Ming-Hsiun Hsieh studied nitrogen (N) sensing and metabolism in the laboratory of Gloria M. Coruzzi, where he worked on the N regulatory protein PII, glutamate receptor-like proteins (GLRs), and asparagine synthetase in Arabidopsis. He discovered genes encoding ACT domain repeat (ACR) and heptahelical transmembrane protein (HHP) families and investigated the methylerythritol phosphate pathway mutants in Arabidopsis under the supervision of Howard M. Goodman. The Hsieh laboratory at Academia Sinica has contributed to the research of RNA metabolism in chloroplasts and mitochondria, vitamin B1 biosynthesis, and glutamine (Gln)/glutamate (Glu) metabolism and signaling in plants.

Gln/Glu signaling in Arabidopsis

Gln and Glu are the first organic nitrogen synthesized in the plant cell. In addition to their role in metabolism, we recently found that Gln and Glu could activate the expression of stress- and defense-responsive genes in plants. We propose a working model in which extracellular Gln and Glu may serve as “danger signals” to activate plant defense (Fig. 1).

Fig. 1 A hypothetical working model for Gln/Glu functions in plant growth and defense. GS, Gln synthetase; GOGAT, Gln:2-oxoglutarate aminotransferase; ACR11, ACT domain repeat 11.

The ACR proteins are putative amino acid sensors in plants. Arabidopsis has twelve ACRs, and their functions are largely unknown. The ACR11 protein interacts and regulates Gln synthetase 2 (GS2) and Fd-dependent Gln:2-oxoglutarate aminotransferase 1 (Fd-GOGAT1), but the molecular mechanisms remain elusive. We are interested in studying the functions of Arabidopsis ACRs, including the mechanism of ACR11 in regulating GS2/Fd-GOGAT1 (Fig. 1).

To study genes involved in Gln/Glu-regulated growth and defense, we screened for Arabidopsis mutants that grew poorly on Gln or Glu as the sole N source (Fig. 2). We are in the process of identifying the mutated genes and the underlying molecular mechanisms.

Fig. 2 Arabidopsis mutants grown on medium containing Gln or Glu as the sole nitrogen source.

Gln/Glu signaling in rice

We previously found that Gln and Glu could rapidly induce the expression of stress-responsive genes in rice (Oryza sativa). To study the functions of NH₄NO₃-, Gln-, and Glu-responsive genes, we use the CRISPR/Cas9 technology to generate a collection of rice mutants (Fig. 3). Further study on the CRISPR mutants may provide insights into the functions of these genes and the molecular mechanism of Gln/Glu-regulated stress response in rice.

Fig. 3 Genotypes and phenotypes of the OsNRG1 CRISPR mutants.