In mammals, TGF-beta signaling plays fundamental roles in cell proliferation, differentiation and apoptosis, and it is also required for normal growth and development in C.elegans. Mutations in
daf-8, which encodes an R-Smad transcription factor, result in constitutive dauer arrest. Previous genetic studies showed that the
daf-8 and
daf-14 Smads inhibit dauer formation, whereas the
daf-3 Smad promotes it. However, the functional relationship of these three proteins has not been described. DAF-8 is strongly associated with itself and weakly with DAF-14 and DAF-3 in vivo and in vitro. DAF-3 represses transcription of
daf-8 and
daf-7 (the gene encoding the TGF-beta-like ligand required for non-dauer development), but not
daf-14 or the genes encoding the type I and type II TGF-beta receptors. Expression of
daf-7 and
daf-8 was up-regulated in
daf-3 mutants and repressed by over-expression of
daf-3. Also, DAF-3 was able to bind the regulatory regions of
daf-7 and
daf-8. This suggests a novel feedback regulation mechanism for TGFbeta signaling in C. elegans that has not been reported in other systems.We examined specific effects of TGF-beta signaling on development. In a transgenic line carrying an integrated
daf-8::gfp translational fusion, we detected GFP from pre-comma embryos through larval stages and adults. In the head, the strongest expression was in ASI and ADL amphid neurons. DAF-8 was also expressed in the phasmid neurons, distal tip cells, gut cells and the excretory cell. The stably integrated
daf-8::gfp transgene conveyed a dauer-defective phenotype and suppressed dauer formation in
daf-8 and
daf-14 mutants. This suppression was reversed by
daf-8 RNAi treatment. In
daf-8 mutants, the mitotic region of the gonad was enlarged compared to that of wild type, and the number of actively mitotic cells was increased, suggesting a modulatory role for
daf-8 in germ line mitosis. The expression of
lag-2, which is required for maintaining mitotic activity near the distal end of the gonad, was increased in
daf-8 mutants and suppressed by overexpression of
daf-8. The expression pattern of
daf-8 and its regulation suggest a cell-autonomous function of TGF-beta signaling in ASI neurons for rapid decision to form dauers under adverse conditions.