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Background Harpins are proteins secreted by the type III secretion system of Gram-negative bacteria during pathogen–plant interactions that can act as elicitors, stimulating defense and plant growth in many types of non-host plants. Harpin-treated plants have higher resistance, quality and yields and, therefore, harpin proteins may potentially have many valuable agricultural applications. Harpins are characterized by high thermal stability at 100 °C. However, it is unknown whether harpins are still active at temperatures above 100 °C or whether different temperatures affect the activity of the harpin protein in different ways. The mechanism responsible for the heat stability of harpins is also unknown. Results We identified a novel harpin, HpaXpm, from the cassava blight bacteria Xanthomonas phaseoli pv. manihotis HNHK. The predicted secondary structure and 3-D structure indicated that the HpaXpm protein has two β-strand domains and two major α-helical domains located at the N- and C-terminal regions, respectively. A phylogenetic tree generated using the maximum likelihood method grouped HpaXpm in clade I of the Hpa1 group along with harpins produced by other Xanthomonas spp. (i.e., HpaG-Xag, HpaG-Xcm, Hpa1-Xac, and Hpa1Xm). Phenotypic assays showed that HpaXpm induced the hypersensitive response (HR), defense responses, and growth promotion in non-host plants more effectively than Hp1Xoo (X. oryzae pv. oryzae). Quantitative real-time PCR analysis indicated that HpaXpm proteins subjected to heat treatments at 100 °C, 150 °C, or 200 °C were still able to stimulate the expression of function-related genes (i.e., the HR marker genes Hin1 and Hsr203J, the defense-related gene NPR1, and the plant growth enhancement-related gene NtEXP6); however, the ability of heat-treated HpaXpm to induce HR was different at different temperatures. Conclusions These findings add a new member to the harpin family. HpaXpm is heat-stable up to 200 °C and is able to stimulate powerful beneficial biological functions that could potentially be more valuable for agricultural applications than those stimulated by Hpa1Xoo. We hypothesize that the extreme heat resistance of HpaXpm is because the structure of harpin is very stable and, therefore, the HpaXpm structure is less affected by temperature.

La clareta (creasing) es una alteración fisiológica que afecta a la piel de los frutos cítricos y se manifiesta en la fase de maduración, sobre todo a partir del cambio de color, observándose unas depresiones en la superficie externa del fruto que le dan un aspecto arrugado y lo deprecian a nivel comercial. El origen de esta alteración no está claramente dilucidado, aunque diversos estudios coinciden en indicar que las principales causas relacionadas con su aparición pueden ser: predisposición genética varietal, estados de déficit nutricionales, especialmente la carencia de calcio, la exposición a estreses abióticos como la sequía y las altas temperaturas durante el cuajado de los frutos. Los elicitores son moléculas que activan rutas de crecimiento y defensa en las plantas, mejorando su estado general y parecen ser una herramienta de acción que podría reducir la incidencia de la clareta. En este estudio preliminar, se ha evaluado el efecto de la aplicación de proteínas Harpin aB, elicitores vegetales, para combatir la aparición de clareta en frutos de naranjas del grupo navel. Los resultados sugieren una tendencia a reducir la incidencia de clareta e incrementar la firmeza de los frutos, así como un aumento del contenido en calcio ligado en estos frutos.

Fruits are one of the most important sources of polyphenols for humans, whether they are consumed fresh or as processed products. To improve the phenolic content of fruits, a novel field of interest is based on results obtained using elicitors, agrochemicals which were primarily designed to improve resistance to plant pathogens. Although elicitors do not kill pathogens, they trigger plant defense mechanisms, one of which is to increase the levels of phenolic compounds. Therefore, their application not only allows us to control plant disease but also to increase the phenolic content of plant foodstuffs. Pre- or post-harvest application of the most commonly used elicitors to several fruits is discussed in this review.

PurposeThe aim of this research is to determine the effects of pre‐harvest harpin (H) and modified atmosphere packaging (MAP) on the storage and fruit quality of cherry tomato (Lycopersicon esculentum Mill. cvs. “Alona” and “Cluster”).Design/methodology/approachH was used at 50 g 100 l‐1 concentration. First application was done on the seedlings in 3‐4 leaf stages and the other four applications were repeated at 20‐day intervals. Then, treated and untreated fruits were stored in plastic film materials with various O2 and CO2 permeabilities.FindingsWeight loss was significantly higher in tomatoes stored under normal atmosphere (NA) compared to MAP. Initial total soluble solids of 3‐4 percent increased to 7‐9 percent in without H (WH)+NA. Firmness was measured as 0.44 (“Alona”) and 1.20 kg (“Cluster”) in H+NA and 0.17 (“Alona”) and 0.30 kg (“Cluster”) in WH+NA at the end of storage. The acidity and ascorbic acid contents of tomatoes decreased during storage. Between H and WH treatments the levels of lycopene and β‐carotene were different significantly. At the end of the study, H gave successful results in both cultivars. This success was even higher especially when the H was combined with MAP. This combination slowed down the changes in TSS, firmness values. Therefore, H+50μ PE (polyethylene) treatment was effective with regard to delaying the maturity along the storage and fruit quality in cherry tomatoes.Originality/valueHe is a plant activator. It is the first bacterial hypersensitive response (HR)‐elicitor characterized. Effectiveness of H is evaluated on quality changes in treated cherry tomato fruits.

