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Towards comprehensive understanding of PLEIOTROPIC REGULATORY LOCUS 1 associated resistance signalling Weihmann, Tabea

Abstract

Plants employ a multi-layered protection system to recognize pathogen presence and act upon intrusion. The conserved MOS4-associated complex (MAC) participates in the triggered signal transduction relay and contributes to the build-up of sound resistance. PLEIOTROPIC REGULATORY LOCUS 1 (PRL1), a MAC component with predicted structural function, is needed for a healthy immune response. Loss of this WD40 protein results in substantially higher pathogen colonization in Arabidopsis mutants. To dissect signalling steps downstream of the MAC, a mutant allele of PRL1 was chosen as the basis for a genetic suppressor screen. From this screen, both dominant and recessive mutants with defects in candidate genes were isolated, and two suppressors were cloned using map-based cloning techniques. Characterization of the first dominant mutant revealed a gain-of-function mutation in PRL2, the homolog of PRL1. Although similar in sequence, the expression of PRL2 is greatly reduced in wild-type plants and functional analysis had not been attempted. Using the dominant prl2-1d allele and complementary mutants, full functional equivalence between the related proteins was established by means of defence –testing assays and evaluation of morphological criteria. This investigation revealed unequal genetic redundancy between the homologs; PRL2 has retained residual but relevant expression levels compared to the higher expressed PRL1. PRL2 also displays modified expression patterns, potentially indicative of developing tissue specificity. The haplo-insufficient SUPPRESSOR OF prl1, 2 (SOP2) gene is an intriguing discovery in PRL1 signal relay. Devoid of known sequence motifs, SOP2 encodes a novel nuclear protein with homologs limited to the plant kingdom. Several lines of evidence support a dosagedependent mechanism, mediated by SOP2, which is prone to interference by a spoiler protein. Both the obtained dominant-negative sop2-1D allele and a recessive sop2 mutation fully suppress prl1-related phenotypes, however neither one causes impaired resistance in single mutant analysis. Although specifics of SOP2 functionality in the context of plant resistance signalling remain to be fully resolved, clues from epistasis analysis point towards a PRL1 centered relationship and do not support SOP2 as a target of the MAC

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