Signalling by TGFβ superfamily factors plays an important role in tissue growth 
and cell proliferation. In Drosophila, the activity of the TGFβ/Activin 
signalling branch has been linked to the regulation of cell growth and 
proliferation, but the cellular and molecular basis for these functions are not 
fully understood. In this study, we show that both the RII receptor Punt (Put) 
and the R-Smad Smad2 are strongly required for cell and tissue growth. Knocking 
down the expression of Put or Smad2 in salivary glands causes alterations in 
nucleolar structure and functions. Cells with decreased TGFβ/Activin signalling 
accumulate intermediate pre-rRNA transcripts containing internal transcribed 
spacer 1 regions accompanied by the nucleolar retention of ribosomal proteins. 
Thus, our results show that TGFβ/Activin signalling is required for ribosomal 
biogenesis, a key aspect of cellular growth control. Importantly, overexpression 
of Put enhanced cell growth induced by Drosophila Myc, a well-characterized 
inducer of nucleolar hypertrophy and ribosome biogenesis.