Endophytic Methylobacterium spp. were inoculated on citrus seed and evaluated for their ability to promote growth of Citrus limonia and Citrus sunki seedlings under commercial nursery conditions. The germination rate and seedlings growth differed according to the combination between Methylobacterium species and citrus rootstock, showing that the interaction depends on their compatibility. Methylobacterium had no effect on germination of both rootstocks, except AR 1.6/2 that reduced the germination of C. limonia. On the other hand, some strains from citrus significantly promoted biomass production and height of aerial part of both rootstocks. The pathway of Indole-3-acetic acid (IAA) biosynthesis was identified in M. mesophilicum SR1.6/6 genome and this ability was confirmed in culture medium, suggesting that this mechanism is probably involved in growth promotion observed in present study. Recovery of strains in culture medium and ARDRA analysis confirmed the endophytic colonization of rootstocks by Methylobacterium. Our analyses of C. limonia and C. sunki seeds revealed that Methylobacterium is not vertically transferred to citrus plants. These results suggest that Methylobacterium can endophytically colonize the plant and have a potential for plant growth promotion under commercial nursery conditions. However, this growth promoting effect depends on specific interactions between Methylobacterium and citrus species.
Key words: Methylobacterium, Citrus, plant-bacteria interaction, growth promotion, Indole-3-acetic acid (IAA) pathway.
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