Abstract: Endophytic diazotrophs isolated from Hongjiang Orange were identified by 16S rRNA sequence analysis. The biodiversity and distribution of endophytic diazotroph in Hongjiang Orange were revealed by Shannon diversity index (H), relative frequency (RF) and similarity index. The differences of the distribution of endophytic diazotroph between the healthy plants and the diseased plants infected by citrus yellow shoot disease were compared. The results showed that the genera of endophytic diazotroph of Hongjiang Orange were abundant. A total of 72 strains were isolated from different tissues of Hongjiang Orange, which belonged to 3 phyla, 4 classes, 5 orders, 8 families, 12 genera. Bacillus spp., Curtobacterium spp., Pantoea spp. and Burkholderia spp. were the dominant endophytic diazotroph of Hongjiang Orange, with relative frequency of 36.11%, 13.89%, 13.89% and 9.72%, respectively. The total Shannon diversity index (H) of endophytic diazotroph was 2.01, and the total evenness index was 0.62. The similarity index between different parts of Hongjiang Orange was C_s(root-stem)=0.29, C_s(stem-leaf)=0.40, C_s(root-leaf)=0.20, respectively, indicating a low similarity of endophytic diazotroph in different parts. There were 9 genera of endophytic diazotroph in leaf, 6 genera in root, and only 1 genus in stem. There was little difference in Shannon diversity index (H_health=1.87 vs. H_disease=1.91) and evenness index (E_health=0.67 vs. E_disease=0.72) between healthy plants and diseased plants, while the relative frequency of two dominant endophytic azotobacter Curtobacterium (RF_health=9.38% vs. RF_disease=17.5%) and Pantoea (RF_health=21.88% vs. RF_disease=7.5%) changed greatly.

Key words: Hongjiang Orange, endophytic diazotroph, community diversity, Bacillus spp., Curtobacterium spp., Pantoea spp.