Auxin-regulated gene expression

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Abstract
During the 1960s a wide range of studies provided an information base that led to the suggestion that auxin-regulated cell processes - especially cell elongation - may be mediated by auxin-regulated gene expression. Indirect evidence from our work, based on the influence of inhibitors of RNA synthesis (e.g. actinomycin D) and of protein synthesis (e.g. cycloheximide) on auxin-induced cell elongation, coupled with correlations of the influence of auxin on RNA synthesis and cell elongation, provided the basis for this suggestion. With the availability of techniques for DNA-DNA and DNA-RNA hybridization, mRNA isolation-translation, in vitro 2D gel analysis of the translation products, and ultimately the cloning by recombinant DNA technologies of genomic DNA and copy DNAs (cDNAs) made to poly(A) + mRNAs, we and others have provided direct evidence for the influence of auxin on the expression of a few genes (i.e. poly(A) + RNA levels). Our laboratory has provided evidence for auxin’s both down-regulating and up-regulating the level of a few poly (A) + mRNAs out of a population of about 4 x 10 4 sequences that are not significantly affected by auxin. In our studies on auxin-regulated cell elongation, two cDNA clones (pJCW1 and pJCW2) were isolated which corresponded to poly (A) + mRNAs that responded during growth transitions in a way consistent with a potential role of their protein products in cell elongation. These mRNAs are most abundant in the elongating zone of the soybean hypocotyl. Upon excision and incubation in the absence of auxin, these mRNAs deplete in concert with a decreasing rate of cell elongation. Addition of auxin to the medium results in both increased levels of these mRNAs and enhanced rates of cell elongation. These mRNAs do not deplete if auxin is added to the medium at the onset of excised incubation, and cell elongation rates remain high. We have isolated and sequenced genomic clones that are homologous to these cDNAs. Of the two genes sequenced, both genes are members of small multigene families. There are regions of high amino acid homology even though the nucleotide sequences are sufficiently different in these regions for cross-hybridization of the clones not to be observed. More recently others, especially Guilfoyle’s laboratory, have shown that auxin selectively and rapidly influences the level of certain mRNAs and proteins. We have worked on other gene systems such as ribosomal proteins and possible cell wall proteins that are responsive to auxin; again the nature of regulation of expression of these genes is not known. The question now is not whether auxin selectively alters gene expression and thus the biology of responsive tissues, but what the mechanism is by which auxin alters expression of specific genes. Additionally, the function of these gene products remains a ‘mystery’. Fortunately, experimental approaches are available to answer these questions and are actively being pursued.