The possibility of P. capsici-derived ABA was of interest because some plant pathogenic fungi produce ABA , and some stramenopiles such as the malarial pathogen, Plasmodium falciparum, are capable of ABA synthesis . However, we did not detect ABA in P. capsici culture filtrates or mycelium by immunoassay , and genes encoding the necessary biosynthetic enzymes are not evident in oomycete genomes . Furthermore, we found no evidence that P. capsici infection further engages the pathway as part of its infection strategy, either in non-stressed or salt-stressed tomato plants. These results indicate that salt stress, but not Phytophthora infection, strongly engages the ABA pathway in tomato roots – NCED1 and TAS14 gene expression, and ABA synthesis and accumulation. The SA-induced tomato PR protein, P4, is homologous to PR-1 in tobacco and Arabidopsis. P4 gene expression is induced in tomato leaves by plant activators , pathogens, including Phytophthora infestans, and the oomycete elicitor arachidonic acid . We found that infection of tomato roots by P. capsici strongly induces P4, but exposure of the roots to salt prior to inoculation essentially abolished P4 expression relative to non-stressed, inoculated plants . Similarly, expression of the JA-induced PI-2 was significantly reduced in infected plants that had been previously salt-stressed. Our findings that salt stress prevents pathogen-induced SA- and JA-regulated gene expression are consistent with results in other plant–microbe interactions that demonstrate ABA-mediated suppression of SA and JA defense responses . Tomato plants suppressed in SA accumulation by the nahG transgene are more susceptible to P. capsici than the wild type control plants in both non-stressed and salt-stressed assay formats . This suggests a role for SA-mediated responses in partially limiting P. capsici colonization.

However,macetas cuadradas the proportional increase in pathogen colonization observed in salt-stressed plants relative to non-stressed plants is the same in both WT and NahG backgrounds. Impairment of SA action by salt stress may contribute to increased pathogen colonization; however, we did not see a compounding effect of the SA-deficiency in NahG plants on stress-induced disease severity. Salicylic acid’s role in tomato resistance to P. capsici is complex. In a study using chemical activators that mimic SA action to induce resistance, we found these activators when applied to roots induced systemic protection of tomato leaves against the bacterial speck pathogen , with and without predisposing salt stress . However, these same plant activator treatments afforded no protection against P. capsici, with or without the salt stress treatment. Pst and P. capsici are quite different in their infection strategies and requirements, as well as the organs they attack in the plant, so interpreting differences in disease outcomes following different treatments is a speculative exercise, at best. P. capsici may simply be a more aggressive pathogen relative to Pst, and our experimental format is highly conducive to root and crown rot disease. So P. capsici attack overwhelms any chemically induced resistance that is otherwise capable of withstanding Pst challenge. It is also possible that there is sub-functionalization within the SA response network in tomato. NahG expression may impair a set of SA-mediated defenses that are effective against P. capsici, but differ from a subset, induced by chemical activators, that are insufficient to resist this pathogen. The JA-deficient tomato mutants acx1 and def1 in the ‘Castlemart’ background are compromised in defense against insects and pathogens . Although severity of the predisposition phenotype can vary among tomato cultivars, we were astonished that ‘Castlemart’ and its JA mutants were not predisposed by salt, strongly trending instead toward enhanced resistance . This suggests a stress response in ‘Castlemart’ that is different from other tomato genotypes we have examined in predisposition studies. The reason for this is unclear, and limited resources precluded our further examining predisposition in this cultivar.

