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Figure 1. 

Phylogenetic relationships of the SWEET family genes in Arabidopsis, Oryza sativa, Populus, Vitis vinifera, and Morus alba. The sequences of the 104 SWEET proteins from the above four plant species were aligned by Clustal Omega, and the phylogenetic tree was constructed by the MEGA 11.0 using the NJ method with 1000 bootstrap replicates. The proteins from Arabidopsis, Oryza sativa, Populus, Vitis vinifera, and Morus alba are indicated with the prefixes of At, Os, Pt, Vv, and Ma, respectively.

Figure 2. 

Distribution of MaSWEET genes in Morus alba chromosomes. The tandem gene pairs are linked by red lines. The block duplications gene pairs are marked by black lines. The scale is provided in megabase (Mb).

Figure 3. 

Multiple sequence alignment of MaSWEET proteins. The positions of the TMHs are underlined. The positions of the active sites of tyrosine (Y) and aspartic acid (D) are indicated by red triangles. The conserved serine (S) phosphorylation sites are indicated by blue triangles.

Figure 4. 

Gene organization of MaSWEETs and cis-elements in promoter regions of MaSWEETs. (a) Phylogenetic tree using 24 MaSWEETs. (b) Exon/intron structures of Morus alba L. SWEETs. (c) Cis-element distribution in the promoter regions of MaSWEETs.

Figure 5. 

Transcript levels of MaSWEETs in leaves, xylem, phloem, and different development stages of fruit. Three technical replicates were analyzed. Error bars represent SE. Different letters indicate statistically significant differences (Duncan's test, p < 0.05).

Figure 6. 

Expression levels of 24 selected MaSWEET genes in response to fungi stress conditions. Three technical replicates were analyzed. Error bars represent SE. Asterisks indicate significant difference as determined by Student’s t-test (* p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001).

Figure 7. 

Expression levels of 24 selected MaSWEET genes in response to 4 °C, 40 °C , drought, and flood stress conditions. Three technical replicates were analyzed. Error bars represent SE. Asterisks indicate significant difference as determined by Student’s t-test (* p < 0.05 ; ** p < 0.01 ; *** p < 0.001 ; **** p < 0.0001 ).

Figure 8. 

Functional characterization of MaSWEET1a/b in tobacco and mulberry. (a) Damage of tobacco overexpressing MaSWEET1a after infection by C. shiraiana. (b) Expression levels of MaSWEET1a in tobacco. MaSWEET1a-T1, T2, T3 are three independent treatments with transient overexpressed MaSWEET1a. (c) Damage of mulberry with knock-down MaSWEET1a/b after infection by C. shiraiana. (d) The expression level of MaSWEET1a/b in mulberry. MaSWEET1a/b-T1, T3, T4 are three independent treatments with down-regulation of MaSWEET1a/b using VIGS.