|Pan, Zhiqiang - Peter|
Submitted to: The Plant Cell
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/9/2010
Publication Date: 3/26/2010
Citation: Cook, D., Rimando, A.M., Clemente, T.E., Schroder, J., Dayan, F.E., Nanayakkara, D., Pan, Z., Noonan, B.P., Fishbein, M., Abe, I., Duke, S.O., Baerson, S.R. 2010. Alkylresorcinol Synthases Expressed in Sorghum Bicolor Root Hairs Play an Essential Role in the Biosynthesis of the Allelopathic Benzoquinone Sorgoleone. The Plant Cell. 22:867-887. Interpretive Summary: The sorghum plant produces compounds in the cells that cover the root system (root hairs) that inhibit the growth of other plant species. Sorgoleone is one of these compounds. Previous work has shown that a type of enzyme known as a polyketide synthase is probably one of the key enzymes involved in manufacturing sorgoleone. A database of gene sequences expressed in root hair cells was assembled to help identify the polyketide synthase gene or genes involved in this process. We have studied the chemical reactions performed by two of the polyketide synthase genes we found in the database, and they appear to have the properties one would expect for a polyketide synthase enzyme involved in sorgoleone production. In the manuscript we propose that these enzymes, which we call SbLRS1 and SbLRS2, could be involved in the production of sorgoleone in sorghum root hair cells.
Technical Abstract: Sorghum bicolor is considered to be an allelopathic crop species, producing phytotoxins such as the lipid benzoquinone sorgoleone, which likely accounts for many of the allelopathic properties of Sorghum spp. Current evidence suggests that sorgoleone biosynthesis occurs exclusively in root hair cells and involves the production of an alkylresorcinolic intermediate (5-[(8’Z,11’Z)-8’,11’,14’- pentadecatrienyl]resorcinol) derived from an unusual 16:3(delta)9,12,15 fatty acyl-CoA starter unit. This has led to the suggestion of the involvement of one or more alkylresorcinol synthases (ARSs), type III polyketide synthases (PKSs) that produce 5-alkylresorcinols using medium to long-chain fatty acyl-CoA starter units via iterative condensations with malonyl-CoA. In an effort to characterize the enzymes responsible for the biosynthesis of the pentadecyl resorcinol intermediate, a previously-described expressed sequence tag database prepared from isolated S. bicolor (genotype BTx623) root hairs was first mined for all PKS-like sequences. Quantitative real-time RT-PCR analyses revealed that three of these sequences were preferentially expressed in root hairs, two of which (designated ARS1 and ARS2) were found to encode ARS enzymes capable of accepting a variety of fatty acyl-CoA starter units in recombinant enzyme studies. Furthermore, RNA interference (RNAi) experiments directed against ARS1 and ARS2 resulted in the generation of multiple independent transformant events exhibiting dramatically reduced sorgoleone levels. Thus, both ARS1 and ARS2 are likely to participate in the biosynthesis of sorgoleone in planta. The sequences of ARS1 and ARS2 were also used to identify several rice genes encoding ARSs, which are likely involved in the production of defense-related alkylresorcinols.