Tomato seed biopriming: Unveiling molecular mechanisms for enhanced biotic stress management

Plants in natural environments are frequently exposed to multiple biotic and abiotic stresses, and their combined effects often trigger complex and non-additive physiological and molecular responses (Suzuki et al., 2014). Among these, parasitic plants impose a major biotic stress, establishing haustoria connections with host roots to extract water, nutrients, and signalling molecules (Albert et al., 2021; Yoshida et al., 2016). Parasitism may profoundly influence the host’s immune competence and interactions with other microbial pathogens. However, the impact of parasitic plants on host immune regulation against microbial infections remains largely unexplored. This study examined the impact of parasitism by the facultative hemiparasite Phtheirospermum japonicum on the immune responses of Arabidopsis thaliana to microbial pathogens. Parasitization assays revealed that A. thaliana plants parasitized by P. japonicum exhibited a significant reduction in growth and biomass, suggesting a substantial physiological burden for the host. To examine whether this effect extends to immune regulation, early pattern-triggered immunity (PTI) responses were analyzed using flg22 and chitin treatments. Measurements of reactive oxygen species (ROS) production, Mitogen-Activated Protein Kinase (MAPK) phosphorylation, and defence gene expression revealed that parasitized plants exhibited significantly reduced PTI activation compared to unparasitized controls. To determine whether this compromised immunity translated into altered disease susceptibility, infection assays were conducted using the hemibiotrophic bacterium Pseudomonas syringae pv. tomato DC3000 and the necrotrophic fungus Botrytis cinerea strain B05.10. Parasitized A. thaliana plants exhibited hypersusceptibility to both pathogens, characterized by higher bacterial titers and larger necrotic lesions compared to unparasitized plants, confirming a broad suppression of immune competence. To elucidate the molecular mechanisms underlying this immune attenuation, transcriptomic profiling (RNA-seq) was performed on parasitized and unparasitized A. thaliana plants treated with flg22 or infected with Pst DC3000. The results revealed that P. japonicum parasitism markedly suppressed flg22- and Pst-triggered transcriptional responses. In parasitized plants, PTI-associated genes and growth-related pathways were downregulated, while nutrient starvation pathways were induced. Moreover, infection with Pst DC3000 triggered strong phosphate starvation responses and repressed key defence pathways involving salicylic acid (SA), jasmonic acid (JA), and glucosinolate biosynthesis. Phosphate starvation is well documented to suppress plant immunity, a phenomenon that appears to be evolutionarily conserved across multiple plant species, where nutrient deprivation often compromises defence signalling and increases susceptibility to pathogens. Finally, to identify which phytohormone signalling networks are altered during parasitism, phytohormone reporter lines of A. thaliana were used to monitor SA, JA, JA/ethylene (JA/ET), and abscisic acid (ABA) activity in roots. Parasitism induced strong ABA signalling, whereas SA and JA/ET pathways remained largely unresponsive. Interestingly, JA reporters revealed a complex pattern, with induction of VSP2 and JAZ9 but no activation of AOS in the host roots. As previous transcriptomic studies have shown AOS induction in P. japonicum haustoria during parasitism (Kokla et al., 2022), this suggests that the observed JA response may originate in the parasite rather than the host, although this remains to be confirmed. Collectively, this study provides novel insights into how P. japonicum modulates host immunity and nutrient signalling. The findings highlight that parasitism induces nutrient starvation responses, particularly phosphate limitation, which are closely linked to suppression of immune signalling. This work underscores the intricate trade-offs between growth, defence, and nutrient acquisition in plants subjected to combined stresses, including parasitism and pathogen attack.

In the last years, the nutritional quality of fruits has been widely evaluated and requested by consumers, mainly because of the health effects they provide. As known, these benefits can be due to micronutrients, as vitamins and minerals, but also to phenolic compounds, as flavonoids and ellagitannins. In this context, strawberries represent a very good choice for a diet low in saturated fats and sodium and, at the same time they are rich in fiber, potassium and other minerals, vitamins, and antioxidant phytochemicals: all elements that are currently considered as the essential constituents of a well-balanced diet. However, the nutritional quality of strawberry fruits can be considerably affected by several pre-harvest and post-harvest conditions, which, in most cases, may decrease the nutrient and the phytochemical contents of this fruit. This paper reviews and updates the current knowledge on the nutritional and phytochemical composition of strawberry, paying particular attention on the role played by the genotype, the maturity, the environment, the storage and the processing on the nutritional quality of this fruit.

