Each group used the model plant Arabidopsis thaliana and each alighted on a sub-group of transcription factors called ethylene response factors (ERFs) as key mediating proteins sensing oxygen shortage. Don’t let the name mislead you. The plant hormone ethylene is neither necessarily involved in the production of sub-group members (ERF subgroupVII to be precise) nor in their activation of key adaptive genes such as alcohol dehydrogenase. Both papers also identify the regulation of ERF protein breakdown as the key oxygen-responsive process. The susceptibility of the protein breakdown mechanism to oxygen shortage is shown to depend on there being an appropriate N-terminal amino acid sequence. As oxygen concentrations fall, this terminal sequence is essential if the ERF is to be protected from the more usual degradation seen in fully aerobic cells. These N-terminal residues are found in proteins of other organisms too where they are already known to be substrates for the so-called N-end rule pathway that quickly degrades them. This pathway has an oxygen-requiring step that permits a process called ubiquitination. This, in turn, leads to breakdown within large protein bodies (proteosomes). Sensing low oxygen in plants thus amounts to blocking oxidation of a key ERF-type transcription factor at the N-terminal end. This, in turn, prolongs its cellular life sufficiently for it to activate adaptive genes needed for enhanced tolerance of oxygen loss. In addition to protecting from degradation when oxygen concentrations are low, there is a targeting of ERF to hypoxia-inducible genes in the nucleus. Furthermore its not just enhanced post-translational stability and targeting that are involved. Transcription of the gene for the ERF known as RAP2.12 is also promoted when air (21 % oxygen) is replaced by 1 % oxygen.
Each of these two articles reinforces the other. The findings are rich in experimental detail and promise new molecular approaches to enhancing flooding tolerance in crop plants of the future. In an increasingly hungry and flood-prone world this can only be very good news.