The Exoribonuclease XRN4 Is a Component of the Ethylene Response Pathway inArabidopsis

Abstract

EXORIBONUCLEASE4 (XRN4), the Arabidopsis thaliana homolog of yeast XRN1, is involved in the degradation of several unstable mRNAs. Although a role for XRN4 in RNA silencing of certain transgenes has been reported, xrn4 mutant plants were found to lack any apparent visible phenotype. Here, we show that XRN4 is allelic to the unidentified components of the ethylene response pathway ETHYLENE-INSENSITIVE5/ACC-INSENSITIVE1 (EIN5/AIN1) and EIN7. xrn4 mutant seedlings are ethyleneinsensitive as a consequence of the upregulation of EIN3 BINDING F-BOX PROTEIN1 (EBF1) and EBF2 mRNA levels, which encode related F-box proteins involved in the turnover of EIN3 protein, a crucial transcriptional regulator of the ethylene response pathway. Epistasis analysis placed XRN4/EIN5/AIN1 downstream of CTR1 and upstream of EBF1/2. XRN4 does not appear to regulate ethylene signaling via an RNA-INDUCED SILENCING COMPLEX-based RNA silencing mechanism but acts by independent means. The identification of XRN4 as an integral new component in ethylene signaling adds RNA degradation as another posttranscriptional process that modulates the perception of this plant hormone.

The role of protein turnover in ethylene biosynthesis and response

“…EBF1 and EBF2 transcripts are upregulated in ein5/xrn4 mutants [67,68]. However, EBF transcript turnover rate is the same between wild type and ein5/xrn4 mutants, indicating that XRN4 fails to directly degrade EBF transcripts [68]. Furthermore, ein5/xrn4 and ein3 mutants display a similar kinetic response to ethylene, where both are defective in the second phase of growth inhibition [68].…”

“…However, EBF transcript turnover rate is the same between wild type and ein5/xrn4 mutants, indicating that XRN4 fails to directly degrade EBF transcripts [68]. Furthermore, ein5/xrn4 and ein3 mutants display a similar kinetic response to ethylene, where both are defective in the second phase of growth inhibition [68]. While the mechanism of how XRN4 regulates EBF1/2 transcript levels is unknown, these data show that XRN4 regulates the ethylene signaling pathway through negative regulation of EBF1/2.…”

“…The EIN5 locus, which was first identified in a screen for ethylene-insensitive Arabidopsis mutants, was found to encode XRN4, and epistasis analysis showed that EIN5/XRN4 lies downstream of CTR1 but upstream of EBF1 and EBF2 [67,68]. EBF1 and EBF2 transcripts are upregulated in ein5/xrn4 mutants [67,68]. However, EBF transcript turnover rate is the same between wild type and ein5/xrn4 mutants, indicating that XRN4 fails to directly degrade EBF transcripts [68].…”

“…The EBF1 and EBF2 proteins seem to have differing roles in ethylene response [64], and the exoribonuclease XRN4 is responsible for regulation of the EBF1 and EBF2 transcripts, but its mode of action is unknown [67,68]. The ein5/xrn4 mutant is not defective in EBF1 or EBF2 mRNA turnover [68], although increased transcript levels are seen in ein5/xrn4 mutants [67,68].…”

“…Recent findings indicate that the exoribonuclease XRN4 regulates EBF1 and EBF2 transcript levels [67,68]. The EIN5 locus, which was first identified in a screen for ethylene-insensitive Arabidopsis mutants, was found to encode XRN4, and epistasis analysis showed that EIN5/XRN4 lies downstream of CTR1 but upstream of EBF1 and EBF2 [67,68]. EBF1 and EBF2 transcripts are upregulated in ein5/xrn4 mutants [67,68].…”