Fruits, frugivores and the evolutionary arms race.

Immune signaling networks must be tunable to alleviate fitness costs associated with immunity and, at the same time, robust against pathogen interferences. How these properties mechanistically emerge in plant immune signaling networks is poorly understood. Here, we discovered a molecular mechanism by which the model plant species Arabidopsis thaliana achieves robust and tunable immunity triggered by the microbe-associated molecular pattern, flg22. Salicylic acid (SA) is a major plant immune signal molecule. Another signal molecule jasmonate (JA) induced expression of a gene essential for SA accumulation, EDS5 Paradoxically, JA inhibited expression of PAD4, a positive regulator of EDS5 expression. This incoherent type-4 feed-forward loop (I4-FFL) enabled JA to mitigate SA accumulation in the intact network but to support it under perturbation of PAD4, thereby minimizing the negative impact of SA on fitness as well as conferring robust SA-mediated immunity. We also present evidence for evolutionary conservation of these gene regulations in the family Brassicaceae Our results highlight an I4-FFL that simultaneously provides the immune network with robustness and tunability in A.thaliana and possibly in its relatives.

SUMMARY The majority of plants establish symbiotic associations with arbuscular mycorrhizal (AM) fungi. The symbiosis provides the plants with an improved mineral nutrition and, to some extent, higher tolerance to biotic and abiotic stresses. In this work we have evaluated whether AM symbiosis modifies the response of tomato plants to the attack of the necrotrophic pathogen Botrytis cinerea. Leaves of tomato plants, colonized or not by the AM fungus Glomus mosseae, were infected with B. cinerea. A higher disease index in control plants (60.3%) compared to mycorrhizal plants (37.5%) was observed. To assess the potential involvement of salicylic acid (SA), jasmonic acid (JA) and abscisic acid (ABA) in this response, the levels of these hormones were also measured in the leaves of mycorrhizal and non mycorrhizal plants. While JA was not detected and no differences were observed in the SA content between the two biological conditions, a statistically significant lower content of ABA was detected in mycorrhizal vs control plants. Our results show that AM symbiosis reduces disease severity in tomato plants infected by B. cinerea and suggest that ABA is one component of the AM-induced lower susceptibility to B. cinerea.

Trichoderma species are well known biocontrol agents that are able to induce responses in the host plants against an array of abiotic and biotic stresses. Here, we investigate, when applied to tomato seeds, the potential of Trichoderma strains belonging to three different species, T. parareesei T6, T. asperellum T25, and T. harzianum T34, to control the fully pathogenic strain Pseudomonas syringae pv. tomato (Pst) DC3000, able to produce the coronatine (COR) toxin, and the COR-deficient strain Pst DC3118 in tomato plants, and the molecular mechanisms by which the plant can modulate its systemic defense. Four-week old tomato plants, seed-inoculated, or not, with a Trichoderma strain, were infected, or not, with a Pst strain, and the changes in the expression of nine marker genes representative of salicylic acid (SA) (ICS1 and PAL5) and jasmonic acid (JA) (TomLoxC) biosynthesis, SA- (PR1b1), JA- (PINII and MYC2) and JA/Ethylene (ET)-dependent (ERF-A2) defense pathways, as well as the abscisic acid (ABA)-responsive gene AREB2 and the respiratory burst oxidase gene LERBOH1, were analyzed at 72 hours post-inoculation (hpi) with the bacteria. The significant increase obtained for bacterial population sizes in the leaves, disease index, and the upregulation of tomato genes related to SA, JA, ET and ABA in plants inoculated with Pst DC3000 compared with those obtained with Pst DC3118, confirmed the COR role as a virulence factor, and showed that both Pst and COR synergistically activate the JA- and SA-signaling defense responses, at least at 72 hpi. The three Trichoderma strains tested reduced the DC3118 levels to different extents and were able to control disease symptoms at the same rate. However, a minor protection (9.4%) against DC3000 was only achieved with T. asperellum T25. The gene deregulation detected in Trichoderma-treated plus Pst-inoculated tomato plants illustrates the complex system of a phytohormone-mediated signaling network that is affected by the pathogen and Trichoderma applications but also by their interaction. The expression changes for all nine genes analyzed, excepting LERBOH1, as well as the bacterial populations in the leaves were significantly affected by the interaction. Our results show that Trichoderma spp. are not adequate to control the disease caused by fully pathogenic Pst strains in tomato plants.

