Genomic evolutionary relationship of SWEET genes and their responses to HLB disease and oxytetracycline treatment in Valencia sweet orange

As a novel characteristic sugar transporter group, the sugars-will-eventually-be-exported-transporters (SWEETs) play important roles in plant growth and development and responses to biotic and abiotic stresses. A comprehensive analysis of SWEETs has been performed in a large number of plant species, but has not been done in citrus. In this study, a total of 27 citrus SWEET genes (CitSWEETs) were identified by homology search, then which expression patterns in response to Huanglongbing (HLB) desease and oxytetracycline (OTC) were analyzed. Our data revealed that the expressions of CitSWEET1 and CitSWEET5 were suppressed in severely HLB-infected leaves, whereas CitSWEET2, CitSWEET3, CitSWEET4, CitSWEET7, CitSWEET8, CitSWEET9, CitSWEET10, CitSWEET12, CitSWEET13, CitSWEET14, CitSWEET15, CitSWEET18, CitSWEET20, CitSWEET21, CitSWEET22, CitSWEET23, CitSWEET24, CitSWEET25 and CitSWEET27 expressions were induced. At 30 d after OTC trunk injection, the expressions of 11 CitSWEET genes, especially CitSWEET2, were strongly inhibited. In addition, genetic relationships of the CitSWEET genes in sweet oranges (Citrus sinesis) and clementines (Citrus clementina) were also analyzed. Taken together, our work provided a basis for breeding with molecular resistance to HLB disease.