لخّصلي

Introduction

The necrotrophic fungus Pyrenophora tritici-repentis (Died.)

Drechs.However, to our knowledge, the in planta production of cate-

narin by parasitic fungi has not been demonstrated.Moreover, in order to

clarify the possible ecological and physiological roles of catenarin

produced by P. tritici-repentis, the phytotoxic and antimicrobial

activities of the pigment were assessed, respectively, on wheat and

a wide range of microorganisms associated with the pathogen

during its saprophytic and parasitic phases of growthIn some wheat production

areas, tan spot often occurs as part of a leaf-spotting complex on

wheat, in conjunction with Phaeosphaeria nodorum (E. Mu?ller)

Hedjaroude (anamorph: Stagonospora nodorum (Berk.) Castellani

and E.G. Germano), Mycosphaerella graminicola (Fuckel) J. Schro?t.The latter condition is also associated with other fungal and

bacterial pathogens, including Alternaria alternata (Fr.:Fr.) Keissl.,

Fusarium spp., Aspergillus spp., Cladosporium spp., and Pseudomonas

syringae pv. atrofaciens (McCulloch) Young et al. [11].It was previously hypothesized that the reddish discoloration of

wheat kernels associated with red smudge results from the secre-

tion and accumulation of catenarin [14], an anthraquinone pro-

duced by some isolates of P. tritici-repentis [14], as well as other

fungi [15-19] and some plants [20].In the absence of

a suitable host, P. tritici-repentis can survive saprophytically on

wheat debris on the soil surface, and the incidence of tan spot has

increased in recent years with the adoption of conservation tillage

practices that retain crop residues [4,5].The bioactivity of catenarin also suggests that in

addition to inhibiting the growth of microorganisms associated

with P. tritici-repentis during its saprophytic phase [14], the pig-

ment suppresses parasites accompanying the tan spot pathogen as

part of the wheat leaf-spotting complex.Evidence that

the pigment is produced in wheat kernels infected by P. tritici-

repentis would support the hypothesis [14] that catenarin causes

the red smudge symptom, and would also allow for a better as-

sessment of the impact of this pathogen on seed health, processing

characteristics, and food safety.On susceptible

host genotypes, P. tritici-repentis differentially induces combina-

tions of tan necrosis and/or extensive chlorosis.in

Cohn (anamorph: Septoria tritici Roberge in Desmaz.), and

Cochliobolus sativus (Ito and Kuribayashi) Drechs.

Introduction

The necrotrophic fungus Pyrenophora tritici-repentis (Died.)

Drechs. (anamorph: Drechslera tritici-repentis (Died.) Shoem.),

causal agent of tan spot, is an important foliar pathogen of wheat.

Yield losses attributed to tan spot range from 3 to 50% on the

central plains of the U.S.A. and Canada [1–3]. In the absence of

a suitable host, P. tritici-repentis can survive saprophytically on

wheat debris on the soil surface, and the incidence of tan spot has

increased in recent years with the adoption of conservation tillage

practices that retain crop residues [4,5]. In some wheat production

areas, tan spot often occurs as part of a leaf-spotting complex on

wheat, in conjunction with Phaeosphaeria nodorum (E. Mu¨ller)

Hedjaroude (anamorph: Stagonospora nodorum (Berk.) Castellani

and E.G. Germano), Mycosphaerella graminicola (Fuckel) J. Schro¨t. in

Cohn (anamorph: Septoria tritici Roberge in Desmaz.), and

Cochliobolus sativus (Ito and Kuribayashi) Drechs. ex Dastur (ana-

morph: Bipolaris sorokiniana (Sacc.) Shoem.) [6]. On susceptible

host genotypes, P. tritici-repentis differentially induces combina-

tions of tan necrosis and/or extensive chlorosis. Accordingly, iso-

lates of the fungus have been classified into eight races based on

their virulence patterns on specific cultivars/lines of a differential

set [7–10].

In addition to foliar symptoms, P. tritici-repentis can also infect

wheat kernels, causing a red discoloration known as red smudge, or

a blackening of the germ end of affected seeds termed black point

[11]. The latter condition is also associated with other fungal and

bacterial pathogens, including Alternaria alternata (Fr.:Fr.) Keissl.,

Fusarium spp., Aspergillus spp., Cladosporium spp., and Pseudomonas

syringae pv. atrofaciens (McCulloch) Young et al. [11]. Red smudge is

commonly found in Kenya and Australia, but occurs less frequently

in North America [12]. Nevertheless, all wheat cultivars registered

in western Canada are susceptible to infection of the kernels [13].

It was previously hypothesized that the reddish discoloration of

wheat kernels associated with red smudge results from the secre-

tion and accumulation of catenarin [14], an anthraquinone pro-

duced by some isolates of P. tritici-repentis [14], as well as other

fungi [15–19] and some plants [20]. Anthraquinones (octaketides)

form the largest group of the quinone class of natural products [21],

and are pigments that were traditionally used as dyes [19]. This

class of compounds includes many 9,10-anthraquinones that ex-

hibit a wide spectrum of biological activities. Catenarin in particular

inhibits DNA-dependent RNA polymerase [18], protein kinase C and

protein tyrosine kinases [22]. The pigment has also been shown to

inhibit the incorporation of certain nucleotides and amino acids by

bacteria [18]. Accordingly, Wakulin´ ski et al. [14] suggested that catenarin also plays an important role in the life strategy of P. tritici-

repentis, inhibiting the growth of microorganisms accompanying

the fungus during its saprophytic phase of development.

However, to our knowledge, the in planta production of cate-

narin by parasitic fungi has not been demonstrated. Evidence that

the pigment is produced in wheat kernels infected by P. tritici-

repentis would support the hypothesis [14] that catenarin causes

the red smudge symptom, and would also allow for a better as-

sessment of the impact of this pathogen on seed health, processing

characteristics, and food safety. Also of concern is the possible oc-

currence of related anthraquinones, especially the diarrheagenic

and genotoxic mycotoxin emodin [23], in infected kernels, given

that their in vitro production by P. tritici-repentis has been dem-

onstrated [14]. The bioactivity of catenarin also suggests that in

addition to inhibiting the growth of microorganisms associated

with P. tritici-repentis during its saprophytic phase [14], the pig-

ment suppresses parasites accompanying the tan spot pathogen as

part of the wheat leaf-spotting complex. If this is the case, catenarin

would be expected to occur in wheat leaves infected by P. tritici-

repentis. Finally, the phytotoxicity of catenarin to wheat has not

been evaluated, although given its bioactivity, it may contribute to

development of non-specific foliar disease symptoms.

The primary objectives of this study were to determine whether

catenarin and related anthraquinones such as emodin are produced

in planta by P. tritici-repentis, and to evaluate whether this pro-

duction occurs both in kernels and leaves. Moreover, in order to

clarify the possible ecological and physiological roles of catenarin

produced by P. tritici-repentis, the phytotoxic and antimicrobial

activities of the pigment were assessed, respectively, on wheat and

a wide range of microorganisms associated with the pathogen

during its saprophytic and parasitic phases of growth