Plants use a plethora of sophisticated detection systems to recognize a variety of attackers and to subsequently initiate defense responses. A well-known paradigm in this context is the perception of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs), a process referred to as pattern triggered immunity (PTI). Additionally, plants also recognize endogenous molecules to induce similar defense responses. These molecules are believed to be released upon enemy attack and are therefore referred to as danger-associated molecular patterns (DAMPs). The best-investigated DAMP so far is systemin, a short peptide capable of inducing defense responses in tomato and required for full-strength defense against insect herbivores. More recently, a family of eight peptides has been discovered in Arabidopsis, named Arabidopsis thaliana danger peptides (AtPeps) 1-8. These AtPeps have been shown to be capable of inducing PTI-like responses and to be expressed upon the detection of various biotic stresses, therefore being considered as DAMPs. Moreover, two PRRs, named Pep-Receptor 1 (PEPR1) and Pep-Receptor 2 (PEPR2) have been identified to perceive AtPeps and to induce defense responses upon receptor-ligand interaction. Despite of eliciting PTI responses and being expressed upon the detection of biotic stress, no direct beneficial involvement of the AtPep-PEPR system to plant defense against attackers has been described so far. Taking advantage of a mutant deficient in both PEPRs and thus fully impaired in AtPep-PEPR signaling, we investigated the potential contribution of a functional AtPep-PEPR system to plant defense responses. In a first approach, we investigated the potential interplay between MAMP and DAMP signaling, especially in the context of DAMPs being believed to act as endogenous amplifiers of MAMP-induced PTI. Doing so, we identified that the AtPep-triggered production of reactive oxygen species (ROS) is strongly enhanced upon previous MAMP detection, indeed indicating a role of the AtPep-PEPR signaling system as an enhancer of MAMP-triggered defense signaling. In a second approach, we compared the AtPep-PEPR system to systemins – well described DAMPs in tomato with generally similar molecular features to AtPeps. Following up the lead that systemins are important mediators of defense responses against herbivorous insects, we checked whether a similar role would apply to the AtPep-PEPR system. Here, we could show that the AtPep-PEPR system is indeed induced by herbivore feeding and strongly interacts with the plant hormone jasmonic acid (JA) to orchestrate defense responses. Accordingly, mutants deficient in AtPep-PEPR signaling are strongly impaired in defense responses against the generalist herbivore Spodoptera littoralis, underlining the importance of AtPep signaling in plant defense against herbivores. Thirdly, we followed up a lead that the expression of some AtPeps as well as both PEPRs is induced upon virus infection. Assessing the potential contribution of the AtPep-PEPR system to plant defense against viruses, we could not observe an increased susceptibility of plants deficient in both PEPRs. However, mutants in BAK1 (BRI1 Associated Kinase 1), a co-receptor required for full-strength AtPep-triggered signaling and many other PRRs, showed a clearly increased susceptibility to all viruses tested. Therefore, we established a first potential line of evidence for a role of PTI in plant defense against viruses. All in all, we provide several lines of evidence that show the contribution of a functional AtPep-PEPR signaling system to plant defense. Therefore, we underline the pivotal importance of DAMP signaling to plant immunity against a plethora of biotic invaders.

