| Summary: | <p>Abstract</p> <p>Background</p> <p>RACK1 is a versatile scaffold protein in mammals, regulating diverse developmental processes. Unlike in non-plant organisms where RACK1 is encoded by a single gene, Arabidopsis genome contains three <it>RACK1 </it>homologous genes, designated as <it>RACK1A, RACK1B </it>and <it>RACK1C</it>, respectively. Previous studies indicated that the loss-of-function alleles of <it>RACK1A </it>displayed multiple defects in plant development. However, the functions of <it>RACK1B </it>and <it>RACK1C </it>remain elusive. Further, the relationships between three <it>RACK1 </it>homologous genes are unknown.</p> <p>Results</p> <p>We isolated mutant alleles with loss-of-function mutations in <it>RACK1B </it>and <it>RACK1C</it>, and examined the impact of these mutations on plant development. We found that unlike in <it>RACK1A</it>, loss-of-function mutations in <it>RACK1B </it>or <it>RACK1C </it>do not confer apparent defects in plant development, including rosette leaf production and root development. Analyses of <it>rack1a</it>, <it>rack1b </it>and <it>rack1c </it>double and triple mutants, however, revealed that <it>rack1b </it>and <it>rack1c </it>can enhance the rack1a mutant's developmental defects, and an extreme developmental defect and lethality were observed in rack1a rack1b rack1c triple mutant. Complementation studies indicated that RACK1B and RACK1C are in principle functionally equivalent to <it>RACK1A</it>. Gene expression studies indicated that three <it>RACK1 </it>genes display similar expression patterns but are expressed at different levels. Further, <it>RACK1 </it>genes positively regulate each other's expression.</p> <p>Conclusion</p> <p>These results suggested that <it>RACK1 </it>genes are critical regulators of plant development and that <it>RACK1 </it>genes function in an unequally redundant manner. Both the difference in <it>RACK1 </it>gene expression level and the cross-regulation are likely the molecular determinants of their unequal genetic redundancy.</p>
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