Application of RNAi to confirm theobromine as the major intermediate for caffeine biosynthesis in coffee plants with potential for construction of decaffeinated varieties

The caffeine biosynthetic pathway in coffee plants has been proposed to involve three distinct N-methyltransferases, xanthosine methyltransferase (XMT), 7-N-methylxanthine methyltransferase (MXMT; theobromine synthase), and 3,7-dimethylxanthine methyltransferase (DXMT; caffeine synthase). We previously isolated all corresponding cDNAs designated as CaXMT1, CaMXMT1, CaMXMT2 and CaDXMT1, respectively, and showed that caffeine was indeed synthesized in vitro by the combination of their gene products. In order to regulate caffeine biosynthesis in planta, we suppressed expression of CaMXMT1 by the double stranded RNA interference (RNAi) method. For this purpose, we first established a protocol for efficient somatic embryogenesis of Coffea arabica and C. canephora, and then Agrobacterium-mediated transformation techniques. The RNAi transgenic lines of embryogenic tissues derived from C. arabica and transgenic plantlets of C. canephora demonstrated a clear reduction in transcripts for CaMXMT1 in comparison with the control plants. Transcripts for CaXMT1 and CaDXMT1 were also reduced in the most cases. Both embryonic tissues and plantlets exhibited a concomitant reduction of theobromine and caffeine contents to a range between 30% and 50% of that of the control. These results suggest that the CaMXMT1-RNAi sequence affected expression of not only CaMXMT1 itself, but also CaXMT1 and CaDXMT1, and that, since the reduction in theobromine content was proportional to that for caffeine, it is involved in the major synthetic pathway in coffee plants. The results also indicate that the method can be practically applied to produce decaffeinated coffee plants.