Moniliophthora perniciosa, the causal agent of witches' broom disease of cacao, interferes with cytokinin metabolism during infection of Micro-Tom tomato and promotes symptom development

Root Development of Tomato Plants Infected by the Cacao Pathogen Moniliophthora perniciosa Is Affected by Limited Sugar Availability

Moniliophthora perniciosa is the causal agent of the witches' broom disease of cacao (Theobroma cacao), and it can infect the tomato (Solanum lycopersicum) 'Micro-Tom' (MT) cultivar. Typical symptoms of infection are stem swelling and axillary shoot outgrowth, whereas reduction in root biomass is another side effect. Using infected MT, we investigated whether impaired root growth derives from hormonal imbalance or sink competition. Intense stem swelling coincided with a reduction in root biomass, predominantly affecting lateral roots. RNA-seq analyses of root samples identified only a few differentially expressed genes involved in hormone metabolism, and root hormone levels were not expressively altered. Inoculation of the auxin highly-sensitive entire mutant genotype maintained the impaired root phenotype; in contrast, the low-cytokinin MT transgenic line overexpressing CYTOKININ OXIDASE-2 (35S::AtCKX2) with fewer symptoms did not exhibit root growth impairment. Genes involved in cell wall, carbohydrate, and amino acid metabolism were downregulated, accompanied by lower levels of carbohydrate and amino acid in roots, suggesting a reduction in metabolite availability. 13CO2 was supplied to MT plants, and less 13C was detected in the roots of infected MT but not in those of 35S::AtCKX2 line plants, suggesting that cytokinin-mediated sugar sink establishment at the infection site may contribute to impaired root growth. Exogenous sucrose application to roots of infected MT plants partially restored root growth. We propose that the impairment of lateral root development is likely attributed to disrupted sugar signalling and photoassimilate supply by establishing a strong sugar sink at the infected stem.

Effects of Varying Nitrogen Concentrations on the Locule Number in Tomato Fruit

Tomato seedlings were treated with nutrient solutions containing varying nitrogen concentrations (50, 150, and 250 mg·L-1) after germination until the completion of flower bud differentiation. The changes in nutrient content, enzyme activity, endogenous hormone levels, and gene expression in the stem apex were analyzed to explore the mechanisms regulating the number of locules in tomatoes at different nitrogen concentrations. The results indicated that an increase in nitrogen concentration facilitated the differentiation of tomato flower buds, increased the number of fruit locules, and increased the contents of soluble sugar, soluble protein, starch, and sucrose, as well as the activities of the enzymes POD, NR, and PPO in the seedling stem apex. The contents of soluble sugars and soluble proteins, as well as the activities of POD, NR, and PPO, were closely correlated with the number of fruit locules. An increase in nitrogen concentration was also found to elevate cytokinin levels while reducing auxin content in the stem apex. The transcriptome analysis screened for peroxidase genes, auxin response genes, and cytokinin synthesis genes. The analysis of gene expression patterns suggests that CKX and LOG6 play significant roles in flower development. Additionally, combined physiological changes indicated that an increase in nitrogen concentration during the tomato seedling stage leads to a higher number of fruit locules, which may be associated with elevated cytokinin content, primarily involving the key genes CKX and LOG6.

Proteomic Analysis of Plants with Binding Immunoglobulin Protein Overexpression Reveals Mechanisms Related to Defense Against Moniliophthora perniciosa

Moniliophthora perniciosa is one of the main pathogens affecting cocoa, and controlling it generally involves planting resistant genotypes followed by phytosanitary pruning. The identification of plant genes related to defense mechanisms is crucial to unravel the molecular basis of plant-pathogen interactions. Among the candidate genes, BiP stands out as a molecular chaperone located in the endoplasmic reticulum that facilitates protein folding and is induced under stress conditions, such as pathogen attacks. In this study, the SoyBiPD gene was expressed in Solanum lycopersicum plants and the plants were challenged with M. perniciosa. The control plants exhibited severe symptoms of witches' broom disease, whereas the transgenic lines showed no or mild symptoms. Gel-free proteomics revealed significant changes in the protein profile associated with BiP overexpression. Inoculated transgenic plants had a higher abundance of resistance-related proteins, such as PR2, PR3, and PR10, along with increased activity of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase, and fungal cell wall-degrading enzymes (glucanases). Additionally, transgenic plants accumulated less H2O2, indicating more efficient control of reactive oxygen species (ROS). The interaction network analysis highlighted the activation of defense-associated signaling and metabolic pathways, conferring a state of defensive readiness even in the absence of pathogens. These results demonstrate that BiP overexpression increases the abundance of defense proteins, enhances antioxidant capacity, and confers greater tolerance to biotic stress. This study demonstrates the biotechnological potential of the BiP gene for genetic engineering crops with increased resistance to economically important diseases, such as witches' broom in cocoa.

