MATERIALS AND METHODSEthylene production and cell wail hydroxyproline-rich glycoprotein (HRGP) biosynthesis are greatly enhanced in melon (Cucumin melo cv.Cantaloup charentais) seedlings infected with CoUetotrichum lagenariwm.Short-term experiments performed in the presence of specific inhibitors of the ethylene pathway from methionine, namely L-canaline and aminoethoxyvinylglycine, indicate that under non-toxic conditions, both ethylene and I'4Clhydroxyproline deposition in the cell wall of infected tissues are significantly lowered. On the contrary, treatment of healthy tissues with 1-aminocyclopropane 1-carboxylic acid, a natural precursor of ethylene, stimulates both the production of the hormone and the incorporation of I'Clhydroxyproline into cell wall proteins.The data provide the first evidence of the in vivo effect of ethylene on the cell wall hydroxyproline-rich glycoprotein biosynthesis in plants.It has been reported in previous papers (10, 11) that melon seedlings respond to a fungal attack by the accumulation of HRGP2 in their cell walls. Special attention has been paid to this modification of the cell surface for it involves glycoproteins, a class of components often mediating cell to cell interactions (5). Availability of plants with higher or lower than normal amounts of cell wall hydroxyproline allowed us to ascertain that the enrichment of diseased plants in HRGP is closely associated to their defense against microorganisms (9). Indirect evidence suggests a role for ethylene in the regulation of this mechanism. When supplied exogenously to intact plants, this hormone promotes an enrichment of the cell wall in hydroxyproline (24) corresponding to an enrichment in HRGP (9). A similar hydroxyproline response occurs after wounding of plant tissues (7), a process otherwise well-known to stimulate the production ofethylene (13). Inasmuch as large amounts of this hormone are often released by infected plants (1,22) Patil and Tang (21), and then diluted to appropriate concentrations with 50 mm phosphate buffer (pH 6.0). AVG and ACC were each dissolved and adjusted to I x lo-3 M in 50 mm phosphate buffer (pH 6.0). Appropriate dilutions were then made with the same buffer.Ethylene Measurement. The production of ethylene was measured from excised seedlings, petiole of the first leaf, and from C. lagenarium, as follows. Four excised seedlings (without roots) were enclosed for 24 h in 570-ml serum flasks stopped with serum caps, and containing 30 ml of the usual growth medium (28). Newly excised seedlings were used everyday. The petioles were cut into 5-mm segments as previously indicated (9), and divided into lots of 2 g. The segments of each lot were incubated for 4 h under the light in 13 ml serum vials, stopped with serum vaccine caps, containing 6 ml of a medium consisting of 2% sucrose in 50 mm K-phosphate buffer (pH 6.0). A CO2 trap made of filter paper soaked with 200 td of 1 N KOH was inserted into each vial.Measurements of ethylene from C. lagenarium were performed on a 6-d-old culture i...