Expansion of cultured Zinnia mesophyll cells in response to hormones and light

Mesophyll suspension cultures of Zinnia elegans L. have been used extensively to investigate the development of tracheary elements. Here we have modified the culture conditions to promote cell expansion and inhibit tracheary element differentiation and cell division. Cell expansion, measured by computer image analysis, was stimulated by auxin ( α ‐naphthyleneacetic acid), cytokinin (N 6 ‐benzylaminopurine), gibberellic acid, brassinosteroid (24‐epibrassinolide), and light, all of which are known to promote cell expansion in whole plants or excised organs. Whereas light stimulated cell expansion primarily during the first 48 h of culture, auxin, cytokinin, gibberellic acid and brassinosteroid had little effect until after 48 h. Treatments also differed in their relative effects on cell elongation and radial cell expansion. Light and cytokinin had a greater effect on radial cell expansion, auxin and epibrassinolide promoted only cell elongation, and gibberellic acid had nearly equal effects on expansion in both directions. We have also shown by combining treatments that the effects of cytokinin and auxin are additive. Neither hormone treatment, however, was additive with the effect of light treatment. Finally, in contrast to xylogenic cultures where expansion occurs by tip growth, cell expansion in non‐differentiating cells was due to diffuse growth. These data show that cell expansion can be induced by hormones in primary mesophyll cultures from Zinnia in contrast to serially transferred plant suspension cultures. Furthermore, they indicate that auxin, cytokinin, and light induce cell expansion by different mechanisms in these cultures.