Bio-refining of perennial ryegrass (Lolium perenne): evaluation of aqueous extracts for plant defence elicitor activity using French bean cell suspension cultures

Background: There is growing interest in the bio-refining of foliage grasses to yield a range of industrial raw products. The aim of this research was to evaluate if aqueous extracts from grasses have the potential to act as crop bio-stimulants by assessing their defence elicitor activity.

Results: Field-grown foliage (500 g fresh weight) from the Lolium perenne varieties Tyrella, Dromara, Abermagic, and Spelga were homogenised in water (2 l) and the run-off liquid filtered through Whatman No. 1 filter paper before freezing. Thawed filtrate was centrifuged at 21,000 rpm for 15 min, to give pellet and supernatant fractions, which were assayed separately for defence elicitor activity by adding them to cell suspension cultures of Phaseolus vulgaris in the early exponential phase of growth (7–10 days into a 14 days culture passage). Elicitor activity was evaluated 24 h after treatment by assessing the extent of necrosis (browning) in the P. vulgaris cells. Supernatant and pellet fractions of all four L. perenne varieties were found to have potent cell defence elicitor activity, although there were relatively small but significant differences between the varieties. In a second series of extractions, homogenisation of tissues in water was compared with screw-pressed juice using the varieties, Malone, Seagoe, and Copeland. Varietal differences in elicitor activity of these extracts were removed if they were first equalised for protein content prior to eliciting the bean cells, although the screw-pressed juice was marginally more active as an elicitor than the homogenised extract. Autoclaving the extracts in attempts to solubilise/release additional elicitor compounds neither increased nor decreased the elicitor activities of the supernatant or pellet fractions. Elicitor activity of the extract was partially reduced by treating with a cation exchange resin, almost entirely removed by adsorption onto activated carbon and completely abolished by ashing at 650 °C.

Conclusion: The L. perenne defence elicitor(s) is therefore organic rather than inorganic, heat-stable, cationic at neutral pH, and readily soluble in water at room temperature.