In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 109, No. 28 ( 2012-07-10)
Kurzfassung:
Our present study settles a long and controversial debate about the origin of P-proteins, their relationship with the SEO gene family, and the evolution of this family in different groups of plants (including the previously described role of a different family of phloem proteins in squash). Of particular interest is our demonstration of the definitive relationship between P-proteins and forisomes, whose unique reactivity has been investigated in detail. This knowledge should lead to further insights into the functions of P-proteins. Our initial experiments showed that tobacco SEO genes are expressed at the same time and in the same place as native P-proteins and that the SEO proteins assemble into mixed complexes with native P-proteins. We did so by linking the SEO gene to an unrelated gene encoding a fluorescent protein to monitor the appearance of fluorescent P-protein complexes. These experiments showed that the SEO proteins were coincident with the P-proteins; however, these data were uninformative regarding SEO function. We next showed that SEO proteins expressed in nonphloem cells (i.e. cells lacking any other factors required for P-protein assembly) were fully capable of assembling into P-protein complexes with the same ultrastructure as native P-proteins. This result showed that SEO proteins are sufficient to form authentic P-protein structures. Finally, we used a technique called “RNAi” to block the expression of the endogenous SEO genes in tobacco, thereby leaving the phloem cells depleted of the corresponding proteins. These plants contained hardly any P-proteins when examined under the microscope. When they were injured, they exuded up to nine times more sugar from cut leaves than did corresponding wild-type plants cut in the same way. This outcome showed that the phloem leaked much more profusely from the plants inhibited by RNAi than from the wild-type plants, pointing to a loss of short-term sieve-element blocking activity. Thus, SEO genes are necessary to form functional P-proteins in plants. 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. We therefore set out to achieve a functional characterization of SEO proteins ( Fig. P1 ) and to confirm they are both necessary and sufficient to function as P-proteins. 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.
Materialart:
Online-Ressource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1202999109
Sprache:
Englisch
Verlag:
Proceedings of the National Academy of Sciences
Publikationsdatum:
2012
ZDB Id:
209104-5
ZDB Id:
1461794-8
SSG:
11
SSG:
12
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