Bean plants contain contractile protein bodies (forisomes) which change their shape in response to calcium (released during stress). In reaction to wounding, forisomes block the sieve plates (structures in the stems which regulate fluid transport) by changing their shape and acting as a 'plug'. The dispersion of the forisome protein prevents fluids from leaking out of the plant (otherwise your plants would 'bleed' to death when you mow your lawn!).
Aphids overcome this blockage by injecting their saliva into the plant. The forisome's shape sensitivity to concentrations of calcium is being investigated further as these proteins could potentially be applied in nanomotor systems and also in the provision of nutrients in microfluidity systems for medical research. Currently, attempts are being made to sequence the forisome proteins involved in this reversible dispersion/contraction by Dr. Michael Knoblauch's group.
Vetch aphids feed off the sieve tubes and are a common infestation to plants, transferring harmful viruses. The reaction of the aphids to the change in fluid flow was investigated with the use of an electrical circuit. When plugging occurred at the sieve plates feeding aphids reacted by producing more saliva. The saliva must contain substances that cause contraction of the forisome protein back to its original state and thus restoring food supplies by clearing the sieve plates.