Lolium perenne engineered for elevated leaf lipids exhibits greater energy density in field canopies under defoliation

Abstract

Co-expression of diacylglycerol O -acyltransferase (DGAT1) and a lipid droplet-stabilizing protein called Cys-oleosin (collectively ‘HME’ technology) increases foliar lipid concentrations and plant biomass, which could elevate energy yields from perennial ryegrass (PR, Lolium perenne L.)-based pastures. Quantifying the feasible benefits of HME PR necessitates that herbage growth and energy density are quantified on a ground area basis under regular defoliation. We examined the translation, from spaced pots indoors to field canopies (swards), of the fatty acid (FA), gross energy (GE), and biomass enhancing traits associated with HME technology expression in PR. Under controlled indoor conditions in spaced pots, hemizygous HME seedlings with 32% higher leaf FA content (1.2%DW) than null controls exhibited a faster rate of leaf extension. These effects translated into a stable 0.2–0.5 kJ/gDW increase in herbage GE concentration, and a 6–10% herbage growth rate advantage when HME ‘miniswards’ were grown under moderate light competition (LAI 7–9) for five defoliation and regrowth cycles (harvests). However, subsequent growth in a dense canopy (LAI>10) eliminated this HME herbage growth advantage. In two field trials in the Midwest United States, hemizygous HME swards exhibited 25–34% (0.8–1%DW) higher end-of-season herbage FA content than null control swards, and 0.2–0.5 kJ/gDW higher end-of-season herbage GE. Herbage growth rates were generally similar for HME and null control swards across both field seasons. This thorough analysis of trait translation shows that the benefits of enhanced FA and GE translate to HME PR canopies grown in the field. The intrinsic growth advantage of HME PR may be diminished in field canopies such that yield benefits are realized only under limited conditions.