Multifunctional Role of Cytokinin in Horticultural Crops

Cytokinins (CKs) are a class of phytohormones identified in the early 1960s and are mainly responsible for stimulating cell division. Following the discovery, research to help understand the pluralistic roles of CKs in plant growth and stress biology increased. With their fascinating ability, CKs serve as an important element in regulating the defense-growth trade-off. Herein, we demonstrate how the CK fine-tuning the organogenesis of different parts of horticultural plants is discussed. CK's role in tailoring reproductive biology (flowering, sex differentiation, fruit set, and fruit attributes) has been presented. An extensive explanation of the CK-mediated response of horticultural crops to abiotic (temperature, drought, and salinity) and biotic stresses (fungal, bacterial, and nematodes) is provided. Finally, we posit the unexplored roles of CKs and highlight the research gaps worth addressing.

Comparative transcriptome analysis of juniper branches infected by Gymnosporangium spp. highlights their different infection strategies associated with cytokinins

BACKGROUND

Gymnosporangium asiaticum and G. yamadae can share Juniperus chinensis as the telial host, but the symptoms are completely different. The infection of G. yamadae causes the enlargement of the phloem and cortex of young branches as a gall, but not for G. asiaticum, suggesting that different molecular interaction mechanisms exist the two Gymnosporangium species with junipers.

RESULTS

Comparative transcriptome analysis was performed to investigate genes regulation of juniper in responses to the infections of G. asiaticum and G. yamadae at different stages. Functional enrichment analysis showed that genes related to transport, catabolism and transcription pathways were up-regulated, while genes related to energy metabolism and photosynthesis were down-regulated in juniper branch tissues after infection with G. asiaticum and G. yamadae. The transcript profiling of G. yamadae-induced gall tissues revealed that more genes involved in photosynthesis, sugar metabolism, plant hormones and defense-related pathways were up-regulated in the vigorous development stage of gall compared to the initial stage, and were eventually repressed overall. Furthermore, the concentration of cytokinins (CKs) in the galls tissue and the telia of G. yamadae was significantly higher than in healthy branch tissues of juniper. As well, tRNA-isopentenyltransferase (tRNA-IPT) was identified in G. yamadae with highly expression levels during the gall development stages.

CONCLUSIONS

In general, our study provided new insights into the host-specific mechanisms by which G. asiaticum and G. yamadae differentially utilize CKs and specific adaptations on juniper during their co-evolution.

Infection by Moniliophthora perniciosa reprograms tomato Micro-Tom physiology, establishes a sink, and increases secondary cell wall synthesis

Witches' broom disease of cacao is caused by the pathogenic fungus Moniliophthora perniciosa. By using tomato (Solanum lycopersicum) cultivar Micro-Tom (MT) as a model system, we investigated the physiological and metabolic consequences of M. perniciosa infection to determine whether symptoms result from sink establishment during infection. Infection of MT by M. perniciosa caused reductions in root biomass and fruit yield, a decrease in leaf gas exchange, and down-regulation of photosynthesis-related genes. The total leaf area and water potential decreased, while ABA levels, water conductance/conductivity, and ABA-related gene expression increased. Genes related to sugar metabolism and those involved in secondary cell wall deposition were up-regulated upon infection, and the concentrations of sugars, fumarate, and amino acids increased. 14C-glucose was mobilized towards infected MT stems, but not in inoculated stems of the MT line overexpressing CYTOKININ OXIDASE-2 (35S::AtCKX2), suggesting a role for cytokinin in establishing a sugar sink. The up-regulation of genes involved in cell wall deposition and phenylpropanoid metabolism in infected MT, but not in 35S::AtCKX2 plants, suggests establishment of a cytokinin-mediated sink that promotes tissue overgrowth with an increase in lignin. Possibly, M. perniciosa could benefit from the accumulation of secondary cell walls during its saprotrophic phase of infection.

Loss of function of a DMR6 ortholog in tomato confers broad-spectrum disease resistance

Plant diseases are among the major causes of crop yield losses around the world. To confer disease resistance, conventional breeding relies on the deployment of single resistance (R) genes. However, this strategy has been easily overcome by constantly evolving pathogens. Disabling susceptibility (S) genes is a promising alternative to R genes in breeding programs, as it usually offers durable and broad-spectrum disease resistance. In Arabidopsis, the S gene DMR6 (AtDMR6) encodes an enzyme identified as a susceptibility factor to bacterial and oomycete pathogens. Here, we present a model-to-crop translational work in which we characterize two AtDMR6 orthologs in tomato, SlDMR6-1 and SlDMR6-2. We show that SlDMR6-1, but not SlDMR6-2, is up-regulated by pathogen infection. In agreement, Sldmr6-1 mutants display enhanced resistance against different classes of pathogens, such as bacteria, oomycete, and fungi. Notably, disease resistance correlates with increased salicylic acid (SA) levels and transcriptional activation of immune responses. Furthermore, we demonstrate that SlDMR6-1 and SlDMR6-2 display SA-5 hydroxylase activity, thus contributing to the elucidation of the enzymatic function of DMR6. We then propose that SlDMR6 duplication in tomato resulted in subsequent subfunctionalization, in which SlDMR6-2 specialized in balancing SA levels in flowers/fruits, while SlDMR6-1 conserved the ability to fine-tune SA levels during pathogen infection of the plant vegetative tissues. Overall, this work not only corroborates a mechanism underlying SA homeostasis in plants, but also presents a promising strategy for engineering broad-spectrum and durable disease resistance in crops.