Oxygen sensing in plants is mediated by an N-end rule pathway for protein destabilization

Abstract

The majority of eukaryotic organisms rely on molecular oxygen for respiratory energy production. When the supply of oxygen is compromised, a variety of acclimation responses are activated to reduce the detrimental effects of energy depletion. Various oxygen-sensing mechanisms have been described that are thought to trigger these responses, but they each seem to be kingdom specific and no sensing mechanism has been identified in plants until now. Here we show that one branch of the ubiquitin-dependent N-end rule pathway for protein degradation, which is active in both mammals and plants, functions as an oxygen-sensing mechanism in Arabidopsis thaliana. We identified a conserved amino-terminal amino acid sequence of the ethylene response factor (ERF)-transcription factor RAP2.12 to be dedicated to an oxygen-dependent sequence of post-translational modifications, which ultimately lead to degradation of RAP2.12 under aerobic conditions. When the oxygen concentration is low-as during flooding-RAP2.12 is released from the plasma membrane and accumulates in the nucleus to activate gene expression for hypoxia acclimation. Our discovery of an oxygen-sensing mechanism opens up new possibilities for improving flooding tolerance in crops.

Nitrogen metabolism in plants under low oxygen stress

“…It is important to note that the expression of these genes under aerobic condition was as high as that in wild type under anaerobic condition (Gibbs et al 2011). In Arabidopsis seedlings, overexpressing RAP2.2 (Hinz et al 2010) or its closest homolog RAP2.12 (Licausi et al 2011) anaerobic carbon metabolism genes were strongly induced during low oxygen stress; changes in gene expression were accompanied by improved survival of the seedlings to flooding-induced low oxygen stress. Microarray analysis followed by quantitative RT-PCR for the comparison of genes induction by hypoxia treatment in wild type and Arabidopsis silenced in both RAP2.2 and RAP2.12 showed a reduced induction of anaerobic carbon metabolism genes in miRNA lines and confirmed that these transcription factors are required for the regulation of hypoxia-induced genes (Licausi et al 2011;Hinz et al 2010).…”

“…Instead it is thought that oxygen availability in plants is sensed via changes in redox status (NADH/NAD ratio), reactive oxygen species (ROS), NO, and energy status of the cell (ATP content). It is only recently that the N-end rule pathway (NERP) for protein destabilization described first in animals (Hu et al 2005) was proposed as an indirect mechanism for oxygen sensing in plants (Gibbs et al 2011;Licausi et al 2011). Transcription factors RAP2.2, RAP2.12, HRE1 and HRE2 of the group-VII Ethylene Response Factors (ERF) revealed as key players in the regulation of low oxygen stress-responsive genes by experiments aiming at comprehensive insight into Arabidopsis transcriptome/translatome responses to hypoxia and anoxia proved to be subjected to oxygen-dependent posttranslational modification through the NERP for protein degradation (Licausi et al , 2011Gibbs et al 2011).…”

“…It is only recently that the N-end rule pathway (NERP) for protein destabilization described first in animals (Hu et al 2005) was proposed as an indirect mechanism for oxygen sensing in plants (Gibbs et al 2011;Licausi et al 2011). Transcription factors RAP2.2, RAP2.12, HRE1 and HRE2 of the group-VII Ethylene Response Factors (ERF) revealed as key players in the regulation of low oxygen stress-responsive genes by experiments aiming at comprehensive insight into Arabidopsis transcriptome/translatome responses to hypoxia and anoxia proved to be subjected to oxygen-dependent posttranslational modification through the NERP for protein degradation (Licausi et al , 2011Gibbs et al 2011). Taking into account the fact that RAP2.2 and RAP2.12 are constitutively expressed while HRE1 and HRE2 are induced by hypoxia, it was proposed that as cellular oxygen concentration decrease, constitutively expressed RAP2.2 and RAP2.12 escape posttranslational modification and N-end rule-mediated proteolysis.…”

“…RAP2.12 is constitutively expressed throughout the entire plant and further up-regulated by hypoxia but not by the ethylene precursor 1-aminocyclopropane 1-carboxylic acid (ACC) (Licausi et al 2011). RAP2.2 is also constitutively expressed with high expression in the roots and weak expression in the shoots where it is induced by darkness (Hinz et al 2010).…”

“…In Arabidopsis seedlings, overexpressing RAP2.2 (Hinz et al 2010) or its closest homolog RAP2.12 (Licausi et al 2011) anaerobic carbon metabolism genes were strongly induced during low oxygen stress; changes in gene expression were accompanied by improved survival of the seedlings to flooding-induced low oxygen stress. Microarray analysis followed by quantitative RT-PCR for the comparison of genes induction by hypoxia treatment in wild type and Arabidopsis silenced in both RAP2.2 and RAP2.12 showed a reduced induction of anaerobic carbon metabolism genes in miRNA lines and confirmed that these transcription factors are required for the regulation of hypoxia-induced genes (Licausi et al 2011;Hinz et al 2010). Interestingly, the overexpression of RAP2.2 and RAP2.12 did not affect the expression of hypoxia-induced genes when seedlings were grown under aerobic condition indicating that additional mechanisms interfere with the 1 3 signaling pathway controlled by these transcription factors.…”