Endoplasmic reticulum-bound ANAC013 factor is cleaved by RHOMBOID-LIKE 2 during the initial response to hypoxia in Arabidopsis thaliana.

Aerobic reactions are essential to sustain plant growth and development. Impaired oxygen availability due to excessive water availability, e.g., during waterlogging or flooding, reduces plant productivity and survival. Consequently, plants monitor oxygen availability to adjust growth and metabolism accordingly. Despite the identification of central components in hypoxia adaptation in recent years, molecular pathways involved in the very early activation of low-oxygen responses are insufficiently understood. Here, we characterized three endoplasmic reticulum (ER)-anchored Arabidopsis ANAC transcription factors, namely ANAC013, ANAC016, and ANAC017, which bind to the promoters of a subset of hypoxia core genes (HCGs) and activate their expression. However, only ANAC013 translocates to the nucleus at the onset of hypoxia, i.e., after 1.5 h of stress. Upon hypoxia, nuclear ANAC013 associates with the promoters of multiple HCGs. Mechanistically, we identified residues in the transmembrane domain of ANAC013 to be essential for transcription factor release from the ER, and provide evidence that RHOMBOID-LIKE 2 (RBL2) protease mediates ANAC013 release under hypoxia. Release of ANAC013 by RBL2 also occurs upon mitochondrial dysfunction. Consistently, like ANAC013 knockdown lines, rbl knockout mutants exhibit impaired low-oxygen tolerance. Taken together, we uncovered an ER-localized ANAC013-RBL2 module, which is active during the initial phase of hypoxia to enable fast transcriptional reprogramming.

Hypoxia reshapes Arabidopsis root architecture by integrating ERF-VII factor response and abscisic acid homoeostasis.

Oxygen limitation (hypoxia), arising as a key stress factor due to flooding, negatively affects plant development. Consequently, maintaining root growth under such stress is crucial for plant survival, yet we know little about the root system's adaptions to low-oxygen conditions and its regulation by phytohormones. In this study, we examine the impact of hypoxia and, herein, the regulatory role of group VII ETHYLENE-RESPONSE FACTOR (ERFVII) transcription factors on root growth in Arabidopsis. We found lateral root (LR) elongation to be actively maintained by hypoxia via ERFVII factors, as erfVII seedlings possess hypersensitivity towards hypoxia regarding their LR growth. Pharmacological inhibition of abscisic acid (ABA) biosynthesis revealed ERFVII-driven counteraction of hypoxia-induced inhibition of LR formation in an ABA-dependent manner. However, postemergence LR growth under hypoxia mediated by ERFVIIs was independent of ABA. In roots, ERFVIIs mediate, among others, the induction of ABA-degrading ABA 8'-hydroxylases CYP707A1 expression. RAP2.12 could activate the pCYC707A1:LUC reporter gene, indicating, combined with single mutant analyses, that this transcription factor regulates ABA levels through corresponding transcript upregulation. Collectively, hypoxia-induced adaptation of the Arabidopsis root system is shaped by developmental reprogramming, whereby ERFVII-dependent promotion of LR emergence, but not elongation, is partly executed through regulation of ABA degradation.

Root Bending Is Antagonistically Affected by Hypoxia and ERF-Mediated Transcription via Auxin Signaling.

When plants encounter soil water logging or flooding, roots are the first organs to be confronted with reduced gas diffusion resulting in limited oxygen supply. Since roots do not generate photosynthetic oxygen, they are rapidly faced with oxygen shortage rendering roots particularly prone to damage. While metabolic adaptations to low oxygen conditions, which ensure basic energy supply, have been well characterized, adaptation of root growth and development have received less attention. In this study, we show that hypoxic conditions cause the primary root to grow sidewise in a low oxygen environment, possibly to escape soil patches with reduced oxygen availability. This growth behavior is reversible in that gravitropic growth resumes when seedlings are returned to normoxic conditions. Hypoxic root bending is inhibited by the group VII ethylene response factor (ERFVII) RAP2.12, as rap2.12-1 seedlings show exaggerated primary root bending. Furthermore, overexpression of the ERFVII member HRE2 inhibits root bending, suggesting that primary root growth direction at hypoxic conditions is antagonistically regulated by hypoxia and hypoxia-activated ERFVIIs. Root bending is preceded by the establishment of an auxin gradient across the root tip as quantified with DII-VENUS and is synergistically enhanced by hypoxia and the auxin transport inhibitor naphthylphthalamic acid. The protein abundance of the auxin efflux carrier PIN2 is reduced at hypoxic conditions, a response that is suppressed by RAP2.12 overexpression, suggesting antagonistic control of auxin flux by hypoxia and ERFVII. Taken together, we show that hypoxia triggers an escape response of the primary root that is controlled by ERFVII activity and mediated by auxin signaling in the root tip.