PII is induced by WRINKLED1 and fine-tunes fatty acid composition in seeds of Arabidopsis thaliana.

The PII protein is an integrator of central metabolism and energy levels. In Arabidopsis, allosteric sensing of cellular energy and carbon levels alters the ability of PII to interact with target enzymes such as N-acetyl-l-glutamate kinase and heteromeric acetyl-coenzyme A carboxylase, thereby modulating the biological activity of these plastidial ATP- and carbon-consuming enzymes. A quantitative reverse transcriptase-polymerase chain reaction approach revealed a threefold induction of the AtGLB1 gene (At4g01900) encoding PII during early seed maturation. The activity of the AtGLB1 promoter was consistent with this pattern. A complementary set of molecular and genetic analyses showed that WRINKLED1, a transcription factor known to induce glycolytic and fatty acid biosynthetic genes at the onset of seed maturation, directly controls AtGLB1 expression. Immunoblot analyses and immunolocalization experiments using anti-PII antibodies established that PII protein levels faithfully reflected AtGLB1 mRNA accumulation. At the subcellular level, PII was observed in plastids of maturing embryos. To further investigate the function of PII in seeds, comprehensive functional analyses of two pII mutant alleles were carried out. A transient increase in fatty acid production was observed in mutant seeds at a time when PII protein content was found to be maximal in wild-type seeds. Moreover, minor though statistically significant modifications of the fatty acid composition were measured in pII seeds, which exhibited decreased amounts of modified (elongated, desaturated) fatty acid species. The results obtained outline a role for PII in the fine tuning of fatty acid biosynthesis and partitioning in seeds.