Herbivore feeding preference corroborates optimal defense theory for specialized metabolites within plants
Investigating into herbivore feeding behaviour on plants, researchers from DynaMo Center of Excellence at University of Copenhagen show that membrane transporters are key for accumulation of glucosinolate defence compounds to highest levels in young leaves, as just published in the prestigious journal Proceedings of the National Academy of Sciences, USA. The finding corroborates the long-standing “optimal defence theory”, since young leaves are most valuable for survival and fitness of a plant.
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Biosynthesis of toxic defence compounds is a big energy investment. Where defence compounds are synthesised, these resources are not available for growth. Instead of investing a little bit in defence throughout the plant, the present study shows that the model plant Arabidopsis use transporters to move glucosinolates to the youngest rosette leaves. Thereby, the distasteful young leaves are protected from the cotton leafworm, Spodoptera littoralis, and the plant has successfully guided the larvae of this generalist herbivore to feed on other tissues.
Experimentally, the key element was to utilise mutants of specific transporters that are involved in loading glucosinolates into the phloem, one of the two transport highway in plants. While in the wildtype plant the larvae mostly fed on old leaves and did not touch the young leaves (left picture), they preferred on mutants to munch on the young leaves – which they consumed almost completely (right picture).
Preference for leaves with lower glucosinolate content is triggered by the fact that larvae feeding on defence compound-containing leaves must invest resources in detoxification. Accordingly, larvae feeding on the glucosinolate-free mutant gained more than double the weight compared with the ones feeding on transporter mutant or wildtype.
Advancing our knowledge about the feeding behavior of this herbivore is important for fighting yield loss as the leafworm is a devastating pest of many economically important crop plants including cotton, tomato and maize.