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21 Auxin and Cytokinin in Arabidopsis

Mark Estelle, Harry J. Klee


The phytohormones auxin and cytokinin are believed to play a critical role in virtually every aspect of plant growth and development (Evans 1984; Davies 1987). At the cellular level, auxin acts by altering both cell elongation and cell division, whereas cytokinin appears to act primarily by stimulating cell division. Perhaps the most striking demonstration of the importance of these two compounds is their effect on cultured plant cells. In general, growth of cells in culture, either as callus on solid medium or as a cell suspension in liquid medium, requires exogenous cytokinin and auxin (Davies 1987). In addition, auxin and cytokinin have dramatic effects on in vitro organogenesis. Skoog and Miller (1957) were the first to demonstrate that increasing the ratio of cytokinin to auxin in the growth medium promoted shoot development from callus tissue, and decreasing this ratio promoted root development.

In the intact plant, the effects of auxin and cytokinin are exceedingly diverse (Davies 1987), and we do not attempt a comprehensive review in this article. One example of an auxin response is the rapid stimulation of cell elongation in excised stem segments (Brummell and Hall 1987). Numerous studies have shown that auxin-induced cell elongation is correlated with two biochemical responses: activation of a plasma membrane-localized proton-pumping ATPase and consequent acidification of the cell wall space (Brummell and Hall 1987) and the stimulation of transcription of specific genes (Key 1989). The exact function of these two responses in auxin-regulated growth is unclear, but there is increasing evidence that...

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