Thesieve element occlusiongene family in dicotyledonous plants

Abstract

Sieve element occlusion (SEO) genes encoding forisome subunits have been identified in Medicago truncatula and other legumes. Forisomes are structural phloem proteins uniquely found in Fabaceae sieve elements. They undergo a reversible conformational change after wounding, from a condensed to a dispersed state, thereby blocking sieve tube translocation and preventing the loss of photoassimilates. Recently, we identified SEO genes in several non-Fabaceae plants (lacking forisomes) and concluded that they most probably encode conventional non-forisome P-proteins. Molecular and phylogenetic analysis of the SEO gene family has identified domains that are characteristic for SEO proteins. Here, we extended our phylogenetic analysis by including additional SEO genes from several diverse species based on recently published genomic data. Our results strengthen the original assumption that SEO genes seem to be widespread in dicotyledonous angiosperms, and further underline the divergent evolution of SEO genes within the Fabaceae.

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

“…In contrast, peptides from P-protein plugs isolated from fascicular phloem tissue led to the identification of an uncharacterized fragment of a protein with a proposed role in wound sealing (22). These peptide sequences helped us identify the protein as CmSEO1, which we included in our analysis to confirm our hypothesis that the widespread SEO gene family encodes subunits of conventional P-proteins in most angiosperms, including cucurbits (28,35). Given its scarcity in sugar-transporting fascicular phloem and its abundance in extrafascicular phloem, PP1 is unlikely to belong to this group of conventional P-proteins.…”

“…The sequence similarity indicates that forisomes and conventional P-proteins have a common evolutionary origin (28,35). This notion is supported by several shared structural characteristics, such as their fibrillar composition (9,56), their ultrastructure in the condensed and dispersed states (5,57), their spatiotemporal expression profiles (24,58), and their cellular function, although forisomes are unique in that the sealing of sieve plates is reversible (29).…”

“…The SEO gene family originally was restricted to genes encoding forisomes but now also includes genes from non-Fabaceae plants, which do not possess forisomes. Because forisomes and conventional P-proteins have several characteristics in common, we previously speculated that the SEO gene family also encodes these widespread nonforisome P-proteins (28,35). We therefore used previously published SEO sequences as BLAST queries to search for homologous sequences in tobacco, and this search revealed several matching ESTs.…”

“…The presence of all three domains is required for inclusion in the SEO family because the individual domains can be present in otherwise unrelated proteins. We recently identified members of the SEO gene family in several non-Fabaceae dicotyledonous angiosperms, indicating that the SEO family is widespread and additionally encodes conventional nonforisome P-proteins (28,35).…”

Uncertain role of MtSEO-F3 in assembly ofMedicago truncatulaforisomes

“…5,6 The group 1 SEO proteins are only present in papilionoid legumes and these represent forisome subunits, whereas members of groups 5 and 6 represent the common P-proteins in most dicot species that probably cause irreversible sieve element occlusion. [5][6][7][8] In our recent study, we expressed group 1 SEO-F genes from different legumes in a heterologous background to study their impact on forisome assembly, including the DpSEO-F1 gene from the basal papilionoid species D. panamensis. 9 The D. panamensis lineage diverged from lineages of the Old World clade, i.e., Canavalia gladiata, Lotus japonicus, and Medicago truncatula, more than 50 million years ago.…”