Role of Arabidopsis receptor for activated C-protein kinase 1 in plant growth, development and abscisic acid responses

In mammalian cells and yeast, RACK1 (Receptor for Activated C Protein Kinase 1) regulates various signaling pathways and cellular processes through its interaction with numerous signaling proteins. However, its functions in plants are poorly understood. My PhD project utilizes a combination of genetic, molecular, biochemical, bioinformatic, and cell biological approaches to study the function of RACK1 in plants using Arabidopsis as a model system. The first part of my study focused on the role of RACK1 genes in plant growth and development. The Arabidopsis genome contains three RACK1 genes, namely RACK1A, RACK1B and RACK1C. Using a genetic complementation approach, I discovered that three Arabidopsis RACK1 genes are functionally equivalent and positively regulate plant root and shoot growth and development. The second part of my study focused on the role of RACK1 genes in abscisic acid (ABA) responses. ABA primarily mediates plant responses to abiotic stress. It is one of the five classic plant hormones. Through physiological and molecular biological assays, I established that the three RACK1 genes function as negative regulators of ABA responses and that they are also involved in salt and drought stress responses. In searching for the molecular function of RACK1 in ABA responses, I first looked into the potential interaction between RACK1 and the heterotrimeric G-protein complex (another negative regulator of ABA responses). Both protein(s) (complex) are highly conserved between Arabidopsis, yeast and mammal and a physical interaction between them were found in non-plant systems. I discovered that Arabidopsis RACK1 and a heterotrimeric G-protein complex appeared to work additively in ABA responses. Moreover, there was no physical interaction detected between the Arabidopsis homologs of RACK1 and the subunits of G-protein complex. These data indicate that Arabidopsis RACK1 and heterotrimeric G-protein complex work in independent manner in regulating ABA responses, distinct from their counterparts in mammalian and yeast cells. I next looked into the potentially evolutionarily-conserved role of RACK1 in regulating protein translation as a candidate mechanism via which RACK1 could negatively influence ABA responses. I found five lines of evidence directly or indirectly supporting this hypothesis: all three Arabidopsis RACK1s complemented the growth defects of the yeast rack1/cpc2 mutant; the rack1 mutation had an additive effect with anisomycin, an inhibitor of protein translation, on root growth; RACK1 physically interacted with Arabidopsis eukaryotic initiation factor 6 (eIF6), also known to regulate ribosome assembly and translation initiation in mammalian cells; rack1 mutants displayed impaired 60S ribosome subunit biogenesis and 80S functional ribosome assembly. In addition, ABA constantly inhibited the expression of RACK1 and eIF6. In summary, my PhD work has advanced our understanding of the versatile role of RACK1 genes in regulating several traits in plant growth and development as well as ABA/stress responses. I also found that Arabidopsis RACK1 and heterotrimeric G-protein complex, different from their counterparts in mammals and yeast, worked independently in regulating ABA responses. In addition, I established a role of Arabidopsis RACK1 in regulating protein translation, which was the first defined cellular process in which RACK1 is involved. At last, the data from my study indicates a role of RACK1 as a molecular link between ABA signaling and its effect on protein translation.

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