Phytopathology. 1998 Mar;88(3):234-44. doi: 10.1094/PHYTO.1998.88.3.234.
ABSTRACT The influence exerted by Pythium group F (a minor pathogen ubiquitous in soilless cultures) and P. uncinulatum (a nonpathogenic species) colonization on tomato roots was investigated. In both interactions, infected roots did not exhibit obvious symptoms; however, major physiological changes occurred within the host tissues colonized by Pythium group F compared to P. uncinulatum. According to our cytological observations, Pythium group F colonization involved a series of events: first, development and growth of the fungus in the epidermis and outer cortex tissues, which was associated with marked host cell disorganization and even breakdown. In colonized roots, symptoms were not easily discernible because alterations were restricted to the epidermis and outer cortex tissues. Second, pathogen ingress in the inner cortex and stele tissues was associated with massive induction of host defense reactions and alteration of invading hyphae. In a complex interaction that involved major metabolic changes in root cells, an array of defense-related reactions was produced, as exemplified by the formation of wall appositions and plugging of host cells with osmiophilic, electron-dense, granular, or fibrillar materials. P. uncinulatum growth was restricted to the epidermis and outer cortex tissues and associated with relatively minor damage to host cells. P. uncinulatum colonization of root tissues did not result in defense events similar to those observed in Pythium group F-infected inner cortex and stele tissues. Moreover, most of the invading P. uncinulatum hyphae were moribund. The results of the current study indicated that symptomless infections can be insignificant in terms of pathology for the plant (i.e., P. uncinulatum) or can be potentially dangerous (i.e., Pythium group F). Our results clearly showed a striking difference between nonpathogenic and minor pathogenic Pythium spp. attacks. The description of the Pythium group F infection process suggests that this interaction is unique, because the fungus behaves as a necrotrophic fungus in the epidermis and outer cortex tissues, whereas it is a potential inducer of plant defense reactions in the inner cortex and stele tissues. To our knowledge, cytological events similar to those observed with Pythium group F have not been described until now, even though descriptions of these events could help elucidate several aspects of the relationships between plants and minor pathogens.
本研究调查了腐霉组 F（一种在无土栽培中普遍存在的次要病原体）和无钩腐霉（一种非致病性物种）定植对番茄根系的影响。在这两种相互作用中，受感染的根未表现出明显的症状；然而，与无钩腐霉相比，腐霉组 F 定植的宿主组织发生了重大的生理变化。根据我们的细胞学观察，腐霉组 F 的定植涉及一系列事件：首先，真菌在表皮和外皮层组织中发育和生长，这与宿主细胞严重紊乱甚至崩溃有关。在定植的根中，由于改变仅限于表皮和外皮层组织，因此不易察觉症状。其次，病原体进入内皮层和中柱组织与宿主防御反应的大量诱导以及侵入菌丝的改变有关。在涉及根细胞中主要代谢变化的复杂相互作用中，产生了一系列防御相关反应，例如细胞壁的附加物形成和用亲脂性、电子致密、颗粒状或纤维状物质堵塞宿主细胞。无钩腐霉的生长仅限于表皮和外皮层组织，对宿主细胞的损伤相对较小。无钩腐霉对根组织的定植不会导致与腐霉组 F 感染的内皮层和中柱组织中观察到的防御事件相似的事件。此外，大多数侵入的无钩腐霉菌丝处于濒死状态。本研究的结果表明，无症状感染在病理学上对植物（即无钩腐霉）可能意义不大，或者可能具有潜在危险（即腐霉组 F）。我们的研究结果清楚地表明，非致病性和弱致病性腐霉属的攻击之间存在显著差异。腐霉组 F 感染过程的描述表明，这种相互作用是独特的，因为真菌在表皮和外皮层组织中表现为坏死真菌，而在中皮层和中柱组织中则是植物防御反应的潜在诱导剂。据我们所知，迄今为止，尚未描述与腐霉组 F 观察到的类似的细胞学事件，尽管这些事件的描述有助于阐明植物与弱病原体之间关系的几个方面。
