Enhanced ethylene production and peroxidase activity in IAA-overproducing transgenic tobacco plants is associated with increased lignin content and altered lignin composition

Abstract

In a previous investigation, the lignin content of the xylem in the tobacco stem was shown to be greater in transgenic IAA-overproducing line C plants than in wild-type plants (Sitbon et al., Plant Physiol. 99 (1992) 1062–1069). Here, we confirm this observation and also show that the lignin composition in the transformants is altered, the ratio of syringyl to guaiacyl units being decreased due to an increase in guaiacyl units. Line C plants displayed an increased ethylene production in leaves and internodes, as well as a greater capacity to evolve ethylene in response to wounding and exogenous IAA. Line C plants also had greater peroxidase (POD) activity, whereas cinnamyl alcohol dehydrogenase and ß-glucosidase activities were similar in the two genotypes. The mRNA level of a tobacco anionic POD, previously associated with increased levels of lignin and related polyphenols when overexpressed in transgenic tobacco plants (Lagrimini, Plant Physiol. 96 (1991) 577–583), was increased in line C plants. It is suggested that the high IAA level in the transformants, through an induction of ethylene synthesis, increases POD activity and hence also lignin deposition.