Alain Toppan

Chitin oligosaccharides as elicitors of chitinase activity in melon plants

Chitin oligosaccharides elicited chitinase activity in melon plants. Hexamer to nonamer were the most efficient elicitors: hexamer for maximal stimulation of colorimetrically assessed chitinase activity, heptamer for maximal stimulation of radiochemically assessed chitinase activity. Chitinase elicitation was a rapid response to these elicitors: it occurred within 6 hours after treatment and was maximal at 12-24 hours. In addition, chitinase induction in melon plants by these oligosaccharides was both local and systemic.

Cell surfaces in plant micro-organism interactions. VIII. Increased proteinase inhibitor activity in melon plants in response to infection by Colletotrichum lagenarium or to treatment with an elicitor fraction from this fungus

Proteinase inhibitor activity had increased sharply in melon seedlings infected by Colletotrichum lagenarium 3 days after inoculation. The activity was associated with heat stable proteins and was effective against the protease produced by the fungus as well as against trypsin. Treatment of healthy melon leaves with an elicitor of ethylene isolated from the pathogen, resulted in a three-fold increase in proteinase inhibitor activity after 24 h. Ethylene production increased early in elicitor-treated leaves and may be involved in the elicitation of proteinase inhibitor activity. In the presence of aminoethoxyvinylglycine, an inhibitor of ethylene biosynthesis, both elicitor-induced ethylene and, to a lesser extent, elicitor-induced proteinase inhibitor activity were inhibited. In contrast, 1-aminocyclopropane-1-carboxylic acid, the direct precursor of ethylene, triggers proteinase inhibitor activity. It is concluded that ethylene is involved in the elicitation of proteinase inhibitor activity, but its exact role remains to be defined.

Systemic induction of chitinase activity and resistance in melon plants upon fungal infection or elicitor treatment

Melon plants locally infected with Colletotrichum lagenarium display a marked increase in chitinase activities (exo- and endo-activities) throughout the whole plant. This increase begins 3 days after inoculation in the inoculated cotyledon, and then occurs sequentially in the non-infected tissues. Both fungal elicitors and plant endogenous elicitors induce a rapid increase in chitinase activity in the treated cotyledon. In other organs, chitinase activity is stimulated, to a lesser extent and after a lag period, only by fungal elicitors. The earlier, more rapid, systemic induction of chitinase activity, produced by treatment with the fungal elicitor is correlated by the increased resistance of the tissues to infection by the pathogen.

An improved approach for the accurate determination of fungal pathogens in diseased plants

Abstract A simple and accurate method for the determination of the mass of chitin-containing fungal pathogens in plants is reported. It is based upon the release of glucosamine from fungal chitin by hydrochloric acid hydrolysis. In order to obtain reliable results, four steps were needed: (1) isolation of the chitin-containing fraction from an infected plant, i.e. the cell wall fraction, (2) selection of a suitable hydrolysis, (3) purification of the hydrolysates and (4) glucosamine determination by a specific method. This procedure was then applied, during the course of infection, to different organs (stems, leaves and cotyledons) of melon seedlings inoculated with Colletotrichum lagenarium. The fungal growth curves in stems and leaves were then compared to the curves of proteolytic activity that developed in the infected stems and leaves at the same time. It is shown that this method enables us to correlate biochemical events appearing in plants after infection with the fungal mass they contain.

Structural organization of str 246C and str 246N, plant defense-related genes from Nicotiana tabacum

We have previously identified a cDNA clone, pNt246, whose corresponding transcripts accumulate in leaves in response to inoculation by compatible and incompatible isolates of the phytopathogenic bacterium Pseudomonas solanacearum [19]. We now describe the nucleotide sequence of a genomic clone, str 246C, corresponding to this cDNA species, and of a related genomic clone, str 246N, which appears to be a pseudogene with a 5′-end deletion. The nucleotide sequence of the str 246C gene was found to be identical to that of the parA gene, previously shown to be regulated by auxin [28, 29]. Upstream of the str 246N gene, sequences homologous to a Bam HI repetitive element described in Vicia faba [15] are present within an ORF showing significant homologies to an integrase-encoding gene of several retroviruses. This observation indicates that this highly repetitive DNA originates from sequences present in transposable mobile elements.

Elicitors and Ethylene Trigger Defense Responses in Plants

Infected plants often produce large amounts of ethylene. The possible role of this hormone in plant-microorganism interaction is poorly understood. We have previously demonstrated that ethylene protects melon plants against Colletotrichum lagenarium, the causal agent of anthracnose (1). This has led to the hypothesis that ethylene could be a message which triggers defense responses in plants. The increased synthesis of proteins associated to defense mechanisms during plant-microorganism interactions, and their elicitation by ethylene, or via ethylene by fungal elicitors, are successively reported. The 3 proteins considered in this study are cell wall hydroxyproline-rich glycoprotein (HRGP, which strengthens the plant cell wall), chitinase (which digests chitin-containing fungal cell wall), and proteolytic inhibitors (which neutralize the trypsin-like proteases of plant pathogens).

Hydroxyproline-Rich Glycoproteins in the Cell Wall of Diseased Plants as a Defense Mechanism

The plant cell wall may be considered as a barrier which separates a protoplast from the surrounding medium. Yet it is not a passive barrier, as indicated by active modifications under natural and stress physiology processes such as growth, maturation, ripening, abscission, senescence, and plant-microorganism interactions. This activity results, in part, from the presence of proteins and glycoproteins inside the cell wall. Although already mentioned sixty years ago(l), cell wall proteins were mainly studied after the discovery in the 1960’s (2), that some of these proteins contain high amounts of hydroxyproline (Hyp). This paper is concerned with the study of these Hyp-containing proteins that we will abbreviate, according to their chemical nature, i.e. HRGP (Hydroxyproline Rich Glycoproteins). The following points will be successively considered: 1 - Hydroxyproline-containing components in plants: their occurrence and structural features; 2 - Hydroxyproline-rich glycoproteins in plant-pathogen interactions; 3 - Significance of the accumulation of hydroxyproline-rich glyco-proteins in diseased plants; 4 - Role of cell surface interaction in the elicitation, via ethylene, of hydroxyproline-rich glycoprotein biosynthesis.