THE INFECTION OF PERIDERM BY ZOOSPORES OF PHYTOPHTHORA CINNAMOMI: A HISTOLOGICAL DETECTIVE STORY.
THE INFECTION OF PERIDERM BY ZOOSPORES OF PHYTOPHTHORA CINNAMOMI: A HISTOLOGICAL DETECTIVE STORY.E O'GAR-A 1 , JA McCOMB 1 , and GESTJ HARDY 1
1 Murdoch University, Perth 6150, Western Australia.
Background and Objectives
In woody plants the periderm provides a formidable barrier to mechanical damage, and only a few fungal pathogens have been shown to directly penetrate the suberised and/or lignified phellem cells. Recent fmdings have shown that when the non-wounded peridenn of Eucalyptus marginata Oarrah) seedlings are exposed to motile zoospores of P. cinnamomi, infection  and ultimately seedling death may occur . The current histological study was undertaken to determine how zoospores of P. cinnamomi circumvent the peridenn.
Materials and Methods
Jarrah seedlings between 6 and 18 months old were inoculated with an axenic suspension of motile zoospores using the method described in . Seedlings were harvested up to 48 hours after inoculation, and were prepared for light (LM), fluorescence (FM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using standard techniques.
Results and Conclusions
SEM examination of inoculated tissue revealed that zoospores were attracted to jarrah tissue at sites of bud emergence, such as in leaf axils or on the lignotuber. The attraction of the pathogen was characterised by concentrated numbers of zoospores bound at these sites, all with genn-tubes orientated directly toward the plant. In contrast, zoospores bound randomly and with no particular orientation to periderin and green stem (tissue with epidermis and cuticle). Zoospores appeared to get trapped in the rough topography of the periderm. Zoospores did not show any attraction to stomata or lenticels.
LM and FM examinations revealed that P. cinnamomi invaded and colonised jarrah stems at the sites where zoospores were attracted. In a 48 hour period, the pathogen penetrated, extensively colonised and macerated the thin-walled meristematic tissue of the emerging buds, and moved into the connecting vascular tissue. The susceptibility of the buds to infection was dependent on the stage of emergence at the time of inoculation. Although zoospores showed no attraction to stomata, FM revealed that zoospores will penetrate through stomata, if binding occurs in close proximity.
Germ-tubes from randomly bound zoospores were occasionally observed to grow between the thin-walled suberised phellem cells. Although, hyphae were never observed to directly penetrate suberised phellem cells, they were occasionally observed within the cells. The direct penetration of the thick-walled lignified cells by hyphae of randomly bound zoospores was observed in TEM.
In conclusion, this study presents new evidence about the mechanisms employed by P. cinnamomi to circumvent the peridenn. It has been demonstrated that: the buds in the lignotuber and lower stem are the Achilles heel of jarrah seedlings; P. cinnamomi can directly penetrate the lignified phellem; and the pathogen will grow between the suberised phellem cells
1. O'Gara E, Hardy GESTJ, McComb JA, 1996. Plant Pathology 45, 955-63.