Developmental restrictions on transcription: determinants of the developmental program and their role in aging

A developing plant system, the soybean hypocotyl has been used to investigate early transcription events which restrict auxin induced cellular proliferation to the appropriate developmental stage. Auxin treatment of 4-day old seedlings resulted in an early (6 hour) activation of chromatin-bound RNA polymerase activity wihich approached 200% of control values by 18 hours. This occurred without a detectable alteration in chromatin template capacity (assayed with exogenous RNA polymerase) and resulted in the synthesis of "induced RNA transcripts" as determined in vitro by nearest neighbor analysis. In contrast, auxin treatment of unresponsive 8-day old seedlings did not alter the chromatin-bound RNA polymerase activity. Hormonal activation did, however, result in the exposure of "induced template" regions in chromatin which could only be transcribed in vitro if exogenous RNA polymerase was included in the transcription reaction. Isoelectric focusing of the endogenous chromatin-bound and soluble RNA polymerase enzymes from successive developmental stages revealed that the chromatin-bound enzymes at the 2-day stage were first released from the chromatin complex and could be recovered in the soluble pool (4-day stage). This was followed by a gradual disappearance of these subspecies (6-day stage) until only a limited ensemble of RNA polymerase subspecies remained bound to chromatin and free in the soluble pool (8-day stage). Similar analyses of both the bound and free enzymes at the 4 and 8-day stages following auxin treatment revealed that the 4-day soluble enzymes could be induced to rebind to the chromatin complex in a defined sequence after hormone treatment while those of the 8-day hypocotyl were unable to do so. These developmental events indicate that the select loss of certain RNA polymerase subspecies serves to restrict the hormone responsivness of this tissue to the early developmental stages. Such restrictions could thus commit the constituent hypocotyl cells to their terminal post-mitotic phase of development.