Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/26/2007
Publication Date: 10/10/2007
Citation: Dayan, F.E., Watson, S.B., Nanayakkara, D. 2007. Biosynthesis of Lipid Resorcinols and Benzoquinones in Isolated Secretory Plant Root Hairs. Journal of Experimental Botany. 58(12):3263-3272. Interpretive Summary: The primary functions of root hairs are to increase the root surface area and to aid plants in water and nutrient uptake. However, some root hairs have developed the ability to produce and release bioactive compounds. Sorghum bicolor root hairs release sorgoleone, a potent allelochemical that prevents the growth of other plants in its surrounding. This paper reports a new method of root hair isolation that permitted to test the enzymes responsible for the synthesis of sorgoleone. The enzymes were tested on several substrates to determine their specificity. Their preferred pH and temperature were alsoe determined. The fact that the entire biosynthesis of sorgoleone is located in the root hair of sorghum will have implications for the genetic engineering of this pathway in other plants species.
Technical Abstract: The primary functions of root hairs are to increase the root surface area and to aid plants in water and nutrient uptake. However, some root hairs also have secretory functions and exude bioactive secondary metabolites. Sorghum bicolor root hairs release a substantial amount of the lipid benzoquinone sorgoleone, a potent allelochemical. The activity of the key enzymes involved in the biosynthesis of sorgoleone and its metabolic precursors was measured directly in isolated root hairs preparations obtained from 6-d old roots. The first committed step in sorgoleone biosynthesis is catalyzed by a type III polyketide synthase. The purified root hairs preparation readily converted long chain acyl-CoA starter units to their corresponding lipid resorcinols. Optimum activity was observed with decanoyl-CoA, which yielded a 5-n-nonyl-resorcinol. The isolated root hair preparation also had high S-adenosyl-L-methionine-dependent O-methyltransferase activity, which catalyzes the next step in the sorgoleone biosynthetic pathway. Several 5-alkyl-resorcinols were readily methylated, but the optimum activity was observed with 5-n-pentyl-resorcinol. The last biochemical step, catalyzed by a hydroxylase (putatively a P450 monooxygenase), produces dihydrosorgoleone. The in situ hydroxylase activity of this enzyme was low relative to the other enzymes in the pathway, but was nonetheless detectable in isolated root hairs. Thus, sorghum root hairs possess the entire metabolic machinery necessary for the biosynthesis of sorgoleone. This will have implications for the genetic engineering of this pathway in sorghum and in other plants species. It also demonstrates that some root hairs can function as specialized cells for the production of bioactive secondary metabolites.