SILENCING OF A BETA-1,3-GLUCANASE TRANSGENE IS OVERCOME DURING SEED FORMATION

Expression of a beta-1,3-glucanase transgene (gn1) driven by the CaMV 35S promoter is silenced in the T17 homozygous tobacco transgenic line . This silencing process is post-transcriptionally regulated and subje ct to developmental control. We have examined this phenomenon to inves tigate the developmental pathways involved in suppression and reactiva tion of gn1 expression as well as to identify the plant tissues where these processes occur. Analysis of beta-1,3-glucanase activity and gen e expression have allowed us to determine that suppression of gn1 is a very efficient process reducing the steady-state gn1 mRNA level, simu ltaneously, in all leaves of the plant. Gene silencing occurs a few we eks after seed germination, and is maintained throughout Vegetative gr owth and floral development. Expression of gn1 is restored in the matu ring fruit some time after fertilization. In situ hybridization analys es show that expression of gn1 is restored within the developing seeds in tissues derived from meiotically divided cells. In contrast to the high level of expression found in seedlings obtained from germinated T17 homozygous seeds, the expression of gn1 is not reactivated in plan tlets regenerated in vitro from leaf explants of suppressed T17 homozy gous plants that is, in plant tissues obtained by mitotic division. Th us, reactivation of gn1 expression specifically occurs along the devel opmental programme controlling sexual reproduction and likely througho ut epigenetic modifications affecting the state of gene expression dur ing meiosis.