Technical Abstract: The role of PROTEIN ISOASPARTYL-METHYLTRANSFERASE (PIMT) in repairing a wide assortment of damaged proteins in a host of organisms has been inferred from the affinity of the enzyme for isoaspartyl residues in a plethora of amino acid contexts. The identification of specific PIMT target proteins in plant seeds, where the enzyme is highly active and proteome long-lived, has been hindered by large amounts of isoaspartate-containing storage proteins. Mature seed phage display libraries circumvented storage protein abundance. Inclusion of the PIMT co-substrate, AdoMet, during panning, permitted PIMT to retain aged phage in greater numbers than controls lacking co-substrate or when PIMT protein binding was poisoned with AdoHcy. At 4 rounds, hitherto increasing phage titer plateaued in AdoMet containing pans (consolidating around a few, highly represented sequences) while titer declined in both controls. This strategy identified 17 in-frame, PIMT target proteins, including a cupin-family protein similar to those identified previously using on-blot methylation (OBM). All recovered phage, regardless of reading-frame, had at least one susceptible D or N residue. Five targets were recovered independently in separate pans, as different clones, and/or from different libraries. Two in-frame targets were produced in E. coli as recombinant protein and shown, by OBM, to acquire isoAsp, becoming a PIMT target. Both gained isoAsp rapidly in solution (one to a high titer) upon thermal insult. Mutant analysis of plants deficient in any of three in-frame PIMT targets resulted in demonstrable phenotypes. An over-representation of clones encoding proteins involved in protein production suggests that the translational apparatus comprises a sub-group for which PIMT-mediated repair is vital for orthodox seed longevity. Impaired PIMT activity would simultaneously hinder protein function in these various targets, possibly resulting in a malaise and poor seed performance (quality).