Citral induces plant systemic acquired resistance against tobacco mosaic virus and plant fungal diseases

Litsea cubeba essential oil (LCO) has a wide range of industrial uses. This study found that LCO has significant anti-TMV activity and that its active components are monoterpenoids. Citral, the main active component of the oil, showed outstanding anti-TMV activity. The protective, inactive, and curative efficacies of citral (500 μg/mL) against TMV were 76.27%, 55.14%, and 50.42%, respectively, which were significantly better than those of the positive control agent, chitosan oligosaccharide. Citral showed no direct effect on TMV particles but did induce hypersensitive response (HR). Bioassay results showed that citral induced significant plant disease resistance activity (68.20% at a concentration of 500 μg/mL). The results of physiological and biochemical experiments showed that citral induced the overexpression of defense enzymes such as SOD, CAT, POD, and PAL in tobacco (which increased by 5.3, 1.9, 3.0, and 4.0 times at a concentration of 500 μg/mL, respectively) as well as induced the overexpression of PR proteins, including NPR1, PR1, PR2, and PR5 (which increased by 23.49, 39.27, 34.47, and 52.07 times at a concentration of 500 μg/mL, respectively). These results suggest that citral could induce systemic acquired resistance (SAR) in tobacco. Further mechanism studies conducted suggest that citral induces the overexpression of genes related to salicylic acid (SA) biosynthesis, including PAL, ICS, and PBS3 (50.74, 30.41, and 31.95 times higher than the control at a concentration of 500 μg/mL, respectively) and the accumulation of SA (5.1 times higher than the control at a concentration of 500 μg/mL). These results suggest that citral activates SAR through the SA pathway. The control efficacies induced by citral (500 μg/mL) on Erysiphe cucurbitacearum, Botrytis cinerea, and Sclerotinia sclerotiorum reached 67.39%, 59.61%, and 63.81%, respectively, indicating that citral can induce plant resistance against fungal diseases. The induced plant disease resistance activity of citral is prominent, with a long duration and a broad spectrum, performing better than the commercial agent chitosan oligosaccharide. Citral thus has the potential to be developed into a new botanical-derived anti-virus agent and plant immunity activator.