Molecular and phylogenetic characterization of the sieve element occlusion gene family in Fabaceae and non-Fabaceae plants

<p>Abstract</p> <p>Background</p> <p>The phloem of dicotyledonous plants contains specialized P-proteins (phloem proteins) that accumulate during sieve element differentiation and remain parietally associated with the cisternae of the endoplasmic reticulum in mature sieve elements. Wounding causes P-protein filaments to accumulate at the sieve plates and block the translocation of photosynthate. Specialized, spindle-shaped P-proteins known as forisomes that undergo reversible calcium-dependent conformational changes have evolved exclusively in the <it>Fabaceae</it>. Recently, the molecular characterization of three genes encoding forisome components in the model legume <it>Medicago truncatula </it>(<it>MtSEO1</it>, <it>MtSEO2 </it>and <it>MtSEO3</it>; SEO = sieve element occlusion) was reported, but little is known about the molecular characteristics of P-proteins in non-<it>Fabaceae</it>.</p> <p>Results</p> <p>We performed a comprehensive genome-wide comparative analysis by screening the <it>M. truncatula</it>, <it>Glycine max</it>, <it>Arabidopsis thaliana</it>, <it>Vitis vinifera </it>and <it>Solanum phureja </it>genomes, and a <it>Malus domestica </it>EST library for homologs of <it>MtSEO1</it>, <it>MtSEO2 </it>and <it>MtSEO3 </it>and identified numerous novel <it>SEO </it>genes in <it>Fabaceae </it>and even non-<it>Fabaceae </it>plants, which do not possess forisomes. Even in <it>Fabaceae </it>some <it>SEO </it>genes appear to not encode forisome components. All <it>SEO </it>genes have a similar exon-intron structure and are expressed predominantly in the phloem. Phylogenetic analysis revealed the presence of several subgroups with <it>Fabaceae</it>-specific subgroups containing all of the known as well as newly identified forisome component proteins. We constructed Hidden Markov Models that identified three conserved protein domains, which characterize SEO proteins when present in combination. In addition, one common and three subgroup specific protein motifs were found in the amino acid sequences of SEO proteins. <it>SEO </it>genes are organized in genomic clusters and the conserved synteny allowed us to identify several <it>M. truncatula </it>vs <it>G. max </it>orthologs as well as paralogs within the <it>G. max </it>genome.</p> <p>Conclusions</p> <p>The unexpected occurrence of forisome-like genes in non-<it>Fabaceae </it>plants may indicate that these proteins encode species-specific P-proteins, which is backed up by the phloem-specific expression profiles. The conservation of gene structure, the presence of specific motifs and domains and the genomic synteny argue for a common phylogenetic origin of forisomes and other P-proteins.</p

P-proteins in Arabidopsis are heteromeric structures involved in rapid sieve tube sealing

Structural phloem proteins (P-proteins) are characteristic components of the sieve elements in all dicotyledonous and many monocotyledonous angiosperms. Tobacco P-proteins were recently evidenced to be encoded by the widespread SEO gene family, and tobacco SEO proteins were shown to be directly involved in sieve tube sealing thus preventing the loss of photosynthate. Analysis of the two Arabidopsis SEO proteins (AtSEOa and AtSEOb) indicated that the corresponding P-protein subunits do not act in a redundant manner. However, there are still pending questions regarding the interaction properties and specific functions of AtSEOa and AtSEOb as well as the general function of structural P-proteins in Arabidopsis. In this study, we characterized the Arabidopsis P-proteins in more detail. We used in planta bimolecular fluorescence complementation assays to confirm the predicted heteromeric interactions between AtSEOa and AtSEOb. Arabidopsis mutants depleted for one or both AtSEO proteins lacked the typical P-protein structures normally found in sieve elements, underlining the identity of AtSEO proteins as P-proteins and furthermore providing the means to determine the role of Arabidopsis P-proteins in sieve tube sealing. We therefore developed an assay based on phloem exudation. Mutants with reduced AtSEO expression levels lost twice as much photosynthate following injury as comparable wild-type plants, confirming that Arabidopsis P-proteins are indeed involved in sieve tube sealing

The sieve element occlusion gene family in dicotyledonous plants

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