Abstract
The Tomato mosaic virus (ToMV) resistance gene Tm-1 encodes a direct inhibitor of ToMV RNA replication to protect tomato from infection. The plant Tm-1-like (Tm-1L) protein is predicted to contain an uncharacterized N-terminal UPF0261 domain and a C-terminal TIM-barrel signal transduction (TBST) domain. Homologous searches revealed that proteins containing both of these two domains are mainly present in charophyte green algae and land plants but absent from glaucophytes, red algae and chlorophyte green algae. Although Tm-1 homologs are widely present in bacteria, archaea and fungi, UPF0261- and TBST-domain-containing proteins are generally encoded by different genes in these linages. A co-evolution analysis also suggested a putative interaction between UPF0261- and TBST-domain-containing proteins. Phylogenetic analyses based on homologs of these two domains revealed that plants have acquired UPF0261- and TBST-domain-encoding genes through two independent horizontal gene transfer (HGT) events before the origin of land plants from charophytes. Subsequently, gene fusion occurred between these two horizontally acquired genes and resulted in the origin of the Tm-1L gene in streptophytes. Our results demonstrate a novel evolutionary mechanism through which the recipient organism may acquire genes with functional interaction through two different HGT events and further fuse them into one functional gene.
Highlights
Green plants, known as Viridiplantae, consist of green algae (Chlorophyta and Streptophyta) and land plants
Experimental evidence has revealed that the Tm-1 N-terminal region was responsible for its inhibitory activity, and a C-terminally truncated Tm-1 protein still possesses inhibitory activity[6,7]. These findings suggest that the full-length tomato Tm-1 has a function other than Tomato mosaic virus (ToMV) resistance[11]
BLAST searches revealed that homologs of the tomato Tm-1 gene are present in various streptophyte groups, including charophyte algae, bryophytes, lycophytes and seed plants
Summary
Known as Viridiplantae, consist of green algae (Chlorophyta and Streptophyta) and land plants (embryophytes). From peaceably aquatic surroundings to harshly terrestrial environments, the ancestor of land plants had to face significant stresses, including dryness, ultraviolet radiation and pathogenic microorganisms other than those in water. During their colonization of land, plants gradually evolved new genes and phenotypic novelties to adapt to and radiate in terrestrial environments[2,3]. Experimental evidence has revealed that the Tm-1 N-terminal region was responsible for its inhibitory activity, and a C-terminally truncated Tm-1 protein still possesses inhibitory activity[6,7] These findings suggest that the full-length tomato Tm-1 has a function other than ToMV resistance[11]. We tried to obtain more insights into the origin of the plant Tm-1-like (Tm-1L) gene by performing an extensive search of its homologs in current sequence databases and by analyzing their phylogeny
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