Unlike the other genotypes used in our study, ‘Castlemart’ is a processing variety with a pedigree that may have incorporated different stress tolerances. It is a determinate variety that was bred for arid climates, and arid zone soils are more commonly associated with salinity . ‘Castlemart’ has been reported to accumulate proteinase inhibitors in response to high salinity . Jasmonic acid and its methyl ester when applied to leaves can induce resistance in tomato to P. infestans . Arabidopsis mutants in JA perception and synthesis are more susceptible to oomycete pathogens. Studies with other oomycete diseases also illustrate JA’s importance in resistance . We found that exogenous JA enabled tomato roots to respond in a manner that partially offset the salt stress impairment of PR-protein gene expression . The induction of P4 only during infection of JA-treated plants is reminiscent of the reported sensitization by methyl jasmonate of the plant’s response to eicosapolyenoic acid elicitors released during infection by Phytophthora species and potentiation of JA signaling by the plant activator β-aminobutryic acid . Our results with the tomato genotypes and treatments used in this and previous studies affirms ABA’s dominant effect relative to the salt-induced impacts on SA and JA action during predisposition to Phytophthora root and crown rot. ABA appears to be necessary to predispose tomato seedlings to this disease following acute salt stress. However, results presented here and previously indicate that priming through chemical activation of the SA and JA response networks may partially offset the stress-induced impairment of defense-related gene expression and the increased susceptibility in tomato to certain pathogens. We recognize that the response pathways modulated by ABA, JA and SA during episodic root stress may interact in subtle ways beyond the resolution afforded by the pathosystem and treatments we selected .

Comparative transcriptomics, proteomics and metabolomics of plants under predisposing stress should help identify key regulatory features . Studies with additional mutants as well as salt- and drought-tolerant genotypes also may reveal additional variation that could be useful to refine our understanding of the abiotic-biotic stress ‘interactome’ . This information could suggest novel targets to mitigate the impact of root stresses that increase severity of soilborne diseases. Enhancing the intrinsic ability of crops to survive and thrive under stressed or marginal conditions is a key pathway to reduce agricultural inputs and sustain crop production in a changing climate. When plants are under stress, the over-accumulation of reactive oxygen species leads to the damage of important bio-molecules such as nucleic acids and proteins, resulting in phytotoxicity and growth inhibition. Therefore, modulating ROS homeostasis is a potential pathway to develop plants with an enhanced tolerance to stress. Nanomaterials have unique physicochemical properties such as small size and excellent catalytic activities that enable unique integration with cellular metabolic processes. NMs with enzyme-like catalytic activities are defined as nanozymes. Until now, a number of NMs have been found to have ROS-scavenging capacities that functionally mimic the activity of antioxidant enzymes. For instance, cerium oxide nanoparticles possess superoxide dismutase -like activities,maceta cuadrada plastico which can catalyse the decomposition of ROS. A previous study proposed to use ROS-scavenging CeO2 NPs to augment plant photosynthesis. The authors reported that negatively charged poly CeO2 NPs inserted into chloroplasts can efficiently increase photosynthetic activities by trapping and quenching free radicals prior to organelle damage. These findings demonstrate the promising potential of using ROS-scavenging nanozymes to augment inherent antioxidant functions and constitutively enhance the stress tolerance abilities of plants. ROS is like a double-edged sword. On one hand, the overproduction of ROS damages cell membranes, DNA and protein. On the other hand, when below the threshold value, ROS act as signalling molecules that play key roles in response to abiotic and biotic stresses, such as stress sensing, the integration of different stress-response signalling networks and the activation of stress-response genes . Given this, we hypothesize that below a certain dose, ROS-triggering NMs could be applied to enhance the stress tolerance of plants by stimulating a broad range of defensive pathways. Different from ROS-scavenging NMs, which serve as a ‘recovery or curative’ strategy , ROS-triggering NMs could be applied as a ‘preventive’ strategy to stimulate plant defence systems through boosted signalling ROS and thereby metabolically prepare the plant for future stresses . To date, the use of ROS-triggering NMs to stimulate plant immunity and enhance stress resistance remains largely unexplored compared with ROS-scavenging nanozymes. Here we provide a comprehensive review of the use of nanobiotechnology-based strategies for plant stress tolerance enhancement. We focus on recent discoveries of using ROS-modulating NMs to enhance stress resistance. We review the current research progress of utilizing ROS-scavenging nanozymes to enhance plant stress tolerance. We present directions of using ROS-triggering or defence-triggering NMs for disease or stress tolerance enhancement. Last, we discuss the current challenges and prospects of these applications as a sustainable tool to enhance agricultural productivity.ROS, such as superoxide anion , hydroxyl radical , hydrogen peroxide and singlet oxygen , consist of radical and non-radical oxygen species formed during the metabolism of oxygen.

