Abstract
Atmospheric CO2 levels influences plant growth and susceptibility to
pathogens and pests but there is little information on the mechanisms
involved. We therefore studied how elevated atmospheric CO2
concentrations (eCO2) effects shoot branching and the performance of the
pea aphid on wild type peas and on mutants that are defective in either
strigolactone (SL) synthesis or signalling. Shoot branching and dry
biomass accumulation were increased under eCO2 conditions in all lines.
However, eCO2 decreased shoot water content only in the wild type peas
but not in the SL mutants. Growth under eCO2 decreased the levels of
salicylic acid and increased jasmonic acid in the wild type peas,
without any significant effect on aphid fecundity. However, aphid
numbers were increased on the SL mutants under both ambient and eCO2
conditions. Of the shoot phytohormones measured in this study, only
gibberellic acid (GA3) was decreased in the SL mutants, an effect that
was exacerbated by eCO2. There was a negative correlation between aphid
fecundity and shoot GA3 levels. Taken together, these studies provide
confirm that eCO2 modifies the levels of defence hormones and provides
the first evidence that SL and GA3 are important regulators of plant
responses to phloem-feeding insects.