The working group led by Dr. Jan Lohmann at the Max Planck Institute for Developmental Biology in Tübingen, Germany has now solved this problem. The object of investigation was Arabidopsis thaliana, the "favorite plant" for molecular and genetic research, whose genome was sequenced years ago. Lohmannâ?super>TMs team now carried out elaborate genetic and biochemical experimentation, and thereby identified four genes, which might serve as a mechanistic connection between plant hormones and the genetic regulatory elements in meristem.
The researchers in Tübingen used gene expression analysis to show that the genes ARR5, ARR6, ARR7 and ARR 15, "Arabidopsis Response Regulators", are subject to genetic regulation via the WUSCHEL gene. In particular, WUSCHEL restricts the activity of ARR7 in the apical meristem. The ARR genes in turn carry out a particularly important task in hormonal regulation: they are part of a negative feedback loop, by which the growth-inducing plant hormone cytokinin limits its own influence. The study shows that the ARR genes play a direct role in regulation of the stem cell pool.
The hormone itself instigates the meristematic stem cells to split; at the same time, it activates various ARR genes, which break the cytokinin signal chain. Jan Lohmann explains that "WUSCHEL supports the cytokinin effect by stopping its negative feedback." That is also the reason for earlier observations, that Arabidopsis samples with defective WUSCHEL genes only develop very small meristems, and have trouble growing. The researchers in Tübingen have now discovered the same effect in mutants whose ARR7 gene is constitutively active.
Cytokinin can only have its full growth-promoting effect in tissue in which the WUSCHEL regulatory gene is active. "Meristematic regulation is a fabulous example of how the effects of free circulating hormon
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Source:Max-Planck-Gesellschaft