J. Gen. Plant Pathol. 66: 281 (2000)

Abstracts of the Research by the Winners of the Young Scientist Award

Role of Plant Cell Walls in Determining Plant-pathogen Specificity'

Akinori KIBA*p* *

The mechanism for determining plant-pathogen specificity is one of the most intriguing issues for plant pathologists. By using a model system with pea and an elicitor and suppressor from a pea pathogen, Mycosphaere- lla pinodes, we analyzed the mechanism for the percep- tion of fungal signal molecules by plants. Generally, fungal signal molecules such as elicitors are thought to be recognized by receptors in the plasma membrane. How- ever, the specific suppressor from M. pinodes even nonspecifically inhibited ATPase activity in plasma mem- branes prepared from non-host plants. Then, we focused on plant cell walls, which are specific to plants and are the most exterior organelle.

1. Specific response of cell wall-bound ATPase to fungal suppressor

The cell wall-bound ATPase differed from that in the plasma membrane in several properties. The elicitor nonspecifically enhanced the activity of the cell wall- bound ATPase from several leguminous plants. Inversely, the suppressor inhibited only the ATPase activity in pea cell wall, but enhanced the activity in nonhosts. These results suggested that receptors for fungal signals may be closely associated with cell wall-bound ATPase.

2. The plant cell wall plays an important role in early defense responses against pathogens

We found a peroxidase-catalyzing 02--generating sys- tem in protein fractions solubilized from cell walls of pea and cowpea. The O,--generation in the fractions was regulated by fungal signals, especially by the suppressor, in a species-specific manner. These results suggested that the plant cell wall may play an important role not only in recognition of fungal signals, but also in the early defense response.

3. Signal transduction of fungal signals via

In animal cells, an interaction between the plasma membrane and extracellular matrix via the RGD sequence is involved in signal transmission. Integrin- and vitronectin-like proteins were found in the pea plasma membrane and cell wall, respectively. The hexapeptide GRGDSP inhibited either the binding of cell wall protein(s) to plasma membrane protein(s) or the induc- tion of defense-related gene expression and phytoalexin production. Therefore, an integrin-mediated connection between cell wall and plasma membrane may play an important role in signal transduction for defense responses.

integrin-like protein

4. Molecular cloning and functional analysis of the fungal signal-binding protein gene

A cDNA encoding the fungal signal-binding protein (FSBP) contained four apyrase conserved regions like those found in apyrases and NTPases in animal and plant cells. The recombinant FSBP, which was over expressed in E. coli, had the activity of NTPase. Its ATPase activity was regulated by both the elicitor and suppressor from M. pinodes. Moreover, the recombinant FSBP specifically bound to these fungal signals. These results indicated that the receptor for the elicitor and suppressor from M. pinodes may be the cell wall-bound NTPase itself.

Together with previous data, these findings indicate that the cell wall may contain the real receptors for fungal signals, regulate downstream of signal transduction cas- cade, evoke defense responses itself and therefore deter- mine host-pathogen specificity.

I extend special thanks to Drs. T. Shiraishi, T. Yamada, Y. Ichinose and K. Toyoda, Okayama University, for their contin- uous support and valuable suggestions throughout the course of this study.

t Abstract of the paper given by the winner of the Young Scientist Award at the 2000 Annual Meeting of the Phytopathological Society of Japan in Okayama

* Laboratory of Plant Pathology and Genetic Engineering, College of Agriculture, Okayama University, Okayama 700-8530, Japan

* * Present address : Iwate Biotechnology Research Center, Kitakami, Iwate 024-0003, Japan