Tomato (Solanum lycopersicum L.) is among the most economically important vegetable crops worldwide, yet its productivity and fruit quality are severely constrained by fungal pathogens employing biotrophic, necrotrophic, or hemibiotrophic lifestyles. Major pathogens-including Oidium neolycopersici, Alternaria solani, Botrytis cinerea, Fusarium oxysporum f. sp. lycopersici (FOL), Verticillium dahliae, and Phytophthora infestans-reshape host physiology through distinct infection strategies. This review synthesizes current knowledge on the early immune responses of tomato to fungal invasion, focusing on pattern-triggered immunity (PTI) initiated by pattern-recognition receptors (PRRs) and subsequent signaling via reactive oxygen species (ROS), Ca²⁺ transients, and MAPK cascades, as well as on effector-triggered immunity (ETI) mediated by intracellular R proteins. We compare host physiological outcomes across pathogen lifestyles, highlighting SA-dominant signaling against biotrophs versus JA/ET-dominant responses against necrotrophs, and the two-phased immune dynamics typical for hemibiotrophs. We further discuss effectors that suppress PTI and the circumstances under which hypersensitive-like cell death may paradoxically benefit necrotrophs (effector-triggered susceptibility). Finally, we outline integrative disease management directions-breeding for durable resistance, immune priming, biological control, and informed fungicide rotation-framed by a mechanistic understanding of PTI-ETI crosstalk in tomato. This mechanistic perspective provides a conceptual basis for developing resistant cultivars and robust integrated disease management strategies. Keywords: Solanum Lycopersicum, Fungal Pathogens, Biotroph, Necrotroph, Hemibiotroph, PTI, ETI, PRR, ROS, Ca²⁺ Signaling, MAPK

Tomato (Solanum lycopersicum L.) serves as a major food source and a model crop for understanding plant responses to stress. Abiotic and biotic stresses, exacerbated by climate change, threaten global tomato production. Stress hormones, including abscisic acid (ABA), ethylene (ET), jasmonates (JAs), and salicylic acid (SA), orchestrate intricate signaling pathways that mediate plant immunity and metabolism. This review synthesizes the roles of these hormones in tomato stress responses. We discuss the biosynthesis and signaling cascades of these stress hormones, and focus on the cellular and metabolic reprogramming they cause and the crosstalk that occurs between them. Increased understanding of these molecular events and interactions provides insights to improve tomato resilience and productivity under environmental challenges.

Bacterial wilt is a devastating disease of tomato caused by soilborne pathogenic bacterium Ralstonia solanacearum. Previous studies found that silicon (Si) can increase tomato resistance against R. solanacearum, but the exact molecular mechanism remains unclear. RNA sequencing (RNA-Seq) technology was used to investigate the dynamic changes of root transcriptome profiles between Si-treated (+Si) and untreated (−Si) tomato plants at 1, 3, and 7 days post-inoculation with R. solanacearum. The contents of salicylic acid (SA), ethylene (ET), and jasmonic acid (JA) and the activity of defense-related enzymes in roots of tomato in different treatments were also determined. The burst of ET production in roots was delayed, and SA and JA contents were altered in Si treatment. The transcriptional response to R. solanacearum infection of the +Si plants was quicker than that of the untreated plants. The expression levels of differentially-expressed genes involved in pathogen-associated molecular pattern-triggered immunity (PTI), oxidation resistance, and water-deficit stress tolerance were upregulated in the Si-treated plants. Multiple hormone-related genes were differentially expressed in the Si-treated plants. Si-mediated resistance involves mechanisms other than SA- and JA/ET-mediated stress responses. We propose that Si-mediated tomato resistance to R. solanacearum is associated with activated PTI-related responses and enhanced disease resistance and tolerance via several signaling pathways. Such pathways are mediated by multiple hormones (e.g., SA, JA, ET, and auxin), leading to diminished adverse effects (e.g., senescence, water-deficit, salinity and oxidative stress) normally caused by R. solanacearum infection. This finding will provide an important basis to further characterize the role of Si in enhancing plant resistance against biotic stress.

This is the second special issue on Plant Hormone Signaling. Abscisic acid (ABA), ethylene and salicylic acid (SA) have long been recognized as the key plant hormones mediating abiotic and biotic stresses, respectively. The underlying mechanisms of action by which these and other hormones modulate the response of plants to stresses have received considerable attention in the recent past. Research findings in the last two decades have been revealing their fascinating modes of action. Furthermore, the intricate crosstalks among various hormones by which they can modulate growth and development in response to diverse environmental stresses have emerged as a common theme in this field. With the identification of specific receptors for individual hormones, convincing mechanisms explaining the modes of action of these hormones have been proposed. The involvement of ethylene and the AP2/ERF family of proteins, PR proteins and the like in biotic stress response has now been well established. Besides ABA and ethylene, it is now known that salicylic acid, jasmonates, brassinosteroids, and even gibberellins and auxins crosstalk extensively to regulate practically all aspects of plant stress responses. Eminent practitioners in the field have provided enthusiastic support for this issue as with the first special issue, which dealt with general aspects of hormone signaling. The sixteen contributions compiled in this issue are articles dealing specifically with abiotic and biotic stresses. The contributions range from how hormones initiate signal cascades to control grain yield, root development, and DNA repair as well as protein modifications such as SUMOylation. Also included are papers on the crosstalks among ABA, ethylene, gibberellins, brassinosteroids, SA, jasmonates etc. to regulate herbivory and other biotic stresses. It is hoped that the timely reviews of the signaling-related topics in the two special issues will be a useful contribution to plant biologists in general. Exciting new findings continue to be made, and work on plant hormone signaling intermediates holds tremendous promises for introducing new traits and protecting high levels of crop productivity in the face of global climate change. We can be confident in declaring that the proposed modes of action of plant hormones will continue to get refined and such studies will help in boosting crop productivity in the coming decades. Hopefully, this effort to put together a collection of useful reference material will be appreciated by our readers.

