Cytokinins are involved in many plant processes, including cell division and shoot and root morphogenesis. They are known to regulate axillary bud growth and apical dominance. According to the "direct inhibition hypothesis", these effects result from the ratio of cytokinin to auxin. This theory states that auxin from apical buds travels down shoots to inhibit axillary bud growth. This promotes shoot growth, and restricts lateral branching. Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth. Simple experiments support this theory. When the apical bud is removed, the axillary buds are uninhibited, lateral growth increases, and plants become bushier. Applying auxin to the cut stem again inhibits lateral dominance. Cytokinins have recently been found to play a role in plant pathogenesis. For example, cytokinins have been described to induce resistance against
Pseudomonas syringae in
Arabidopsis thaliana and
Nicotiana tabacum. Also in context of
biological control of plant diseases cytokinins seem to have potential functions. Production of cytokinins by
Pseudomonas fluorescens G20-18 has been identified as a key determinant to efficiently control the infection of
A. thaliana with
P. syringae.. While cytokinin action in
vascular plants is described as
pleiotropic, this class of plant hormones specifically induces the transition from apical growth to growth via a three-faced apical cell in
moss protonema. This bud induction can be pinpointed to
differentiation of a specific single cell, and thus is a very specific effect of cytokinin. ==Mode of action==