Harpin proteins are well known as eliciters that induce multiple responses in plants, such as systemic acquired resistance, hypersensitive response, enhancement of growth, resistance to the green peach aphid, and tolerance to drought. Overexpression of Harpin-encoding genes enhances plant resistance to diseases in tobacco, rice, rape, and cotton; however, it is not yet known whether the expression of Harpin-encoding genes in vivo improves plant tolerance to abiotic stresses. The results of this study showed that overexpression of a Harpin-encoding gene hrf1 in rice increased drought tolerance through abscisic acid (ABA) signalling. hrf1- overexpression induces an increase in ABA content and promotes stomatal closure in rice. The hrf1 transgenic rice lines exhibited a significant increase in water retention ability, levels of free proline and soluble sugars, tolerance to oxidative stress, reactive oxygen species-scavenging ability, and expression levels of four stress-related genes, OsLEA3-1, OsP5CS, Mn-SOD, and NM_001074345, under drought stress. The study confirmed that hrf1 conferred enhanced tolerance to drought stress on transgenic crops. These results suggest that Harpins may offer new opportunities for generating drought resistance in other crops.

The type III secretion system (T3SS) of plant and animal bacterial pathogens directs the secretion and injection of proteins into host cells. Some homologous genes of T3SS were found also in nonpathogenic bacteria, but the organization of its machinery and basic function are still unknown. In this study, we identified a T3SS gene cluster from the plant growth-promoting Pseudomonas fluorescens 2P24 and isolated the corresponding T3SS apparatus. The T3SS gene cluster of strain 2P24 is similar organizationally to that of pathogenic P. syringae, except that it lacks the regulator hrpR and the hrpK1 and hrpH genes, which are involved in translocation of proteins. Electron microscopy revealed that the T3SS supramolecular structure of strain 2P24 was comprised of two distinctive substructures: a long extracellular, filamentous pilus and a membrane-embedded base. We show that strain 2P24 deploys a harpin homologue protein, RspZ1, to elicit a hypersensitive response when infiltrated into Nicotiana tabacum cv. xanthi leaves with protein that is partially purified, and by complementing the hrpZ1 mutation of pHIR11. The T3SS of strain 2P24 retained ability to secrete effectors, whereas its effector translocation activity appeared to be excessively lost. Mutation of the rscC gene from 2P24 T3SS abolished the secretion of effectors, but the general biocontrol properties were unaffected. Remarkably, strain 2P24 induced functional MAMP-triggered immunity that included a burst of reactive oxygen species, strong suppression of challenge cell death, and disease expansion, while it was not associated with the secretion functional T3SS.

Salt stress is a major constraint for many crop plants, such as the moderately salt-sensitive economically important fruit crop grapevine. Plants have evolved different strategies for protection against salinity and drought. Jasmonate signalling is a central element of both biotic and abiotic stress responses. To discriminate stress quality, there must be cross-talk with parallel signal chains. Using two grapevine cell lines differing in salt tolerance, the response of jasmonate ZIM/tify-domain (JAZ/TIFY) proteins (negative regulators of jasmonate signalling), a marker for salt adaptation Na(+)/H(+) EXCHANGER (NHX1), and markers for biotic defence STILBENE SYNTHASE (StSy) and RESVERATROL SYNTHASE (RS) were analysed. It is shown that salt stress signalling shares several events with biotic defence including activity of a gadolinium-sensitive calcium influx channel (monitored by apoplastic alkalinization) and transient induction of JAZ/TIFY transcripts. Exogenous jasmonate can rescue growth in the salt-sensitive cell line. Suppression of jasmonate signalling by phenidone or aspirin blocks the induction of JAZ/TIFY transcripts. The rapid induction of RS and StSy characteristic for biotic defence in grapevine is strongly delayed in response to salt stress. In the salt-tolerant line, NHX1 is induced and the formation of reactive oxygen species, monitored as stress markers in the sensitive cell line, is suppressed. The data are discussed in terms of a model where salt stress signalling acts as a default pathway whose readout is modulated by a parallel signal chain triggered by biotic factors downstream of jasmonate signalling.

HrpN, the hypersensitive response elicitor from Erwinia amylovora, stimulated K+ outward rectifying currents in Arabidopsis thaliana suspension cells. It also decreased anion currents. These data demonstrate the ability of harpin to regulate different plasma membrane ion channels, putative components of signal transduction chains leading to defense responses and programmed cell death.

Erwinia amylovora strain CFBP1430 secretes a protein called HrpW in a Hrp-dependent manner. HrpW was detected in culture supernatant of the wild-type strain grown on solid inducing hrp medium. This protein shares structural similarities with elicitors of the hypersensitive response such as HrpN of Erwinia amylovora and PopA of Ralstonia solanacearum. Furthermore, the C-terminal region of HrpW is homologous to class III pectate lyases. An hrpW mutant is as aggressive as the wild-type strain on pear and apple seedlings. It elicits the hypersensitive response on tobacco at a lower concentration than the wild-type strain.