Unravelling the molecular basis of functional specialization amongst Arabidopsis thaliana poly(A) polymerase isoforms

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In plants and animals, organs grow to characteristic sizes and shapes that depend on their identity, yet how the identity programme modulates growth patterns in primordia to generate these specific sizes and shapes remains poorly understood. We have isolated a mutation in Arabidopsis thaliana that reduces the size of leaves, but increases the size of floral organs; this opposite effect depends on the identity, not the position of the organ on the shoot. The mutation represents an amino-acid exchange in PAP1, one of the three canonical nuclear poly(A)-polymerases responsible for polyadenylating precursor messenger RNAs (pre-mRNAs). While full loss of PAP1 function appears to be lethal, mutations in the two closely related other nuclear poly(A)-polymerases (PAP2, PAP4) do not show a phenotype in isolation. We therefore propose that the different PAP isoforms process specific subsets of cellular pre-mRNAs and that the PAP1-dependent subset contains organ-specific transcripts encoding regulators of growth. Thus, the main aims of this proposal are (1) to determine the set of PAP1-dependent transcripts in leaves and flowers using a genome-wide approach; (2) to understand how the encoded proteins contribute to the opposite growth phenotypes of the pap1 mutant; (3) to understand the basis for the identity-dependence of the growth phenotype; (4) to characterize the functions of PAP2 and PAP4, and the subset of transcripts they preferentially process; and (5) to determine the molecular basis for the functional specialization between the different PAP isoforms.