Alberto Coego

Nitric Oxide Sensing in Plants Is Mediated by Proteolytic Control of Group VII ERF Transcription Factors

Summary Nitric 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.

An Arabidopsis Homeodomain Transcription Factor, OVEREXPRESSOR OF CATIONIC PEROXIDASE 3, Mediates Resistance to Infection by Necrotrophic Pathogens

The mechanisms controlling plant resistance to necrotrophic fungal pathogens are poorly understood. We previously reported on Ep5C, a gene shown to be induced by the H2O2 generated during a plant–pathogen interaction. To identify novel plant components operating in pathogen-induced signaling cascades, we initiated a large-scale screen using Arabidopsis thaliana plants carrying the β-glucuronidase reporter gene under control of the H2O2-responsive Ep5C promoter. Here, we report the identification and characterization of a mutant, ocp3 (for overexpressor of cationic peroxidase 3), in which the reporter construct is constitutively expressed. Healthy ocp3 plants show increased accumulation of H2O2 and express constitutively the Glutathione S-transferase1 and Plant Defensine 1.2 marker genes, but not the salicylic acid (SA)–dependent pathogenesis-related PR-1 gene. Strikingly, the ocp3 mutant shows enhanced resistance to the necrotrophic pathogens Botrytis cinerea and Plectosphaerella cucumerina. Conversely, resistance to virulent forms of the biotrophic oomycete Hyaloperonospora parasitica and the bacterial pathogen Pseudomonas syringae pv tomato DC3000 remains unaffected in ocp3 plants when compared with wild-type plants. Consistently with this, ocp3 plants are not affected in SA perception and express normal levels of PR genes after pathogen attack. To analyze signal transduction pathways where ocp3 operates, epistasis analyses between ocp3 and pad4, nahG, npr1, ein2, jin1, or coi1 were performed. These studies revealed that the resistance signaling to necrotrophic infection in ocp3 is fully dependent on appropriate perception of jasmonic acid through COI1 and does not require SA or ethylene perception through NPR1 or EIN2, respectively. The OCP3 gene encodes a homeodomain transcription factor that is constitutively expressed in healthy plants but repressed in response to infection by necrotrophic fungi. Together, these results suggest that OCP3 is an important factor for the COI1-dependent resistance of plants to infection by necrotrophic pathogens.

Drought tolerance in Arabidopsis is controlled by the OCP3 disease resistance regulator

Water scarcity and corresponding abiotic drought stress is one of the most important factors limiting plant performance and yield. In addition, plant productivity is severely compromised worldwide by infection with microbial pathogens. Two of the most prominent pathways responsible for drought tolerance and disease resistance to fungal pathogens in Arabidopsis are those controlled by the phytohormones abscisic acid (ABA) and the oxylipin methyl jasmonate (MeJA), respectively. Here, we report on the functional characterization of OCP3, a transcriptional regulator from the homeodomain (HD) family. The Arabidopsis loss-of-function ocp3 mutant exhibits both drought resistance and enhanced disease resistance to necrotrophic fungal pathogens. Double-mutant analysis revealed that these two resistance phenotypes have different genetic requirements. Whereas drought tolerance in ocp3 is ABA-dependent but MeJA-independent, the opposite holds true for the enhanced disease resistance characteristics. These observations lead us to propose a regulatory role of OCP3 in the adaptive responses to these two stresses, functioning as a modulator of independent and specific aspects of the ABA- and MeJA-mediated signal transduction pathways.

MYB46 Modulates Disease Susceptibility to Botrytis cinerea in Arabidopsis

In this study, we show that the Arabidopsis (Arabidopsis thaliana) transcription factor MYB46, previously described to regulate secondary cell wall biosynthesis in the vascular tissue of the stem, is pivotal for mediating disease susceptibility to the fungal pathogen Botrytis cinerea. We identified MYB46 by its ability to bind to a new cis-element located in the 5′ promoter region of the pathogen-induced Ep5C gene, which encodes a type III cell wall-bound peroxidase. We present genetic and molecular evidence indicating that MYB46 modulates the magnitude of Ep5C gene induction following pathogenic insults. Moreover, we demonstrate that different myb46 knockdown mutant plants exhibit increased disease resistance to B. cinerea, a phenotype that is accompanied by selective transcriptional reprogramming of a set of genes encoding cell wall proteins and enzymes, of which extracellular type III peroxidases are conspicuous. In essence, our results substantiate that defense-related signaling pathways and cell wall integrity are interconnected and that MYB46 likely functions as a disease susceptibility modulator to B. cinerea through the integration of cell wall remodeling and downstream activation of secondary lines of defense.

