CIRAD, UMR BGPI, TA A-54/K, Montpellier Cedex 5 34398, France.
Open Biol. 2014 Feb 12;4(2):130116. doi: 10.1098/rsob.130116.
Xanthomonas albilineans, the causal agent of sugarcane leaf scald, is missing the Hrp type III secretion system that is used by many Gram-negative bacteria to colonize their host. Until now, this pathogen was considered as strictly limited to the xylem of sugarcane. We used confocal laser scanning microscopy, immunocytochemistry and transmission electron microscopy (TEM) to investigate the localization of X. albilineans in diseased sugarcane. Sugarcane plants were inoculated with strains of the pathogen labelled with a green fluorescent protein. Confocal microscopy observations of symptomatic leaves confirmed the presence of the pathogen in the protoxylem and metaxylem; however, X. albilineans was also observed in phloem, parenchyma and bulliform cells of the infected leaves. Similarly, vascular bundles of infected sugarcane stalks were invaded by X. albilineans. Surprisingly, the pathogen was also observed in apparently intact storage cells of the stalk and in intercellular spaces between these cells. Most of these observations made by confocal microscopy were confirmed by TEM. The pathogen exits the xylem following cell wall and middle lamellae degradation, thus creating openings to reach parenchyma cells. This is the first description of a plant pathogenic vascular bacterium invading apparently intact non-vascular plant tissues and multiplying in parenchyma cells.
黄单胞菌属白叶枯病菌，是引起甘蔗叶枯病的病原菌，缺少许多革兰氏阴性菌用于定殖宿主的 Hrp Ⅲ型分泌系统。到目前为止，这种病原体被认为严格局限于甘蔗的木质部。我们使用共聚焦激光扫描显微镜、免疫细胞化学和透射电子显微镜（TEM）来研究甘蔗叶枯病菌在患病甘蔗中的定位。用绿色荧光蛋白标记的病原菌菌株接种甘蔗植物。对有症状叶片的共焦显微镜观察证实了病原体存在于原木质部和后生木质部中；然而，也观察到黄单胞菌属白叶枯病菌存在于感染叶片的韧皮部、薄壁组织和泡状细胞中。同样，感染甘蔗茎的维管束也被黄单胞菌属白叶枯病菌入侵。令人惊讶的是，该病原体还存在于茎中明显完整的储存细胞和这些细胞之间的细胞间隙中。共聚焦显微镜观察到的大多数结果都得到了 TEM 的证实。病原体通过细胞壁和中层片的降解离开木质部，从而产生到达薄壁组织细胞的开口。这是首例描述一种植物病原血管细菌入侵明显完整的非血管植物组织并在薄壁组织细胞中繁殖的现象。
