An investigation into plant type III polyketide synthases: A styrylpyrone synthase from Equisetum hyemale and anther-specific chalcone synthase-like enzymes from Physcomitrella patens and Arabidopsis thaliana

Type III polyketide synthases (PKSs) produce secondary metabolites that playa variety of roles in plants. Three new type III PKS have been investigated: a putative styrylpyrone synthase (SPS) from Equisetum hyemale and anther-specific alkylpyrone synthases from Physcomitrella patens and Arabadopsis thaliana.;Anther-specific chalcone synthase (CHS)-like (ASCL) enzymes are type III PKSs which are essential for the development of fertile pollen in male plants, although the functional role of ASCLs is poorly understood. ASCLs are related to, but phylogenetically distinct, from CHS enzymes. ASCL enzymes are highly expressed during structural development of pollen, indicating that ASCLs may be involved in the biosynthesis of sporopollenin in the exine, the outer layer of the pollen cell wall. In this study, an ASCL enzyme from A. thaliana and a primitive ASCL homologue from the moss P. patens were characterized. Despite the distant evolutionary relationship between these plants, the A. thaliana and P. patens ASCL enzymes exhibited a conserved activity, with unusually broad substrate specificity and a somewhat different cyclization strategy as compared to typical plant type III PKSs. In vitro, the ASCL enzymes generated alkylpyrone products, although this may not occur in vivo. A. thaliana dihydroflavonol 4-reductase-like 1 (AthDRL1), another enzyme involved in sporopollenin biosynthesis, was also examined to determine if AthDRL1 and ASCLs function together in this pathway. It appears that AthDRL1 may act on the product of the ASCLs. The fundamental importance of ASCL enzymes to pollen and spore development in plants is highlighted by the evolutionary relationship and conserved activity of ASCLs.;SPS has unusual cyclization and condensation activity for a type III PKS, which makes it a promising target for investigation of the control mechanisms for these activities in type III PKSs. A better understanding of this could be applied to similar antibiotic-producing type I PKS systems, which may be useful in engineering these systems to generate novel products. In this study, a putative SPS from E. hyemale was identified, cloned and partially characterized. However, the cloned enzyme did not exclusively exhibit SPS activity in vitro. Therefore, further study is needed before the mechanism can be elucidated.