Myo-Inositol Biosynthesis Genes in Arabidopsis: Differential Patterns of Gene Expression and Role in Cell Death

Myo-inositol is an important cellular metabolite that forms the structural basis of a number of lipid signaling molecules that function in diverse pathways, including stress responses and the regulation of cell death, auxin perception, cell wall biosynthesis, and synthesis of ascorbic acid. The rate-limiting step in its biosynthesis is catalyzed by l-myo-inositol 1-phosphate synthase (MIPS), which exists in multiple isoforms in plants. Arabidopsis has three genes encoding MIPS proteins (MIPS1-3), and these appear to have undergone functional divergence to some extent. For example, Murphy et al. (2008) found that loss-of-function mips2 mutant plants showed enhanced susceptibility to diverse viral, bacterial, and fungal pathogens, whereas mips1 mutants showed no such effect, although both mutant lines exhibited similar depletion in myo-inositol phosphate. More recently, Meng et al. (2009) found that mips1 mutants exhibit spontaneous cell death and enhanced resistance to the oomycete pathogen Hyaloperonospora arabidopsis.

Donahue et al. (pages 888–903) now provide a detailed analysis of the expression patterns and mutant phenotypes of the three genes in Arabidopsis. MIPS1 became the main focus of the study, as it was the most highly expressed in all tissues examined and the only one of the three genes expressed outside vascular tissue in mature plants (see figure), and analysis of T-DNA insertion single mutant lines showed that only mips1 produced an obvious phenotype. All three proteins were found to have similar catalytic properties in vitro, and complementation of mutant lines with β-glucuronidase fusion constructs suggested localization to the cytoplasm for all three, pointing to a divergence in tissue-specific expression patterns as the main cause of functional differences between the three family members.

Spatial expression patterns of MIPS genes. The promoters of MIPS1, MIPS2, or MIPS3 were used to drive β-glucuronidase expression in transgenic plants. Images were taken from 19-d-old plants. Bars = 5 mm. (From Figure 2 of Donahue et al. [2010].)

A major finding of this study was that MIPS1 contributes the most to myo-inositol biosynthesis in mature plants, and this pathway regulates ceramide accumulation in leaves and plays a role in the regulation of cell death in response to various stresses. Loss-of-function mips1 mutant plants were found to have lower levels of myo-inositol relative to the wild type, and this was correlated with significantly higher levels of ceramides, spontaneous lesion formation, and increased sensitivity to treatments known to enhance production of reactive oxygen species.

Ceramide is a sphingolipid known to induce cell death in plants, whereas its phosphorylated derivatives block cell death (Liang et al., 2003). Thus, cell death may be regulated by the metabolism of ceramide into a number of derivative forms, including ceramide phosphate (catalyzed by ceramide kinase) and inositol phosphorylceramide (IPC; catalyzed by IPC synthase). Arabidopsis mutants of IPC synthase expressing the resistance gene RPW8 have elevated ceramide levels and show enhanced cell death (Wang et al., 2008). Donahue et al. hypothesize that loss of MIPS1 function leads to the accumulation of ceramides and the associated induction of cell death due to a reduction in myo-inositol–derived substrate for IPC synthase. This work thus provides important information on the functional divergence of MIPS genes in Arabidopsis and the role of myo-inositol biosynthesis in regulating ceramide metabolism and cell death.