In plant and animal cells, amino-terminal cysteine oxidation controls selective proteolysis via an oxygen-dependent branch of the N-end rule pathway. It remains unknown how the N-terminal cysteine is specifically oxidized. Here we identify plant cysteine oxidase (PCO) enzymes that oxidize the penultimate cysteine of ERF-VII transcription factors by using oxygen as a co-substrate, thereby controlling the lifetime of these proteins. Consequently, ERF-VII proteins are stabilized under hypoxia and activate the molecular response to low oxygen while the expression of anaerobic genes is repressed in air. Members of the PCO family are themselves targets of ERF-VII transcription factors, generating a feedback loop that adapts the stress response according to the extent of the hypoxic condition. Our results reveal that PCOs act as sensor proteins for oxygen in plants and provide an example of how proactive regulation of the N-end rule pathway balances stress response to optimal growth and development in plants.

No abstract

Reactive oxygen species (ROS) play an important role as triggers of gene expression during biotic and abiotic stresses, among which is low oxygen (O 2 ). Previous studies have shown that ROS regulation under low O 2 is driven by a RHO-like GTPase that allows tight control of hydrogen peroxide (H 2 O 2 ) production. H 2 O 2 is thought to regulate the expression of heat shock proteins, in a mechanism that is common to both O 2 deprivation and to heat stress. In this work, we used publicly available Arabidopsis (Arabidopsis thaliana) microarray datasets related to ROS and O 2 deprivation to define transcriptome convergence pattern. Our results show that although Arabidopsis response to anoxic and hypoxic treatments share a common core of genes related to the anaerobic metabolism, they differ in terms of ROS-related gene response. We propose that H 2 O 2 production under O 2 deprivation is a trait present in a very early phase of anoxia, and that ROS are needed for the regulation of a set of genes belonging to the heat shock protein and ROS-mediated groups. This mechanism, likely not regulated via the N-end rule pathway for O 2 sensing, is probably mediated by a NADPH oxidase and it is involved in plant tolerance to the stress.

Plant survival is greatly impaired when oxygen levels are limiting, such as during flooding or when anatomical constraints limit oxygen diffusion. Oxygen sensing in Arabidopsis thaliana is mediated by Ethylene Responsive Factor (ERF)-VII transcription factors, which control a core set of hypoxia- and anoxia-responsive genes responsible for metabolic acclimation to low-oxygen conditions. Anoxic conditions also induce genes related to reactive oxygen species (ROS). Whether the oxygen-sensing machinery coordinates ROS production under anoxia has remained unclear. Here we show that a low-oxygen-responsive universal stress protein (USP), Hypoxia Responsive Universal Stress Protein 1 (HRU1), is induced by RAP2.12 (Related to Apetala 2.12), an ERF-VII protein, and modulates ROS production in Arabidopsis. We found that HRU1 is strongly induced by submergence, but that this induction is abolished in plants lacking RAP2.12. Mutation of HRU1 through transfer DNA (T-DNA) insertion alters hydrogen peroxide production, and reduces tolerance to submergence and anoxia. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) analyses reveal that HRU1 interacts with proteins that induce ROS production, the GTPase ROP2 and the NADPH oxidase RbohD, pointing to the existence of a low-oxygen-specific mechanism for the modulation of ROS levels. We propose that HRU1 coordinates oxygen sensing with ROS signalling under anoxic conditions.

Plants often experience low oxygen conditions as the consequence of reduced oxygen availability in their environment or due to a high activity of respiratory metabolism. Recently, an oxygen sensing pathway was described in Arabidopsis thaliana which involves the migration of an ERF transcription factor (RAP2.12) from the plasma membrane to the nucleus upon hypoxia. Moreover, RAP2.12 protein level is controlled through an oxygen-dependent branch of the N-end rule pathway for proteasomal degradation. Inside the nucleus, RAP2.12 induces the expression of genes involved in the adaptation to reduced oxygen availability. In the present study, we describe the oxygen concentration and time-resolved characterization of RAP2.12 activity. A reduction of the oxygen availability to half the concentration in normal air is sufficient to trigger RAP2.12 relocalization into the nucleus, while nuclear accumulation correlates with the first induction of the molecular response to hypoxia. Nuclear presence of RAP2.12 may not only depend on relocalization of existing protein, but involves de novo synthesis of the transcription factor as well. After re-oxygenation of the tissue, degradation of RAP2.12 in the nucleus was observed within 3 h, concomitant with reduction in hypoxia responsive gene transcripts to normoxic levels.

A plant's eventual size depends on the integration of its genetic program with environmental cues, which vary on a daily basis. Both efficient carbon metabolism and the plant hormone gibberellin are required to guarantee optimal plant growth. Yet, little is known about the interplay between carbon metabolism and gibberellins that modulates plant growth. Here, we show that sugar starvation in Arabidopsis thaliana arising from inefficient starch metabolism at night strongly reduces the expression of ent-kaurene synthase, a key regulatory enzyme for gibberellin synthesis, the following day. Our results demonstrate that plants integrate the efficiency of photosynthesis over a period of days, which is transduced into a daily rate of gibberellin biosynthesis. This enables a plant to grow to a size that is compatible with its environment.

While ethylene controls aerenchyma formation in the fast-elongating 'Arborio Precoce' variety, in 'FR13A' ROS accumulation plays an important role.

Low oxygen stress often occurs during the life of green organisms, mostly due to the environmental conditions affecting oxygen availability. Both plants and algae respond to low oxygen by resetting their metabolism. The shift from mitochondrial respiration to fermentation is the hallmark of anaerobic metabolism in most organisms. This involves a modified carbohydrate metabolism coupled with glycolysis and fermentation. For a coordinated response to low oxygen, plants exploit various molecular mechanisms to sense when oxygen is either absent or in limited amounts. In Arabidopsis thaliana, a direct oxygen sensing system has recently been discovered, where a conserved N-terminal motif on some ethylene responsive factors (ERFs), targets the fate of the protein under normoxia/hypoxia. In Oryza sativa, this same group of ERFs drives physiological and anatomical modifications that vary in relation to the genotype studied. The microalga Chlamydomonas reinhardtii responses to low oxygen seem to have evolved independently of higher plants, posing questions on how the fermentative metabolism is modulated. In this review, we summarize the most recent findings related to these topics, highlighting promising developments for the future.