Deciphering evolutionary dynamics of SWEET genes in diverse plant lineages

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.

SWEET Gene Family in Medicago truncatula: Genome-Wide Identification, Expression and Substrate Specificity Analysis

“…In this study, we identified 25 SWEET genes in the genome of M. truncatula , and the number was comparable to that in various plant species, such as Arabidopsis (17 genes) [1,2], cucumber (17 genes) [5,35], rice (21 genes) [30], sorghum (23 genes) [34], banana (25 genes) [32], tomato (29 genes) [32], cabbage (30 genes) [43], potato (35 genes) [33], and rubber tree (36 genes) [36]. Gene duplication, including tandem and segmental duplication events, can be a crucial factor that affects plant genome evolution [56,57], and contributes to the expansion of SWEET gene family in various plant species [12,58]. For example, two and seven pairs of tandemly duplicated genes were detected in cucumber and cotton, respectively [35,59].…”

SWEET Transporters for the Nourishment of Embryonic Tissues during Maize Germination

“…According to Eom et al [35] and Li et al [36], 24 genes comprise the SWEET family in maize. All putative maize SWEET proteins possess two MtN3/saliva domains, and most are predicted to have the seven transmembrane helices typical of SWEET transporters, except for ZmSWEET4a and ZmSWEET6a, which are predicted to have six helices (Figure S4).…”

“…It has been reported that SWEET members are grouped in four clades [35,36,37]. Maize SWEETs are distributed among the four clades, but most fall into clade III, which also groups most of the SWEET transporters from A. thaliana and rice.…”

Role of transporters in plant disease resistance

“…Most of these genes went through a purifying selection during evolution, a process that selectively removes the deleterious alleles (Li et al, 2018). Besides, the significantly large number of SWEET genes in higher plants was attributed to the presence of segmental and tandem duplications in comparison to the lower land plants (Li et al, 2018). Based on the phylogenetic distribution, SWEET transporters are grouped into four clades (Chandran, 2015).…”

“…In sweet potato, Fusarium oxysporum inflicts huge commercial losses to the farmers. The SWEET gene IbSWEET10 conferred resistance to Fusarium wilt and characterized using RNAi (RNA interference) approach (Li et al, 2018). Besides bacterial and fungal pathogens, the role of SWEET genes in viral disease resistance has also been reported.…”