Antisense suppression of the lignin biosynthetic enzyme, caffeate O-methyltransferase, improves in vitro digestibility of the tropical pasture legume, Stylosanthes humilis

The high lignin content of tropical forage plants reduces digestibility and voluntary feed intake by ruminants. We have used antisense technology to s uppress caffeate O-methyltransferase (COMT EC, 2.1.1.68), a lignin biosynth etic enzyme in the tropical forage legume, Stylosanthes humilis Kunth. Plan ts were transformed using a Ti binary vector containing an antisense COMT c onstruct under the control of the CaMV 35S promoter. From 50 transgenic pla nts, five were selected on the basis of normal morphology, high levels of a ntisense gene expression and altered lignin histochemistry. No plants with altered lignin were observed in a population of 20 transgenic plants derive d using a binary vector that lacked the COMT cDNA insert. The progeny of li gnin-altered plants were analysed for COMT enzyme activity and lignin histo chemistry. A variety of COMT and lignin phenotypes was observed. In several T-1 plants, COMT activity was specifically suppressed by more than 95% com pared to controls. In these plants, expression of antisense mRNA was high w hile sense mRNA could not be detected on northern blots. The overall lignin content of these plants was unchanged but histochemical tests showed abnor mally low levels of the syringyl component, mimicking the pattern of young tissue. Digestibility of these transgenic plants was assessed by incubation of stem material with rumen fluid and acid pepsin in vitro. The digestibil ity of the antisense material was increased dramatically compared to that o f equivalent samples from control transformed plants (72 vs 62%).