Characterization of the auxin-regulated par gene from tobacco mesophyll protoplasts

The auxin-regulated par gene from tobacco mesophyll protoplasts was characterized in detail to deduce its possible function. An homology search of the par gene in the NBRF databases revealed that the pargene has homology to the stringent starvation protein (ssp) gene of Escherichia coli, which is induced under starved conditions and binds in an equimolar ratio to a holo-enzyme of RNA polymerase. Hence, it is supposed that the par gene product could play a similar role to that of ssp. Although sequence homology of the pargene to the Gmhsp 26-A gene from soybean was observed, both genes were shown to respond differently to plant hormones and stresses. Gmhsp 26-A is induced by heat shock, 2,4-dichlorophenoxyacetic acid (2,4-D), cytokinin and abscisic acid (ABA), whereas the par gene was induced only by auxins. Furthermore, cycloheximide treatment prevents 2,4-D-mediated accumulation of Gmhsp 26-A mRNA, but not that of par mRNA. Both par and Gmhsp 26-A respond to CdCl₂, but splicing of the par pre-mRNA proceeded in a normal way, whereas splicing off the Gmhsp 26-A pre-mRNA was inhibited. Hence, the par and Gmhsp 26-A genes should have a common ancestor, but have evolved in different directions. Detailed time-course experiments confirmed that the pargene was induced immediately after the addition of auxin and expressed upon the initiation of meristematic activity in tobacco mesophyll protoplasts. As the par gene was induced by the sole treatment of cycloheximide, it was proposed that the par gene belongs to a category of 'superinduction' genes. Furthermore, the par gene belongs to a small gene family and is expressed in roots of a whole plant body.