ROS are continuously produced by a variety of metabolic pathways in plant cells. The production of ROS in plant cells results in both detrimental and beneficial effects. When ROS over-accumulate, these analytes can damage cell membranes and inhibit photosynthesis. Plants have endogenous ROS elimination systems consisting of enzymatic , ascorbate peroxidase , glutathione reductase , dehydroascorbate reductase , glutathione peroxidase and non-enzymatic , phenolic acids, alkaloids, flavonoids, carotenoids, α-tocopherol, non-protein amino acids antioxidants. These antioxidant systems maintain the balance between ROS production and scavenging. ROS have traditionally been considered as undesirable by-products of metabolic processes generated in different cellular compartments. However, recently the roles of ROS in response to abiotic stress and immunity have been reported. Separate from their damaging activities, ROS appear to play a central role in the acclimation process of plants to abiotic stress. Plants possess ROS-producing enzymes oxidases in the plasma membrane, which are critical factors in the response to hormonal and environmental signals. In addition, ROS can transport signals to the nucleus through the mitogen-activated protein kinase pathway to increase tolerance against diverse abiotic stresses3 . Taken together, ROS-scavenging enzymes are crucial for stress alleviation, while ROS-producing enzymes are responsible for enhancing immunity to defend against pathogens or abiotic stresses . Understanding the balance between ROS-producing and ROS-eliminating mechanisms in plants could better inform nanobionic approaches to mitigate damage from external stressors.Given that excessive ROS can negatively impact photosynthesis, some recent studies show that delivering antioxidant NMs to chloroplasts can protect plants against ROS damage. For example, one study reported that foliar injection of CeO2 NPs effectively protected the photosynthetic system of Arabidopsis thaliana from oxidative damage associated with excessive light, heat and chilling. Another study further demonstrated that poly-coated CeO2 NPs with catalytic scavenging • OH capacity are effective in alleviating salinity stress in A. thaliana. The study reports that CeO2 NPs applied to leaf mesophyll can significantly increase carbon assimilation rates , quantum efficiency of photo system II and chlorophyll content compared with controls after exposure to 100 mM NaCl for 3 days. The authors of another study observed that CeO2 NPs alleviated drought-induced oxidative stress in sorghum by eliminating free radicals. Foliar-applied CeO2 NPs at 10 mg l−1 significantly increased leaf carbon assimilation rates by 38%, pollen germination by 31% and seed yield by 31% in drought-stressed plants relative to controls. A number of additional studies have investigated the use of CeO2 NPs to mitigate salinity stress in crop species such as maize, cotton, grape and Moldavian balm. The collective results highlight the strong potential of using ROS-scavenging CeO2 NPs as antioxidants to alleviate a number of abiotic stresses in plants. This demonstrated ability of CeO2 NPs to scavenge ROS can enable plants to cope with extreme environmental conditions. CeO2 NPs possess SOD-like and/or CAT-like activities. The multifunctional catalytic activities of CeO2 NPs are largely due to the coexistence of two valence states . The alternation between III/IV valence enables the reaction with oxygen radicals and hydrogen peroxide, catalysing a range of reversible redox reactions. Compared with small molecular antioxidants or natural antioxidant enzymes that are consumed as they scavenge ROS, CeO2 nanozymes are conserved as they catalyse the ROS-elimination reactions. Nanozymes are able to regenerate the catalytic sites of ROS scavenging, and this cycling/reusability property enables the action over days or weeks, providing plants with long-term protection under stress conditions. In addition, the high stability and durability of nanozymes enable them to cope with extreme environmental conditions, such as heat, cold and drought events.