The addition of p -coumaric acid ( p CA) to lignin molecules is frequently found in members of the grass family. The role of this addition is not clearly understood, but is thought to potentially aid in the formation of syringyl-type lignin. This is because the incorporation is as a conjugate of p CA ester linked to sinapyl alcohol, a major component of lignin. The forage legume alfalfa ( Medicago sativa L.) does not contain appreciable levels of p CA in its more heavily lignified stem tissues. The maize p -coumaryltransferase ( p CAT) gene was used to transform alfalfa to determine its impact upon lignin composition and its potential to alter cell wall digestibility. A constitutive expression vector using the cassava vein mosaic virus (CsVMV) promoter was used to drive expression of maize p CAT in alfalfa. Expression of the p CAT transgene was detected in both leaves and stems. Though there was a range of p CAconcentration in transformed alfalfa stems (0.2 - 1.79 micrograms (μg)), this was a clear increase over bound p CA in control stems (0.15 - 0.2 mean = 0.17 micrograms (μg)). This did not lead to consistent responses concerning total lignin in the stem tissues. Leaf tissue, on the other hand, already has a relatively high level of p CA (0.85 - 1.2, mean = 0.99 micrograms (μg)) and those expressing p CAT gene showed on average a small increase, but there is a wide range of values among the transformants (0.38 - 1.55, mean = 1.06 micrograms (μg)). Lignin in leaves did not appear to be significantly impacted. However, incorporation of p CA into the wall appears to cause a shift in lignin composition. Testing the p CAT expressing stem cell walls for digestibility using a rumen in vitro system showed there was no change in the digestibility of the stem compared to empty vectors and control alfalfa stems. Although expression of p CAT gene in alfalfa changes the amount of wall bound p CA, it does not appear to change lignin levels or impact digestibility.
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