Studies on Auxin Protectors X. Protector Levels and Lignification in Sunflower Crown Gall Tissue

Abstract

AbstractThe lignification and differentiation of phloem fibers in sunflower stems is inhibited by growing crown gall tumors. Crown gall tumor tissue has previously been shown to contain large quantities of auxin protectors. Since auxin protectors are antioxidants which inhibit peroxidase‐catalyzed reactions, and since the formation of lignin is known to involve a peroxidase‐catalyzed reaction, an investigation was undertaken to examine the relationship between auxin protectors and lignification in sunflower crown gall tissue.Sunflower crown gall tissue placed into media low in mineral content, rapidly lignifies. In the low mineral media, protectors appear in the medium within an hour or two, implying that endogenously‐synthesized protectors rapidly leak out of the tissue. In control media, the tissue neither lignified appreciably, nor did it exhibit an excessive amount of protector release. The addition of Ca2+ to the low mineral medium markedly slowed, but did not entirely prevent lignification; similarly Ca2+ markedly slowed the release of protector into the low mineral medium. Auxin protectors added to the low mineral medium did not inhibit lignification apparently because, in the medium, the protectors are rapidly oxidized to quinones. The addition of catechol, a substance which mimics protector, also failed to inhibit lignification and also formed a colored compound in the medium suggesting o‐qui none formation. In contrast, dithiothreitol, a strong anti‐oxidant which upon oxidation does not form a strong oxidant (such as o‐quinone), when added to the low mineral medium does inhibit lignification.It is suggested that in the in vitro situation lignification and senescence occurs in low mineral media because the protectors leak out rapidly causing the cell's metabolism to favor peroxidase‐catalyzed oxidations including those leading to lignification, while in the in vivo situation the excess protectors produced by crown gall tumor tissue diffuse into surrounding tissue, maintaining a reduced state in such tissues and thereby inhibiting differentiation and lignification. The synthesis of large quantities of protectors by the tumor tissue therefore could account for the anaplasia of the bundle caps observed in sunflower internodes in the vicinity of growing crown gall tumors.