Chemically reactive, oxygen-containing molecules called reactive oxygen species (ROS) are central to cell function. Plant cells generate various ROS, including hydrogen peroxide (H 2 O 2 ), which has a key role in cell signalling. It is produced in an extracellular space between the plasma membrane and cell wall called the apoplast, in response to a range of factors, including stressors, plant hormones such as abscisic acid, and physical or chemical changes outside the cell1. But whether and how this extracellular H 2 O 2 (eH 2 O 2 ) is sensed at the cell surface is unknown. Writing in Nature, Wu et al.2 identify the first known cell-surface H 2 O 2 receptor in plants.

Read the paper: Hydrogen peroxide sensor HPCA1 is an LRR receptor kinase in Arabidopsis

The apoplast and cell wall act as a dynamic interface between plant cells and the outside world, with all its threats, challenges and opportunities. Some eH 2 O 2 moves from the apoplast into the cytoplasm through channel proteins called aquaporins3. However, unlike the cytoplasm, the apoplast contains relatively few molecules that counteract oxidation1 — and so ROS, including H 2 O 2 , can survive for much longer in the apoplast than in the cytoplasm. This is a compelling reason to suspect that there is a sensor for eH 2 O 2 in the apoplast.

Although little is known about the initial target of eH 2 O 2 , the consequences of its production are much better defined4. It is clear that eH 2 O 2 triggers an influx of calcium ions (Ca2+) into the cell, which then leads to the systemic transmission of signals between cells in waves, activating processes such as pathogen resistance or acclimation to stress across the entire plant5. In addition, eH 2 O 2 signals regulate the polarized growth of pollen tubes and root hairs6, and control the opening and closing of stomata3 — pores on the outer layer of the leaf formed by two guard cells. Stomata enable the free passage of molecules such as carbon dioxide and oxygen into the plant when open, and can close to prevent water loss from the plant.

Wu et al. set out to identify cell-surface receptors for eH 2 O 2 that trigger Ca2+ signalling, using a ‘forward’ genetic-screen approach. They treated seeds of the plant Arabidopsis thaliana with a chemical that induces DNA mutations, then screened the resulting plants to identify mutants that showed low Ca2+ influxes in response to H 2 O 2 . They named these mutant plants hydrogen-peroxide-induced Ca2+ increases 1 (hpca1).

The authors then identified the HPCA1 protein. They report that HPCA1 is a membrane-spanning enzyme of a protein family known as leucine-rich repeat (LRR) receptor kinases. The group also showed that HPCA1 has two special pairs of cysteine (Cys) amino-acid residues in its extracellular domain. The thiol groups of Cys residues are known7 to be a target for oxidation by H 2 O 2 . The authors demonstrate that the presence of eH 2 O 2 leads to oxidation of the extracellular Cys residues of HPCA1 in guard cells. This modification activates HPCA1’s intracellular kinase activity, triggering Ca2+-channel activation and Ca2+ influx, followed by stomatal closure (Fig. 1).

Figure 1 | The HPCA1 protein. Wu et al.2 have identified the first extracellular sensor of hydrogen peroxide (H 2 O 2 ) in plants, HPCA1. The protein has an intracellular kinase enzyme domain, and an extracellular domain that protrudes into the apoplast — the compartment between a plant cell’s plasma membrane and the cell wall. HPCA1 has two special pairs of cysteine (Cys) amino-acid residues. The authors demonstrate that H 2 O 2 oxidizes thiol groups (not shown) on these residues, forming sulfenic acid (SOH; not shown) and disulfide bonds. This oxidation triggers a conformational change and kinase activity, which, through unknown mechanisms, lead to the opening of calcium-ion (Ca2+) channels and Ca2+ influx into the cell, triggering intrinsic and systemic signalling pathways.

In the absence of eH 2 O 2 , the hpca1 seedlings showed no differences from wild-type seedlings. However, their guard cells were less sensitive to eH 2 O 2 than were those of the wild-type seedlings, showing lower than wild-type levels of Ca2+ influx in response to eH 2 O 2 . HPCA1 is therefore required to convert the eH 2 O 2 signal into a physiological response. Moreover, the abscisic acid-dependent production of eH 2 O 2 by guard cells was defective in the hpca1 mutants. Of note, the function of HPCA1 in eH 2 O 2 signalling was not limited to guard cells, and the authors provided evidence that eH 2 O 2 signalling helps to transmit environmental signals to the nucleus of various cell types to regulate gene expression.

Oxidation of Cys by H 2 O 2 leads to the formation of a sulfenic acid (SOH), which is at the heart of reduction–oxidation (redox) signalling. Sulfenic acids are rather unstable intermediates that can be further oxidized to sulfinic (SO 2 H) and sulfonic (SO 3 H) acid, or can undergo ‘exchange reactions’ to form disulfide bonds. For HPCA1 to function properly as a receptor for eH 2 O 2 , the Cys oxidation process must be readily reversible, re-forming thiol residues that can be oxidized again. However, the factors that mediate reduction of the oxidized HPCA1 are unknown. One candidate is a membrane-bound electron-transport system, such as the one that reduces an oxidized form of the antioxidant molecule ascorbic acid in the apoplast8. Membrane-bound and apoplastic thioredoxin-like proteins are also putative candidates, given that thioredoxin is a well-characterized reducing agent for oxidized Cys residues of proteins.

How plants perceive salt

Wu and colleagues have uncovered a receptor-kinase-mediated eH 2 O 2 sensing mechanism that does not resemble any known eH 2 O 2 receptors or sensors reported in other organisms. Nonetheless, HPCA1 might be part of a much wider portfolio of sensors used by plants to perceive and respond to environmental changes through ROS signals. The identification of such receptors has proved challenging, not least because likely candidates are members of very large protein families. Sophisticated screens, such as that used by Wu et al., will be required to tease out the family members that have ROS sensing and signalling roles. Once these sensors have been identified, it should be relatively easy to manipulate their properties to produce model plants and crops that have, for example, increased or depressed sensitivity to environmental H 2 O 2 signals, and so show altered tolerance to environmental threats.