The Arabidopsis mutant wrky33 is highly susceptible to Botrytis cinerea. We identified >1680 Botrytis-induced WRKY33 binding sites associated with 1576 Arabidopsis genes. Transcriptional profiling defined 318 functional direct target genes at 14 hr post inoculation. Comparative analyses revealed that WRKY33 possesses dual functionality acting either as a repressor or as an activator in a promoter-context dependent manner. We confirmed known WRKY33 targets involved in hormone signaling and phytoalexin biosynthesis, but also uncovered a novel negative role of abscisic acid (ABA) in resistance towards B. cinerea 2100. The ABA biosynthesis genes NCED3 and NCED5 were identified as direct targets required for WRKY33-mediated resistance. Loss-of-WRKY33 function resulted in elevated ABA levels and genetic studies confirmed that WRKY33 acts upstream of NCED3/NCED5 to negatively regulate ABA biosynthesis. This study provides the first detailed view of the genome-wide contribution of a specific plant transcription factor in modulating the transcriptional network associated with plant immunity.DOI: http://dx.doi.org/10.7554/eLife.07295.001

Introduction to a Virtual Special Issue on cell biology at the plant-microbe interface.

Improved nutrient use efficiency together with the use of biostimulants have been little explored thus far to improve fruit yield and quality in economically relevant crops. The aim of this study was to determine the additive or synergistic effects, if any, of the application of an enzyme hydrolyzed animal protein biostimulant (Pepton) combined with priming with low nitrogen (N) in the production and quality of greenhouse tomatoes. Biostimulant treatment (Pepton at a dose equivalent of 4 kg/ha) was applied by ferti-irrigation for 2 months during the vegetative phase both in controls (watered with nutrient solution) and nutrient efficient crop (NEC), in which plants were primed with low N by exposing them to a 30% N deficiency for 2 months, and then recovered for 1 month before fruit production. Foliar water and N contents, pigments, maximum PSII efficiency (Fv/Fm ratio), and phytohormones [including abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), and cytokinins] were measured prior and at 4 and 8 weeks after the first application. Fruit production and quality [as indicated by total soluble sugars (TSS) and acidity (TA), and the contents of lycopene, vitamin E, and vitamin C] were measured 1 month later at harvest. Priming with low N availability (NEC plants) doubled (p < 0.001) fruit production (due to an increase in the number of fruits), tended to increase (p = 0.057) by 20% the amount of TSS and increased (p < 0.05) the contents of lycopene (by 90%) and vitamin E (by 40%). Pepton displayed a tendency, almost significant, to improve (p = 0.054) total fruit production both in control and NEC plants, thus showing an additive effect to low N priming in boosting fruit production. Pepton maintained fruit quality in terms of sugar accumulation, total acidity and the contents of carotenoids, vitamins C and E. Pepton-related improvement in fruit production seemed to be related, at least partially, to an increased accumulation of cytokinins and photosynthetic pigments in leaves, which might favor vegetative vigor and ultimately fruit yield. In conclusion, Pepton application was effective in improving the yield of greenhouse tomatoes showing additive effect with low N priming, without negatively affecting fruit quality.

Salicylic acid improves acclimation to salt stress by stimulating abscisic aldehyde oxidase activity and abscisic acid accumulation, and increases Na + content in leaves without toxicity symptoms in Solanum lycopersicum L.