Slack Suzanne M, Zeng Quan, Outwater Cory A, Sundin George W
All authors: Department of Plant, Soil, and Microbial Sciences, Michigan State University, East Lansing 48824; and second author: Department of Plant Pathology and Ecology, Connecticut Agricultural Experiment Station, New Haven 06504.
Phytopathology. 2017 Apr;107(4):403-411. doi: 10.1094/PHYTO-09-16-0352-R. Epub 2017 Feb 28.
Fire blight, caused by the pathogen Erwinia amylovora, is the most devastating bacterial disease of pome fruit in North America and worldwide. The primary method of dispersal for E. amylovora is through ooze, a mass of exopolysaccharides and bacterial cells that is exuded as droplets from infected host tissue. During the 2013 and 2014 field seasons, 317 ooze droplets were collected from field-inoculated apple trees. Populations of E. amylovora in ooze droplets were 10 CFU/μl on average. Ooze droplets harboring larger (>10 CFU/μl) cell populations were typically smaller in total volume and had darker coloring, such as orange, red, or dark red hues. Examination of apple host tissue at the site of emergence of ooze droplets using scanning electron microscopy revealed that ooze was not exuding through natural openings; instead, it was found on erumpent mounds and small (10-μm) tears in tissue. These observations suggested that E. amylovora-induced wounds in tissue provided the exit holes for ooze extrusion from the host. Analyses of E. amylovora populations in ooze droplets and within the stems from which ooze droplets emerged indicated that approximately 9% of the total bacterial population from infected stems is diverted to ooze. Gene expression analyses indicated that E. amylovora cells in stem sections located above ooze droplets and in ooze droplets were actively expressing critical pathogenicity genes such as hrpL, dspE, and amsK. Thus, our study identified ooze as a source of large, concentrated populations of E. amylovora that emerged from the host by rupturing host tissue. Because the cells in ooze droplets are expressing genes required for pathogenesis, they are already primed for infection should they be dispersed from ooze to new infection courts.
火疫病由病原菌梨火疫欧文氏菌引起，是北美乃至全球最具毁灭性的仁果类细菌性病害。梨火疫欧文氏菌的主要传播方式是通过菌脓，菌脓是一团胞外多糖和细菌细胞，以液滴形式从受感染的宿主组织中渗出。在2013年和2014年的田间季节，从田间接种的苹果树上收集了317个菌脓液滴。菌脓液滴中梨火疫欧文氏菌的数量平均为10 CFU/μl。携带较大细胞群体（>10 CFU/μl）的菌脓液滴总体积通常较小，颜色较深，如橙色、红色或深红色。使用扫描电子显微镜检查菌脓液滴出现部位的苹果宿主组织发现，菌脓不是通过自然开口渗出的；相反，它出现在组织中的隆起丘和小（10μm）伤口处。这些观察结果表明，梨火疫欧文氏菌在组织中诱导的伤口为菌脓从宿主中挤出提供了出口孔。对菌脓液滴和产生菌脓液滴的茎干内的梨火疫欧文氏菌群体分析表明，受感染茎干中约9%的细菌总数转移到了菌脓中。基因表达分析表明，位于菌脓液滴上方的茎段和菌脓液滴中的梨火疫欧文氏菌细胞正在积极表达关键致病基因，如hrpL、dspE和amsK。因此，我们的研究确定菌脓是梨火疫欧文氏菌大量集中群体的来源，这些群体通过破坏宿主组织从宿主中出现。由于菌脓液滴中的细胞正在表达致病所需的基因，它们一旦从菌脓中分散到新的感染部位就已经准备好进行感染。
