Abstract
Polyphenol oxidases (PPOs) catalyze the oxidation of phenolics to quinones, the secondary reactions of which lead to oxidative browning and postharvest losses of many fruits and vegetables. PPOs are ubiquitous in angiosperms, are inducible by both biotic and abiotic stresses, and have been implicated in several physiological processes including plant defense against pathogens and insects, the Mehler reaction, photoreduction of molecular oxygen by PSI, regulation of plastidic oxygen levels, aurone biosynthesis and the phenylpropanoid pathway. Here we review experiments in which the roles of PPO in disease and insect resistance as well as in the Mehler reaction were investigated using transgenic tomato (Lycopersicon esculentum) plants with modified PPO expression levels (suppressed PPO and overexpressing PPO). These transgenic plants showed normal growth, development and reproduction under laboratory, growth chamber and greenhouse conditions. Antisense PPO expression dramatically increased susceptibility while PPO overexpression increased resistance of tomato plants to Pseudomonas syringae. Similarly, PPO-overexpressing transgenic plants showed an increase in resistance to various insects, including common cutworm (Spodoptera litura (F.)), cotton bollworm (Helicoverpa armigera (Hübner)) and beet army worm (Spodoptera exigua (Hübner)), whereas larvae feeding on plants with suppressed PPO activity had higher larval growth rates and consumed more foliage. Similar increases in weight gain, foliage consumption, and survival were also observed with Colorado potato beetles (Leptinotarsa decemlineata (Say)) feeding on antisense PPO transgenic tomatoes. The putative defensive mechanisms conferred by PPO and its interaction with other defense proteins are discussed. In addition, transgenic plants with suppressed PPO exhibited more favorable water relations and decreased photoinhibition compared to nontransformed controls and transgenic plants overexpressing PPO, suggesting that PPO may have a role in the development of plant water stress and potential for photoinhibition and photooxidative damage that may be unrelated to any effects on the Mehler reaction. These results substantiate the defensive role of PPO and suggest that manipulation of PPO activity in specific tissues has the potential to provide broad-spectrum resistance simultaneously to both disease and insect pests, however, effects of PPO on postharvest quality as well as water stress physiology should also be considered. In addition to the functional analysis of tomato PPO, the application of antisense/sense technology to decipher the functions of PPO in other plant species as well as for commercial uses are discussed.
Highlights
Plants are exposed to a great many abiotic and biotic stresses in their environments that can, together or separately, reduce plant fitness
Pupal weights and mortality did not differ among treatments, but development time was extended on overexpressing PPO activity (OP) plants and attenuated on suppressed PPO activity (SP) plants compared to controls (Figure 2D, Bhonwong et al, unpublished manuscript)
The fact that the effects of Polyphenol oxidases (PPOs) over- and underexpression on larvae differed for larvae feeding on young and old leaves is consistent with previous studies showing that the effects of PPO on nutritional quality are dependent on quality and quantity of dietary protein (e.g., Felton et al [39]), because young and old leaves used in this study likely differed in both quality and quantity of protein
Summary
Plants are exposed to a great many abiotic and biotic stresses in their environments that can, together or separately, reduce plant fitness. All seven genes lack introns, a feature that is common to most PPO genes, except for a banana PPO gene that appears to contain a 85-bp intron in its sequence and two wheat PPO genes that contain 2 introns [9c, 12] These genes are highly conserved (70-100% identity in their coding regions), they exhibit spatially and temporally differential expression in vegetative and reproductive organs of tomato as well as differential inducibility in response to various biotic and abiotic agents [9c, 6c, 10a, 5d, 13]. Multiple PPO cDNAs were isolated and characterized from leaves and tubers, indicating a gene family with specific temporal and spatial expression patterns of individual members, and encode polypeptides whose sequences are highly homologous to tomato PPO [9b, 9g]. PPO has been frequently implicated in resistance to diverse pathogens since quinones, its primary products, are highly reactive electrophiles that can undergo complex secondary reaction pathways, in particular the 1,4 addition of o-quinones to cellular nucleophiles and the reversed disproportionation of quinones to semiquinone radicals that may lead to generation of reactive oxygen species (ROS)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
