Sieve element occlusion (SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

摘要

Higher plants transport sugar in their sap through tubes of end-to-end cells known as "phloem sieve elements." This process is called "translocation." When plants are injured, the sap might leak out, causing the loss of nutrients. We studied specialized phloem proteins (P-proteins) whose role is to block damaged sieve elements after injury. P-proteins are produced in most flowering plants, but their origin has been unclear. We carried out functional experiments and found conclusive proof that P-proteins are encoded by the sieve element occlusion (SEO) gene family and that the ability to block damaged sieve elements is lost if these proteins are absent. The SEO gene family originally was thought to encode a very specialized group of proteins called "forisomes," which are present only in leguminous plants and which expand and contract like a muscle to block sieve elements reversibly after injury (1, 2). Related genes have been found more recently in many nonleguminous plant species and are proposed to encode P-proteins, the phloem proteins that block sieve elements immediately after injury to prevent the loss of sap (3, 4). The main problem with this hypothesis is that it is based on the circumstantial evidence of similar expression profiles and genetic regulatory elements.