Abstract
SWEET/MtN3/saliva genes are prevalent in cellular organisms and play diverse roles in plants. These genes are widely considered as evolutionarily conserved genes, which is inconsistent with their extensive expansion and functional diversity. In this study, SWEET genes were identified from 31 representative plant species, and exhibited remarkable expansion and diversification ranging from aquatic to land plants. Duplication detection indicated that the sharp increase in the number of SWEET genes in higher plants was largely due to tandem and segmental duplication, under purifying selection. In addition, phylogeny reconstruction of SWEET genes was performed using the maximum-likelihood (ML) method; the genes were grouped into four clades, and further classified into 10 monocot and 11 dicot subfamilies. Furthermore, selection pressure of SWEET genes in different subfamilies was investigated via different strategies (classical and Bayesian maximum likelihood (Datamonkey/PAML)). The average dN/dS for each group were lower than one, indicating purifying selection. Individual positive selection sites were detected within 4 of the 21 sub-families by both two methods, including two monocot subfamilies in Clade III, harboring five rice SWEET homologs characterized to confer resistance to rice bacterial blight disease. Finally, we traced evolutionary fate of SWEET genes in clade III for functional characterization in future.
Highlights
The Sugars Will Eventually Be Exported Transporters (SWEET) gene family, is ubiquitous in plants, and plays diverse physiological and biological roles[1,2,3,4,5,6,7]
SWEET genes were detected in unicellular aquatic algae, which was indicative of its ancient origin and functional conservation
Most of the SWEET genes were observed in the legume plant, G. max and the rosids plant E. grandis, which harbored 53 and 52 SWEET homologs, respectively
Summary
The Sugars Will Eventually Be Exported Transporters (SWEET) gene family, is ubiquitous in plants, and plays diverse physiological and biological roles[1,2,3,4,5,6,7]. Members of the MtN3/saliva gene family have been predicted or characterized to be involved in various physiological processes in plants[1,2,3,4,5,6,7]. Sucrose, which is the predominant type of fixed carbon transported in plants[9,10], is synthesized in mesophyll cells, imported into phloem cells and subsequently transported to heterotrophic “sinks” (meristems, roots, flowers, and seeds) In this “phloem loading” process, sucrose is first effluxed from phloem parenchyma cells by SWEETs and loaded into the sieve element-companion cell complex (SE/CC) via active proton-coupled sucrose transporters (SUTs)[11,12]. Phylogenetic reconstruction and molecular evolution analyses of SWEET genes revealed their evolutionary genetic basis
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