Diverse functional interactions between nitric oxide and abscisic acid in plant development and responses to stress

The extensive support for abscisic acid (ABA) involvement in the complex regulatory networks controlling stress responses and development in plants contrasts with the relatively recent role assigned to nitric oxide (NO). Because treatment with exogenous ABA leads to enhanced production of NO, it has been widely considered that NO participates downstream of ABA in controlling processes such as stomata movement, seed dormancy, and germination. However, data on leaf senescence and responses to stress suggest that the functional interaction between ABA and NO is more complex than previously thought, including not only cooperation but also antagonism. The functional relationship is probably determined by several factors including the time- and place-dependent pattern of accumulation of both molecules, the threshold levels, and the regulatory factors important for perception. These factors will determine the actions exerted by each regulator. Here, several examples of well-documented functional interactions between NO and ABA are analysed in light of the most recent reported data on seed dormancy and germination, stomata movements, leaf senescence, and responses to abiotic and biotic stresses.

The TRANSPLANTA collection of Arabidopsis lines: a resource for functional analysis of transcription factors based on their conditional overexpression

Transcription factors (TFs) are key regulators of gene expression in all organisms. In eukaryotes, TFs are often represented by functionally redundant members of large gene families. Overexpression might prove a means to unveil the biological functions of redundant TFs; however, constitutive overexpression of TFs frequently causes severe developmental defects, preventing their functional characterization. Conditional overexpression strategies help to overcome this problem. Here, we report on the TRANSPLANTA collection of Arabidopsis lines, each expressing one of 949 TFs under the control of a β-estradiol-inducible promoter. Thus far, 1636 independent homozygous lines, representing an average of 2.6 lines for every TF, have been produced for the inducible expression of 634 TFs. Along with a GUS-GFP reporter, randomly selected TRANSPLANTA lines were tested and confirmed for conditional transgene expression upon β-estradiol treatment. As a proof of concept for the exploitation of this resource, β-estradiol-induced proliferation of root hairs, dark-induced senescence, anthocyanin accumulation and dwarfism were observed in lines conditionally expressing full-length cDNAs encoding RHD6, WRKY22, MYB123/TT2 and MYB26, respectively, in agreement with previously reported phenotypes conferred by these TFs. Further screening performed with other TRANSPLANTA lines allowed the identification of TFs involved in different plant biological processes, illustrating that the collection is a powerful resource for the functional characterization of TFs. For instance, ANAC058 and a TINY/AP2 TF were identified as modulators of ABA-mediated germination potential, and RAP2.10/DEAR4 was identified as a regulator of cell death in the hypocotyl-root transition zone. Seeds of TRANSPLANTA lines have been deposited at the Nottingham Arabidopsis Stock Centre for further distribution.

OCP3 is an important modulator of NPR1-mediated jasmonic acid-dependent induced defenses in Arabidopsis

BackgroundUpon appropriate stimulation, plants increase their level of resistance against future pathogen attack. This phenomenon, known as induced resistance, presents an adaptive advantage due to its reduced fitness costs and its systemic and broad-spectrum nature. In Arabidopsis, different types of induced resistance have been defined based on the signaling pathways involved, particularly those dependent on salicylic acid (SA) and/or jasmonic acid (JA).ResultsHere, we have assessed the implication of the transcriptional regulator OCP3 in SA- and JA-dependent induced defenses. Through a series of double mutant analyses, we conclude that SA-dependent defense signaling does not require OCP3. However, we found that ocp3 plants are impaired in a Pseudomonas fluorescens WCS417r-triggered induced systemic resistance (ISR) against both Pseudomonas syrinagae DC3000 and Hyaloperonospora arabidopsidis, and we show that this impairment is not due to a defect in JA-perception. Likewise, exogenous application of JA failed to induce defenses in ocp3 plants. In addition, we provide evidence showing that the over-expression of an engineered cytosolic isoform of the disease resistance regulator NPR1 restores the impaired JA-induced disease resistance in ocp3 plants.ConclusionsOur findings point to a model in which OCP3 may modulate the nucleocytosolic function of NPR1 in the regulation of JA-dependent induced defense responses.

Nitric oxide responses in Arabidopsis hypocotyls are mediated by diverse phytohormone pathways

An NO-sensing mechanism controlling hypocotyl growth in etiolated seedlings requires the biosynthesis and/or signaling of ethylene, strigolactones, salicylate, abscisic acid, and brassinosteroids.