SummaryNitric oxide (NO) is an important signaling compound in prokaryotes and eukaryotes. In plants, NO regulates critical developmental transitions and stress responses. Here, we identify a mechanism for NO sensing that coordinates responses throughout development based on targeted degradation of plant-specific transcriptional regulators, the group VII ethylene response factors (ERFs). We show that the N-end rule pathway of targeted proteolysis targets these proteins for destruction in the presence of NO, and we establish them as critical regulators of diverse NO-regulated processes, including seed germination, stomatal closure, and hypocotyl elongation. Furthermore, we define the molecular mechanism for NO control of germination and crosstalk with abscisic acid (ABA) signaling through ERF-regulated expression of ABSCISIC ACID INSENSITIVE5 (ABI5). Our work demonstrates how NO sensing is integrated across multiple physiological processes by direct modulation of transcription factor stability and identifies group VII ERFs as central hubs for the perception of gaseous signals in plants.

SummaryAn integral part of global environment change is an increase in the atmospheric concentration of CO2 ([CO2]) [1]. Increased [CO2] reduces leaf stomatal apertures and density of stomata that plays out as reductions in evapotranspiration [2–4]. Surprisingly, given the importance of transpiration to the control of terrestrial water fluxes [5] and plant nutrient acquisition [6], we know comparatively little about the molecular components involved in the intracellular signaling pathways by which [CO2] controls stomatal development and function [7]. Here, we report that elevated [CO2]-induced closure and reductions in stomatal density require the generation of reactive oxygen species (ROS), thereby adding a new common element to these signaling pathways. We also show that the PYR/RCAR family of ABA receptors [8, 9] and ABA itself are required in both responses. Using genetic approaches, we show that ABA in guard cells or their precursors is sufficient to mediate the [CO2]-induced stomatal density response. Taken together, our results suggest that stomatal responses to increased [CO2] operate through the intermediacy of ABA. In the case of [CO2]-induced reductions in stomatal aperture, this occurs by accessing the guard cell ABA signaling pathway. In both [CO2]-mediated responses, our data are consistent with a mechanism in which ABA increases the sensitivity of the system to [CO2] but could also be explained by requirement for a CO2-induced increase in ABA biosynthesis specifically in the guard cell lineage. Furthermore, the dependency of stomatal [CO2] signaling on ABA suggests that the ABA pathway is, in evolutionary terms, likely to be ancestral.

Stomatal pores evolved more than 410 million years ago [1, 2] and allowed vascular plants to regulate transpirational water loss during the uptake of CO(2) for photosynthesis [3]. Here, we show that stomata on the sporophytes of the moss Physcomitrella patens [2] respond to environmental signals in a similar way to those of flowering plants [4] and that a homolog of a key signaling component in the vascular plant drought hormone abscisic acid (ABA) response [5] is involved in stomatal control in mosses. Cross-species complementation experiments reveal that the stomatal ABA response of a flowering plant (Arabidopsis thaliana) mutant, lacking the ABA-regulatory protein kinase OPEN STOMATA 1 (OST1) [6], is rescued by substitution with the moss P. patens homolog, PpOST1-1, which evolved more than 400 million years earlier. We further demonstrate through the targeted knockout of the PpOST1-1 gene in P. patens that its role in guard cell closure is conserved, with stomata of mutant mosses exhibiting a significantly attenuated ABA response. Our analyses indicate that core regulatory components involved in guard cell ABA signaling of flowering plants are operational in mosses and likely originated in the last common ancestor of these lineages more than 400 million years ago [7], prior to the evolution of ferns [8, 9].

SummaryAbiotic stresses impact negatively on plant growth, profoundly affecting yield and quality of crops. Although much is known about plant responses, very little is understood at the molecular level about the initial sensing of environmental stress. In plants, hypoxia (low oxygen, which occurs during flooding) is directly sensed by the Cys-Arg/N-end rule pathway of ubiquitin-mediated proteolysis, through oxygen-dependent degradation of group VII Ethylene Response Factor transcription factors (ERFVIIs) via amino-terminal (Nt-) cysteine [1, 2]. Using Arabidopsis (Arabidopsis thaliana) and barley (Hordeum vulgare), we show that the pathway regulates plant responses to multiple abiotic stresses. In Arabidopsis, genetic analyses revealed that response to these stresses is controlled by N-end rule regulation of ERFVII function. Oxygen sensing via the Cys-Arg/N-end rule in higher eukaryotes is linked through a single mechanism to nitric oxide (NO) sensing [3, 4]. In plants, the major mechanism of NO synthesis is via NITRATE REDUCTASE (NR), an enzyme of nitrogen assimilation [5]. Here, we identify a negative relationship between NR activity and NO levels and stabilization of an artificial Nt-Cys substrate and ERFVII function in response to environmental changes. Furthermore, we show that ERFVIIs enhance abiotic stress responses via physical and genetic interactions with the chromatin-remodeling ATPase BRAHMA. We propose that plants sense multiple abiotic stresses through the Cys-Arg/N-end rule pathway either directly (via oxygen sensing) or indirectly (via NO sensing downstream of NR activity). This single mechanism can therefore integrate environment and response to enhance plant survival.

