▪ Abstract Fungal plant pathogens have evolved diverse mechanisms for penetrating into host plant tissue, ranging from entry through natural plant openings to various mechanisms of direct penetration through the outer surface. The filamentous fungus Magnaporthe grisea can cause disease on many species of the grass (Poaceae) family. The disease on rice, Rice Blast, is of enormous economic importance and biological interest. The mechanism used by this pathogen for breaching the formidable host surface barriers has been studied cytologically and genetically as a model for plant pathology, and represents a remarkably sophisticated achievment of nature. The single-celled appressorium of M. grisea acts as a vessel for the generation and application of perhaps the highest turgor pressures known. The fungus requires and utilizes melanin-derived, osmotically generated pressures estimated at 80 bars to drive an actin-rich cellular protuberance through the surface of a rice leaf or plastic coverslip.

BREAKING AND ENTERING: Host Penetration by the Fungal Rice Blast Pathogen Magnaporthe grisea

Infection structures of phytopathogenic fungi are modified hyphae specialized for the invasion of plant tissues. Initial events are adhesion to the cuticle and directed growth of the germ tube on the plant surface. At the site of penetration, appressoria are often formed that may have melanized walls and develop high turgor pressure to support the penetration process. The penetration hypha accumulates components of the cytoskeleton in the tip and secretes a variety of cell wall‐degrading enzymes in a highly regulated fashion in order to penetrate the cuticle and the plant cell wall. This article reviews recent papers on the cytology, physiology, and molecular biology of the penetration process.

/pdf/morphogenesis-and-mechanisms-of-penetration-by-plant-4a74p0yaon.pdf

Morphogenesis and mechanisms of penetration by plant pathogenic fungi

Molecular genetic analysis of the rice blast fungus, magnaporthe grisea.

Cells tolerate exposure to cytotoxic compounds through the action of ATP-driven efflux pumps belonging to the ATP-binding cassette (ABC) superfamily of membrane transporters. Phytopathogenic fungi encounter toxic environments during plant invasion as a result of the plant defense response. Here we demonstrate the requirement for an ABC transporter during host infection by the fungal plant pathogen Magnaporthe grisea. The ABC1 gene was identified in an insertional mutagenesis screen for pathogenicity mutants. The ABC1 insertional mutant and a gene-replacement mutant arrest growth and die shortly after penetrating either rice or barley epidermal cells. The ABC1-encoded protein is similar to yeast ABC transporters implicated in multidrug resistance, and ABC1 gene transcripts are inducible by toxic drugs and a rice phytoalexin. However, abc1 mutants are not hypersensitive to antifungal compounds. The non-pathogenic, insertional mutation in ABC1 occurs in the promoter region and dramatically reduces transcript induction by metabolic poisons. These data strongly suggest that M.grisea requires the up-regulation of specific ABC transporters for pathogenesis; most likely to protect itself against plant defense mechanisms.

An ATP-driven efflux pump is a novel pathogenicity factor in rice blast disease.

Gene cloning of chitinase A1 from Bacillus circulans WL-12 revealed its evolutionary relationship to Serratia chitinase and to the type III homology units of fibronectin.

Inhibition of the biosynthesis of plant cell wall materials, especially cellulose biosynthesis, by coumarin

A survey of the toxin production by several strains of blast fungus was carried out. Among the strains tested only one strain, C1(THU 69-03) produced α-picolinic acid with good yield and other strains produced neither α-picolinic acid nor piricularin. On the other hand, tenuazonic acid and its Fe-cbelate compound were isolated as novel blast fungus toxins from the surface culture brothes of these strains, but they could not be isolated from the shaking culture brothes. The phytotoxic effect of tenuazonic acid on rice plants in regard to varietal difference was also studied.

https://www.tandfonline.com/doi/pdf/10.1080/00021369.1972.10860315

Investigation on the Toxin Production by Several Blast Fungus Strains and Isolation of Tenuazonic Acid as a Novel Toxin

Cell walls were prepared from Piricularia oryzae P2. Lytic enzymes were produced when Bacillus circulans WL 12 was grown with P. oryzae cell wall as a sole source of carbon. Mode of lysis of P. oryzae cell walls was compared with the lysis of walls of Aspergillus oryzae and Neurospora crassa. Activities of β-1→3-glucanase, chitinase, and β-1→6-glucanase were demonstrated in the culture filtrate. Modes of lysis of cell walls of P. oryzae P2 by a single and combined action of lytic enzymes were compared. Combination of β-1→3-glucanase and chitinase gave the most rapid and extensive lysis.

Cell walls of pirigularia oryzae

The occurence of tenuazonic acid (T. A.), which had been isolated from the culture broth of blast fungus, in blast-diseased rice plants was surveyed to ascertain whether or not this substance is one of the vivotoxins. T. A. was detected in four of six samples of blast-diseased rice plants, two of which had relatively high T. A. contents; 379 and 91mg per kg of the samples (dry weight). Besides T. A., coumarin, o-coumaric acid and piricularin were also isolated from blast-diseased rice plants. The molecular formula of the last substance, which was tentatively presented in a previous paper, was corrected to C18H30N2O5 from the results of high resolution mass spectrometry.

Kinjiro Tamari

Papers

Inhibition of the biosynthesis of plant cell wall materials, especially cellulose biosynthesis, by coumarin

Investigation on the Toxin Production by Several Blast Fungus Strains and Isolation of Tenuazonic Acid as a Novel Toxin

Cell walls of pirigularia oryzae

Toxins in Blast-diseased Rice Plants

β-1,4-Glucan occurring in homogenate of Phaseolus aureus seedlings. Possible nascent stage of cellulose biosynthesis in vivo.