BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. TRANSPORT IN PLANTS

13.8. Translocation of organic substances in the phloem

13.8.5. First-aid mechanisms: a possible function of sieve plates, P-protein and plastids

Tissue damage by herbivorous animals poses a serious threat to plants. If a sieve tube is ruptured, the leakage of high-calorie substances such as sucrose will lead to significant energy losses. Typically, damaged sieve tube elements are isolated from the rest of the transport system within minutes by callose deposits on the sieve plates, which plug their pores. This is likely another important function of sieve plates. It is believed that P-protein serves as an additional first-aid mechanism, blocking the sieve pores of damaged elements even more rapidly. Cutting a sieve tube causes a pressure drop that triggers a sharp influx of its contents toward the injury site, including insoluble matter. Rushing through the pores, this material automatically clogs them, thereby halting any further leakage of assimilates.