The N-end rule pathway is a highly conserved constituent of the ubiquitin proteasome system, yet little is known about its biological roles. Here we explored the role of the N-end rule pathway in the plant immune response. We investigated the genetic influences of components of the pathway and known protein substrates on physiological, biochemical and metabolic responses to pathogen infection. We show that the glutamine (Gln) deamidation and cysteine (Cys) oxidation branches are both components of the plant immune system, through the E3 ligase PROTEOLYSIS (PRT)6. In Arabidopsis thaliana Gln-specific amino-terminal (Nt)-amidase (NTAQ1) controls the expression of specific defence-response genes, activates the synthesis pathway for the phytoalexin camalexin and influences basal resistance to the hemibiotroph pathogen Pseudomonas syringae pv tomato (Pst). The Nt-Cys ETHYLENE RESPONSE FACTOR VII transcription factor substrates enhance pathogen-induced stomatal closure. Transgenic barley with reduced HvPRT6 expression showed enhanced resistance to Ps. japonica and Blumeria graminis f. sp. hordei, indicating a conserved role of the pathway. We propose that that separate branches of the N-end rule pathway act as distinct components of the plant immune response in flowering plants.

(K.S.); 0000-0002-6438-9188 (R.S.); 0000-0002-9703-0745 (A.J.F.); 0000-0001-9972-5156 (J.E.G.).Stomata are formed by a pair of guard cells which have thickened, elastic cell walls to withstand the large increases in turgor pressure that have to be generated to open the pore that they surround. We have characterized FOCL1, a guard cell-expressed, secreted protein with homology to Hyp-rich cell wall proteins. FOCL1-GFP localizes to the guard cell outer cuticular ledge and plants lacking FOCL1 produce stomata without a cuticular ledge. Instead the majority of stomatal pores are entirely covered over by a continuous fusion of the cuticle, and consequently plants have decreased levels of transpiration and display drought tolerance. The focl1 guard cells are larger and less able to reduce the aperture of their stomatal pore in response to closure signals suggesting that the flexibility of guard cell walls is impaired. FOCL1 is also expressed in lateral root initials where it aids lateral root emergence. We propose that FOCL1 acts in these highly specialized cells of the stomata and root to impart cell wall strength at high turgor and/or to facilitate interactions between the cell wall and the cuticle.

Drought stress triggers remarkable physiological changes and growth impediments, which significantly diminish plant biomass and crop yield. However, certain plant species show notable resilience, maintaining nearly normal yields under severe water deficits. For example, sorghum is a naturally drought-tolerant crop, which is ideal for studying plant adaptive responses to drought. Here we used sorbitol treatments to simulate drought-induced osmotic stress in sorghum cell suspension cultures and analysed fractions enriched for extracellular matrix proteins using isobaric tags for relative and absolute quantification technology. Sorbitol induced an overall increase in protein secretion, with putative redox proteins, proteases, and glycosyl hydrolases featuring prominently among the responsive proteins. Gene expression analysis of selected candidates revealed regulation at the transcriptional level. There was a notable differential gene expression between drought-tolerant and drought-sensitive sorghum varieties for some of the candidates. This study shows that protein secretion is a major component of the sorghum response to osmotic stress. Additionally, our data provide candidate genes, which may have putative functions in sorghum drought tolerance, and offer a pool of genes that could be developed as potential biomarkers for rapid identification of drought tolerant lines in plant breeding programs.

In plants, stomata control water loss and CO 2 uptake. The aperture and density of stomatal pores, and hence the exchange of gases between the plant and the atmosphere, are controlled by internal factors such as the plant hormone abscisic acid (ABA) and external signals including light and CO 2 . In this study, we examine the importance of ABA catabolism in the stomatal responses to CO 2 and light. By using the ABA 8′-hydroxylase-deficient Arabidopsis thaliana double mutant cyp707a1 cyp707a3 , which is unable to break down and instead accumulates high levels of ABA, we reveal the importance of the control of ABA concentration in mediating stomatal responses to CO 2 and light. Intriguingly, our experiments suggest that endogenously produced ABA is unable to close stomata in the absence of CO 2 . Furthermore, we show that when plants are grown in short day conditions ABA breakdown is required for the modulation of both elevated [CO 2 ]-induced stomatal closure and elevated [CO 2 ]-induced reductions in leaf stomatal density. ABA catabolism is also required for the stomatal density response to light intensity, and for the full range of light-induced stomatal opening, suggesting that ABA catabolism is critical for the integration of stomatal responses to a range of environmental stimuli.