Alkylresorcinol biosynthesis in plants: new insights from an ancient enzyme family?

Abstract

Alkylresorcinols are members of an extensive family of bioactive compounds referred to as phenolic lipids, which occur primarily in plants, fungi, and bacteria. In plants, alkylresorcinols and their derivatives are thought to serve important roles as phytoanticipins and allelochemicals, although direct evidence for this is still somewhat lacking. Specialized type III polyketide synthases (referred to as 'alkylresorcinol synthases'), which catalyze the formation of 5-alkylresorcinols using fatty acyl-CoA starter units and malonyl-CoA extender units, have been characterized from several microbial species, however until very recently little has been known concerning their plant counterparts. Through the use of sorghum and rice EST and genomic data sets, significant inroads have now been made in this regard. Here we provide additional information concerning our recent report on the identification and characterization of alkylresorcinol synthases from Sorghum bicolor and Oryza sativa, as well as a brief consideration of the emergence of this intriguing subfamily of enzymes.

Figure 1

Additional ARS-like sequences identified in O. sativa and transcriptional profiling data for the three functionally-characterized O. sativa ARS enzymes are shown. (A) O. sativa Genome Browser return (http://www.phytozome.net) is shown, indicating three predicted ARS-like sequences (encoded by LOC_ Os05g12190, LOC_ Os05g12210 and LOC_ Os05g12240) closely-linked to ARS Os05g12180. (B) Alignment of functionally-characterized S. bicolor and O. sativa ARS sequences with the O. sativa ARS-like sequences shown in (A) and chalcone synthase (CHS) sequences from M. sativa, O. sativa and S. bicolor (accession nos. P30074, BAB39764 and AAD 41874, respectively). Numbering is shown according to the M. sativa CHS2 sequence; deviations from the CHS-type consensus at positions Thr132, Met137 and Gly256 are boxed. Positions 131–137 correspond to the displaced area 2 loop (outlined by rectangle) critical for the formation of the hydrogen bond network formed by Thr132, Glu192 and Ser338 (highlighted in grey). The three residues in area 2 (positions 132, 133 and 137) in direct contact with the active site cavity are indicated by closed circles. (C) Signal intensity values for ARSs Os10g8620, Os05g12180 and Os10g0704015 in different O. sativa tissues (http://www.ricearray.org), based on transcriptional profiling studies performed by Jain et al. SAM, shoot apical meristem; DAP, days after pollination.

Figure 2

Reactions catalyzed by stilbene synthase, alkylresorcinol synthase and chalcone synthase-type III PKS enzymes. All of these PKSs utilize a starter unit and perform three condensation reactions with malonyl-CoA as the extender unit, yielding a tetraketide intermediate. Different ring-folding mechanisms are then employed, as discussed in the text. Alkylresorcinol synthases can also generate tetraketide pyrone side-products via C5 oxygen → C1 intramolecular lactonization. STS, stilbene synthase; ARS, alkylresorcinol synthase; CHS, chalcone synthase.