Molecular and physiological characterization of AtHIGD1 in Arabidopsis.

Flooding is a principal stress that limits plant productivity. The sensing of low oxygen levels (hypoxia) plays a critical role in the signaling pathway that functions in plants in flooded environments. In this study, to investigate hypoxia response mechanisms in Arabidopsis, we identified three hypoxia-related genes and subjected one of these genes, Arabidopsis thaliana HYPOXIA-INDUCED GENE DOMAIN 1 (AtHIGD1), to molecular characterization including gene expression analysis and intracellular localization of the encoded protein. AtHIGD1 was expressed in various organs but was preferentially expressed in developing siliques. Confocal microscopy of transgenic plants harboring eGFP-tagged AtHIGD1 indicated that AtHIGD1 is localized to mitochondria. Importantly, plants overexpressing AtHIGD1 exhibited increased resistance to hypoxia compared to wild type. Our results represent the first report of a biological function for an HIGD protein in plants and indicate that AtHIGD1 is a mitochondrial protein that plays an active role in mitigating the effects of hypoxia on plants.

A novel rice protein family of OsHIGDs may be involved in early signalling of hypoxia-promoted stem growth in deepwater rice.

KEY MESSAGE: OsHIGDs was identified as a novel hypoxia-responsive protein family. Among them, OsHIGD2 is characterized as a mitochondrial protein and is related to hypoxia signalling through interacting with mitochondrial proteins of critical functions in reducing cell damages caused by hypoxia. Recent evidence supports ethylene as a key factor in modulating plant responses to submergence stress. Meanwhile, there has been general consent that ethylene is not the only signal for the submergence-induced stem growth. In this study, we confirmed that hypoxia also promotes stem elongation in deepwater rice even in the absence of ethylene. As components of ethylene-independent hypoxia signalling, five HIGD (hypoxia-induced gene domain) protein genes were identified. Among the genes, OsHIGD2 showed the fastest and strongest induction by hypoxia as well as submergence. Co-expression analysis indicated that OsHIGD2 had a simultaneous expression pattern with fermentation-related genes, such as ADH1 (alcohol dehydrogenase 1) and PDC2 (pyruvate decarboxylase 2). Transient expression of OsHIGD2 in leaf epidermal cells of Nicotiana benthamiana provided evidence that the protein is localized to mitochondria. We further identified OsHIGD2-interacting proteins through the yeast two-hybrid assay using OsHIGD2 as bait. As a result, three mitochondrial proteins were discovered that function in the regulation of redox potential or reduction of protein damages caused by reactive oxygen species. In this report, we propose that OsHIGD2 is a mitochondrial protein which takes part in the early stage of hypoxia signalling by interacting with proteins that are related to oxygen utilization.