxopyran-4-olate [Kim10a].
The genes involved in this pathway, CYP704B1, CYP703A2, ACOS5, PKSA, PKSB, TKPR1 and TKPR2, are tightly co-expressed.
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Ahlers03: Ahlers F, Lambert J, Wiermann R (2003). "Acetylation and silylation of piperidine solubilized sporopollenin from pollen of Typha angustifolia L." Z Naturforsch C 58(11-12);807-11. PMID: 14713155
Ariizumi03: Ariizumi T, Hatakeyama K, Hinata K, Sato S, Kato T, Tabata S, Toriyama K (2003). "A novel male-sterile mutant of Arabidopsis thaliana, faceless pollen-1, produces pollen with a smooth surface and an acetolysis-sensitive exine." Plant Mol Biol 53(1-2);107-16. PMID: 14756310
deAzevedo09: de Azevedo Souza C, Kim SS, Koch S, Kienow L, Schneider K, McKim SM, Haughn GW, Kombrink E, Douglas CJ (2009). "A novel fatty Acyl-CoA Synthetase is required for pollen development and sporopollenin biosynthesis in Arabidopsis." Plant Cell 21(2);507-25. PMID: 19218397
Dobritsa09: Dobritsa AA, Shrestha J, Morant M, Pinot F, Matsuno M, Swanson R, Moller BL, Preuss D (2009). "CYP704B1 is a long-chain fatty acid omega-hydroxylase essential for sporopollenin synthesis in pollen of Arabidopsis." Plant Physiol 151(2);574-89. PMID: 19700560
Grienenberger10: Grienenberger E, Kim SS, Lallemand B, Geoffroy P, Heintz D, Souza Cde A, Heitz T, Douglas CJ, Legrand M (2010). "Analysis of TETRAKETIDE α-PYRONE REDUCTASE function in Arabidopsis thaliana reveals a previously unknown, but conserved, biochemical pathway in sporopollenin monomer biosynthesis." Plant Cell 22(12);4067-83. PMID: 21193572
Kim10a: Kim SS, Grienenberger E, Lallemand B, Colpitts CC, Kim SY, Souza Cde A, Geoffroy P, Heintz D, Krahn D, Kaiser M, Kombrink E, Heitz T, Suh DY, Legrand M, Douglas CJ (2010). "LAP6/POLYKETIDE SYNTHASE A and LAP5/POLYKETIDE SYNTHASE B encode hydroxyalkyl α-pyrone synthases required for pollen development and sporopollenin biosynthesis in Arabidopsis thaliana." Plant Cell 22(12);4045-66. PMID: 21193570
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