Lignification and Xylogenesis inLactucaPith Explants CulturedIn Vitroin the Presence of Auxin and Cytokinin: A Role for Endogenous Ethylene

Abstract

Lignification and xylogenesis were studied in lettuce (Lactuca sativa L. cv. Romaine) pith parenchyma explants cultured on a Murashige and Skoog basal medium supplemented with indole-3-acetic acid, kinetin, and glucose. Lignin formation was observed to occur in two distinct phases, one preceding xylem differentiation (days 0–3 of culture) and another coincident with maximal xylogenesis (days 4–7). The rate of soluble phenolic production by these explants increased concomitant with the first phase of lignification, then decreased during the second phase. Addition of silver, an ethylene antagonist, to the culture medium inhibited the second phase of lignification and markedly reduced wall-bound peroxidase activity. Exogenous L-methionine, an ethylene precursor, completely reversed the inhibitory effect of silver on ligm6cation and wall-bound peroxidase activity. Silver increased phenylalanine ammonia-lyase activity, but had no effect on soluble phenolic production or soluble pcroxidase activity. These results suggest that ethylene may play a role in controlling lignification during xylogenesis by inducing wall-bound peroxidase activity.