Cytokinins, as their name suggests, promote cytokinesis in tissues such as tobacco pith, carrot phloem, and soybean stems. Cytokinins also have been found to influence disease processes caused by certian pathogenic bacteria and fungi. Cytokinin production by nonpathogenic fungi and bacteria is thought to influence mutualistic relations with plants such as formation of mycorhizae and root nodules. (Salsibury and Ross, 1992)
Researchers looking for a cytokinin binding protein have found a binding protein but it does not bind with high enough specificity or affinity to account for the phsiological low concentration of cytokinin (0.01-1uM). Preliminary evidence has suggested that a barley leaf protein exists that binds zeatin ( a cytokinin) with high affinity (Romanov etal, 1988). More evidence from other species must be obtained to confirm this protien as common cytokinin binding protien. (Salisbury and Ross, 1992)
Looking at what inhibits cytokinins and what changes are caused by cytokinins suggests information on how this homone transduces a signal and where the cytokinin binding protein is located. Evidence for a cytosolic cytokinin receptor comes from:
1) cytokinin caused increase in polysome levels (large clusters of ribosomes)
2) cytokinin caused faster incorporation of radiolabled amino acid in proteins
3) inhibition of the physiological responses of cytokinin by inhibitors of protein sythesis.
All of this evidence suggests that cytokinin is active in the cytosol however other research gives evidence for cytokinin's involvment in transcription and mRNA stabilization. The blocking of cytokinin activity by inhibitors of RNA sythesis suggests a role for cytokinin in the nucleus. This nuclear involment does not seem supprising for a hormone that causes cytokinesis, although we can imagine the homone controlling gene activity at various places in the cell and points in the gene pathway (see figure 1).
Congradulations you finished! Now return to the introduction and pick another topic!