Editors' ChoicePlant biology

N-End Rules Again

Sci. Signal.  11 Feb 2014:
Vol. 7, Issue 312, pp. ec44
DOI: 10.1126/scisignal.2005166

Group VII ethylene response factor (ERF) transcription factors function as homeostatic sensors of oxygen and are degraded by an N-end rule pathway in the presence of oxygen and accumulate under hypoxic conditions. The N-end rule degradative pathways involve the recognition of proteins with specific N-terminal residues that can be posttranslationally modified, such as cysteine, which is oxidized and then arginylated (the Arg/N-end rule pathway). Gibbs et al. showed that the Arg/N-end rule pathway also regulates the stability of a subset of ERF transcription factors in response to nitric oxide (NO) and thereby regulates seed germination and contributes to other developmental processes. Treatment of seedlings with NO scavengers stabilized Arg/N-end rule reporter proteins, whereas NO donors promoted the degradation of the reporters. The stability of the reporters was unchanged in prt6 or ate1ate2 plants lacking enzymes in the Arg/N-end rule pathway. To determine whether group VII ERFs were also subject to Arg/N-end rule degradation in response to NO, the abundance of methionine-cysteine (MC)–RAP2.3 tagged with hemagglutinin (HA) on the C terminus (MC-RAP2.3-HA) or a mutant initiated with methionine-alanine (MA) was assessed. MC-RAP2.3-HA, but not MA-RAP2.3-HA, was stabilized by exposure to an NO scavenger, and this response was compromised in plants deficient in the Arg/N-end rule pathway. The abundance of MC-RAP2.3-HA or another group VII ERF MC-HRE2-HA was increased by hypoxia (induced by submersion in water) or in plants deficient in the enzymes needed to produce NO. NO stimulates seed germination by reducing the response to abscisic acid (ABA), and NO donors stimulated germination of dormant seeds from wild-type plants but not those from prt6 or ate1ate2 mutants. If the effect of loss of PRT6 function is an increase in the abundance of group VII ERFs, then knocking out the relevant ERFs in the prt6 background should reduce seed dormancy and reduce the sensitivity to ABA. Seeds from plants in which rap2.12, rap2.2, and rap2.3 were knocked out required less time under cold conditions to stimulate germination (thus indicating reduced dormancy) and required higher concentrations of ABA to inhibit germination. These effects were not seen in seeds from plants in which hre1 and hre2 were knocked out. Analysis of a reporter with the promoter of abi5 (encoding ABI5, an ABA-responsive transcription factor that inhibits germination) and chromatin immunoprecipitation experiments indicated that the RAP subfamily stimulated the expression of this gene. Furthermore, in seeds, the activity of the reporter was inhibited by exposure to an NO donor and was enhanced in prt6 seeds. Thus, Arg/N-end rule mediated degradation of a subset of ERFs functions in plant responses to both oxygen and NO.

D. J. Gibbs, N. Md Isa, M. Movahedi, J. Lozano-Juste, G. M. Mendiondo, S. Berckhan, N. Marín-de la Rosa, J. Vicente Conde, C. Sousa Correia, S. P. Pearce, G. W. Bassel, B. Hamali, P. Talloji, D. F. A. Tomé, A. Coego, J. Beynon, D. Alabadí, A. Bachmair, J. León, J. E. Gray, F. L. Theodoulou, M. J. Holdsworth, Nitric oxide sensing in plants is mediated by proteolytic control of group VII ERF transcription factors. Mol. Cell 53, 369–379 (2014). [PubMed]