Xu P L, Iwata M, Leong S, Sequeira L
Department of Plant Pathology, U.S. Department of Agriculture, Agricultural Research Station, Madison, Wisconsin.
J Bacteriol. 1990 Jul;172(7):3946-51. doi: 10.1128/jb.172.7.3946-3951.1990.
Extracellular polysaccharide (EPS) has long been regarded as one of the most important factors involved in wilting of plants by Pseudomonas solanacearum. By means of transposon Tn5 mutagenesis, we have isolated a class of mutants that have an afluidal colony morphology but retain the ability to cause severe wilting and death of tobacco plants. One such mutant, KD700, was studied in detail. By marker exchange mutagenesis, the altered colony morphology was shown to be the result of a single Tn5 insertion in a 14.3-kilobase EcoRI fragment. This defect could be corrected by introducing a homologous clone from a cosmid library of the wild-type, parental strain K60. The Tn5-containing fragment was introduced into other P. solanacearum wild-type strains by marker exchange, and these altered strains had the same afluidal phenotype as KD700. N-Acetylgalactosamine (GalNac), the major constituent of EPS of all wild-type strains of P. solanacearum, was not detected by gas chromatography-mass spectrometry analysis of vascular fluids from wilting plants infected by KD700. In contrast, GalNac was readily detected in similar fluids of plants infected by K60. Polysaccharides extracted from culture filtrates of KD700 contained approximately one-fifth of the GalNac present in polysaccharides from K60. No differences in growth rates in culture or in planta between the mutant and the parental strains were observed. Since strains that are deficient in EPS production can remain highly virulent to tobacco, we conclude that EPS, or at least its GalNac-containing component, may not be required for disease development by P. solanacearum.
胞外多糖（EPS）长期以来一直被视为青枯菌导致植物萎蔫的最重要因素之一。通过转座子Tn5诱变，我们分离出了一类具有无流动性菌落形态但仍保留导致烟草植株严重萎蔫和死亡能力的突变体。对其中一个这样的突变体KD700进行了详细研究。通过标记交换诱变，发现菌落形态的改变是由于单个Tn5插入到一个14.3千碱基的EcoRI片段中所致。通过引入来自野生型亲本菌株K60的黏粒文库的同源克隆，可以纠正这一缺陷。通过标记交换将含Tn5的片段引入其他青枯菌野生型菌株，这些改变后的菌株具有与KD700相同的无流动性表型。通过气相色谱 - 质谱分析感染KD700的萎蔫植物的维管液，未检测到青枯菌所有野生型菌株EPS的主要成分N - 乙酰半乳糖胺（GalNac）。相比之下，在感染K60的植物的类似液体中很容易检测到GalNac。从KD700培养滤液中提取的多糖所含的GalNac约为K60多糖中GalNac含量的五分之一。未观察到突变体与亲本菌株在培养物中或植物体内的生长速率有差异。由于缺乏EPS产生的菌株对烟草仍可能具有高毒性，我们得出结论，EPS，或至少其含GalNac的成分，可能不是青枯菌致病所必需的。
