In plants, most sugar is transported in the phloem passing through units stacked the length of the plant called sieve tubes. Each sieve tube contains valves that can open or block flow of sugar. The valves are formed by proteins called “forisomes” that contract and block transport through the sieve tube in the presence of calcium and open when calcium concentration is low. These valves keep a plant from losing its liquid when it is cut. Cutting a plant stem causes sieve tube blockage on both sides of the cut. This is why lawn grass does not dehydrate when mowed.
Aphids have the ability to feed on the phloem tissue of plants. The stylets of their sucking mouthparts are inserted into a phloem sieve tube and the aphids ingest the sugary phloem. How do aphids prevent blockage of sieve tubes?
When stylets pierce a sieve tube, the aphid secretes a watery saliva that contains calcium chelating proteins. The secreted proteins bind to calcium inside the sieve tube and lower the concentration to levels at which the forisomes are open. These secretions allow aphids to feed continuously on phloem with interruption of flow by the plant’s defenses.