Cytokinin induced mRNAs

Using differential screening, Tanaka et al. [34] found cDNAs representing cytokinin-induced mRNAs in the tobacco thin layer system. One cDNA encodes S-adenosyl-L-homocysteine hydrolase (SAH hydrolase), which is involved in regulating intracellular transmethylation reactions. The promoter sequence of the gene was fused to the 3-glucuronidase (GUS) reporter gene and introduced in suspension-cultured cells, which rendered expression of GUS inducible by kinetin. 

In most cases, a detectable induction of mRNA levels is observed within 15-30 min following auxin exposure. The levels of three soybean mRNAs (detected by the cDNA clones 6, lOA and 15) are induced by 2.5 min of treatment of excised elongating soybean hypocotyl sections with the synthetic auxin 2,4-D [8]. These mRNAs and others identified in soybean [5, 16], pea [13] and tobacco [14] are specifically induced by auxins such as lAA and NAA, and not induced by nonauxin analogs. With some exceptions, these auxin-induced sequences do not accumulate in response to other plant growth regulators such as GA, ABA, cytokinins and ethylene, or to environmental stresses such as heat shock or cold... 

The unexpected result of this work is that heat induced increases in the ipt mRNA (Figs. 2 and 3), but did not produce effective alterations in plant development. Preliminary analysis of cytokinin levels indicates that even without heat induction, the transgenic plants contain cytokinin levels 8-fold over that of control plants, while cytokinin levels in heat-induced plants is 60-170-fold over that found in control plants (Morgan, unpubl.). One explanation for these results is that the cytokinin levels after heat shock may be beyond the ability of the plant to respond. The supraoptimal cytokinin levels after hs may negate the developmental effects of cytokinins that were found at control temperatures. Our current work is focusing more closely on defining the optimal response times and processes. 

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