توصيفگر ها :
سوختگي باكتريايي برگ پياز , كروناتين , ژنهاي خانه دار , P. syringae pv. alliifistulosi
چكيده انگليسي :
Abstract
In recent years, a type of bacterial leaf blight disease has emerged in the onion (Allium sp.) crops under center pivot irrigation in the Tirran, Damesh, Daran, and Semirom regions of Isfahan, which has caused significant damage to the production of this crop. Initial symptoms of the disease appear as white, spindle-shaped spots on the leaves. As the disease progresses, these spots initially become water-soaked and then necrotic, ultimately merging to create a complete blight of the leaves. Through separate cultivation of bacterial ooze from the white spots (A), necrosis (B), and leaf blight (C) of each of the 57 collected onion samples, a gram-negative bacterium was predominantly isolated, displaying creamy-colored, spindle-shaped, non-fluorescent colonies with slow growth on Nutrient Agar and King’s B media. The bacterial isolates obtained from each type of lesion in all samples exhibited identical DNA amplification patterns in genetic fingerprinting tests using ERIC and BOX, with no observed differences among them. In pathogenicity tests, selected representatives from these bacterial isolates exhibited white spots on onion leaves and onion seedlings over two weeks, which progressively developed into necrotic areas. The similarity of the nucleotide sequence in the 16S rRNA region, the coronatine-producing gene (cfl), and the house-keeping genes gyrB and rpoD in the genus Pseudomonas indicated that these bacterial isolates are over 99% similar to P. syringae pv. alliifistulosi and P. coronafaciens, which are reported as causal agents of leaf blight and yellow bud disease in onions, respectively. The phylogenetic tree drawn based on the housekeeping genes gyrB, rpoD, and glt1 showed that the bacterial isolates responsible for onion blight in Isfahan formed a branch with P. syringae pv. alliifistulosi, in proximity to the strains of P. coronafaciens and P. syringae pv. tremae. In further experiments, the hrpZ group IV primers amplified a 780 nucleotide fragment in the bacterial isolates from Isfahan, indicating their affiliation with group IV of P. syringae. Since P. syringae pv. tremae is recognized as an indicator of group V and has not been reported as a pathogen in onion family plants, the affiliation of the Isfahan isolates to P. syringae pv. tremae can be ruled out. Additionally, given the differences in symptoms on onion leaves (yellowing caused by P. syringae pv. coronafaciens versus leaf blight in onions from Isfahan), the ability of P.coronafaciens to produce fluorescein and hydrolyze esculin, contrary to the non-fluorescent and esculin-hydrolyzing incapacity of the Isfahan isolates, as well as the presence of the pCOR1 plasmid in the bacteria causing yellow bud disease and its absence in the Isfahan bacterial isolates, the initial hypothesis implicating P. syringae pv. coronafaciens in the occurrence of leaf blight disease in Isfahan province was dispelled. The 100% similarity of the Isfahan bacterial isolates with P. syringae pv. alliifistulosi, their placement in a single branch of the phylogenetic tree, the affiliation of both bacteria to group IV of P. syringae, the presence of the coronatine gene in both groups, and the amplification of 230 bp DNA fragment (100% homology) in the Isfahan bacterial isolates with specific primer pairs recommended for identifying P. syringae pv. alliifistulosi confirm that identifying the causal agent of bacterial blight in onion leaves in Isfahan as P. syringae pv. alliifistulosi is rational. However, the fluorescence and avirulence of P. syringae pv. alliifistulosi on cereal plants contrasted with the non-fluorescence and pathogenic capabilities of Isfahan blight bacterial isolates on wheat, barley, rye, and oats indicate that there are differences that require further investigation.
Keywords: Bacterial leaf blight of onion, Coronatine, Housekeeping genes, P. syringae pv. alliifistulosi
