Ethylene Biosynthesis and Xylogenesis inLactucaPith Explants CulturedIn Vitroin the Presence of Auxin and Cytokinin: The Effect of Ethylene Precursors and Inhibitors

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The possible involvement of ethylene in the induction of xylem differentiation was studied in lettuce (Lactuca sativa L. cv. Romaine) pith parenchyma expiants. The addition of the ethylene precursors L-methionine (0-25 //M), S-adenosylmethionine (25 /rM) and 1-aminocyclopropane-l-carboxylic acid (0-01 /?M), or the ethylene-releasing agent 2-chloroethylphosphonic acid (1-0 //M), to a standard IAA-kinetin-containing medium enhanced xylogenesis compared to control expiants cultured in the absence of these compounds. In the presence of the ethylene inhibitors aminoethoxyvinylglycine, Co(N03)2 and AgN03, xylogenesis was inhibited. Inhibition of xylogenesis by aminoethoxy vinylglycine (75 /?M), Co(N03)2 (50 //M) and AgN03 (6-0 //M) was reversed by exogenous 1-aminocyclopropane-l-carboxylic acid (0-01 //M), 2-chloroethylphosphonic acid (5-0 ,uM) and L-methionine (0-25 //M), respectively. Ethylene production by expiants cultured on media containing L-methionine or 1-aminocyclopropane-l-carboxylic acid was greater than the biosynthesis of ethylene by expiants cultured in the absence of these compounds. The incorporation of 2-chloroethyl phosphonic acid into the culture medium resulted in higher rates of ethylene production compared to expiants cultured on the IAA-kinetin medium. The presence of either aminoethoxyvinylglycine or Co(N03)2 inhibited ethylene production by expiants cultured on the IAA-kinetin medium. The data support the hypothesis that ethylene plays a positive role in the initiation of xylem differentiation.