RNAi Blockage of Desoxyhemigossypol-6-Omt Decreases the Amounts of Methylated Sesquiterpenoids in Transgenic Cotton Roots

When plants are attacked by pathogens, they produce compounds, called phytoalexins, that are toxic to these microorganisms.  In the case of the cotton plant, these include desoxyhemigossypol (dHG), desoxyhemigossypol-6-methyl ether (dMHG), hemigossypol (HG), and hemigossypol-6-methyl ether (MHG).  Of these compounds, dHG is the most toxic, followed by dMHG, then HG, and lastly MHG, with the non-methylated compounds being more toxic to fungal spores and hyphae than their methylated counterparts. We previously identified, cloned and sequenced the desoxyhemigossypol-6-O-methyltransferase (dHG-6-OMT) gene from Gossypium barbadense that is responsible for the conversion of dHG to dMHG.  Our objective is to suppress expression of dHG-6-OMT using an RNAi construct, and then determine the disease resistance of these RNAi lines.  Eleven plants from 8 independent transformation events were generated.  Analysis of a small amount of roots from these healthy plants indicated the methyl transferase was suppressed.  Between 32 and 38% of total sesquiterpenoids are methylated in wild type roots.  In contrast, only 0 to 22% of the total sesquiterpenoids are methylated in the roots of T₀ RNAi plants.  Moreover, the total concentration of terpenoids in RNAi roots is reduced to about 5 - 50% compared to WT levels.  Thus, feed-back regulation may prevent the continual synthesis and build-up of HG and dHG.