Impaired Inosine Monophosphate Dehydrogenase Leads to Plant-Specific Ribosomal Stress Responses in Arabidopsis thaliana

Abstract

Nucleotides are the building blocks of living organisms and their biosynthesis must be tightly regulated. Inosine monophosphate dehydrogenase (IMPDH) is a rate-limiting enzyme in GTP synthesis that is essential for biological activities, such as RNA synthesis. In animals, the suppression of IMPDH function causes ribosomal stress (also known as nucleolar stress), a disorder in ribosome biogenesis that results in cell proliferation defects and apoptosis. Despite its importance, plant IMPDH has not been analyzed in detail. Therefore, we analyzed the phenotypes of mutants of the two IMPDH genes in Arabidopsis thaliana and investigated their relationship with ribosomal stress. Double mutants of IMPDH1 and IMPDH2 were lethal, and only the impdh2 mutants showed growth defects and transient chlorophyll deficiency. These results suggested that IMPDH1 and IMPDH2 are redundant and essential, whereas IMPDH2 has a crucial role. In addition, the impdh2 mutants showed a reduction in nucleolus size and resistance to several translation inhibitors, which is a known response to ribosomal stress. Furthermore, the IMPDH1/impdh1 impdh2 mutants showed more severe growth defects and phenotypes such as reduced plastid rRNA levels and abnormal processing patterns than the impdh2 mutants. Finally, multiple mutations of impdh with as2, which has abnormal leaf polarity, caused the development of needle-like leaves because of the enhancement of the as2 phenotype, which is a typical effect observed in mutants of genes involved in ribosome biogenesis. These results indicated that IMPDH is closely related to ribosome biogenesis, and that mutations in the genes lead to not only known responses to ribosomal stress, but also plant-specific responses.