TITLE:
Impact of Expressing p-Coumaryl Transferase in Medicago sativa L. on Cell Wall Chemistry and Digestibility
AUTHORS:
Jane M. Marita, Dave Rancour, Ronald Hatfield, Paul Weimer
KEYWORDS:
Alfalfa, p-Coumarate, Cell Wall, Lignin, Digestibility
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.7 No.17,
December
30,
2016
ABSTRACT: The addition of p-coumaric acid (pCA) 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 pCA ester linked to sinapyl alcohol, a major
component of lignin. The forage legume alfalfa (Medicago sativa L.) does not contain
appreciable levels of pCA in its more heavily lignified stem tissues. The maize p-coumaryltransferase
(pCAT) 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 pCAT in alfalfa. Expression of the pCAT
transgene was detected in both leaves and stems. Though there was a range of pCAconcentration
in transformed alfalfa stems (0.2 - 1.79 micrograms (μg)), this was a
clear increase over bound pCA 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 pCA (0.85
- 1.2, mean = 0.99 micrograms (μg)) and those expressing pCAT 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 pCA into the wall appears to cause a
shift in lignin composition. Testing the pCAT 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 pCAT gene in alfalfa changes the amount of wall bound pCA, it does not appear
to change lignin levels or impact digestibility.