Involvement of plasma membrane peroxidases and oxylipin pathway in the recovery from phytoplasma disease in apple (Malus domestica)

Signaling Cross-Talk between Salicylic and Gentisic Acid in the 'Candidatus Phytoplasma Solani' Interaction with Sangiovese Vines

"Bois noir" disease associated with 'Candidatus Phytoplasma solani' seriously compromises the production and survival of grapevines (Vitis vinifera L.) in Europe. Understanding the plant response to phytoplasmas should help to improve disease control strategies. Using a combined metabolomic and transcriptomic analysis, this work, therefore, investigated the phytoplasma-grapevine interaction in red cultivar Sangiovese in a vineyard over four seasonal growth stages (from late spring to late summer), comparing leaves from healthy and infected grapevines (symptomatic and symptomless). We found an accumulation of both conjugate and free salicylic acids (SAs) in the leaves of 'Ca. P. solani'-positive plants from early stages of infection, when plants are still asymptomatic. A strong accumulation of gentisic acid (GA) associated with symptoms progression was found for the first time. A detailed analysis of phenylpropanoids revealed a significant accumulation of hydroxycinnamic acids, flavonols, flavan 3-ols, and anthocyanin cyanidin 3-O-glucoside, which are extensively studied due to their involvement in the plant response to various pathogens. Metabolomic data corroborated by gene expression analysis indicated that phenylpropanoid biosynthetic and salicylic acid-responsive genes were upregulated in 'Ca. P. solani-positive plants compared to -negative ones during the observed period.

Identification of High Tolerance to Jujube Witches' Broom in Indian Jujube (Ziziphus mauritiana Lam.) and Mining Differentially Expressed Genes Related to the Tolerance through Transcriptome Analysis

The jujube witches' broom (JWB) disease is a severe threat to jujube trees, with only a few cultivars being genuinely tolerant or resistant to phytoplasma. The defense mechanism of jujube trees against phytoplasma is still unclear. In this study, we aimed to investigate the tolerance mechanism of Indian jujube 'Cuimi' to JWB and identify the key genes that contribute to JWB high tolerance. Based on the symptoms and phytoplasma concentrations after infection, we confirmed the high tolerance of 'Cuimi' to JWB. Comparative transcriptome analysis was subsequently performed between 'Cuimi' and 'Huping', a susceptible cultivar of Chinese jujube. Unique gene ontology (GO) terms were identified in 'Cuimi', such as protein ubiquitination, cell wall biogenesis, cell surface receptor signaling pathway, oxylipin biosynthetic process, and transcription factor activity. These terms may relate to the normal development and growth of 'Cuimi' under phytoplasma infection. We identified 194 differential expressed genes related to JWB high tolerance, involved in various processes, such as reactive oxygen species (ROS), Ca²⁺ sensors, protein kinases, transcription factors (TFs), lignin, and hormones. Calmodulin-like (CML) genes were significantly down-regulated in infected 'Cuimi'. We speculated that the CML gene may act as a negative regulatory factor related to JWB high tolerance. Additionally, the cinnamoyl-CoA reductase-like SNL6 gene was significantly up-regulated in infected 'Cuimi', which may cause lignin deposition, limit the growth of phytoplasma, and mediate immune response of 'Cuimi' to phytoplasma. Overall, this study provides insights into the contribution of key genes to the high tolerance of JWB in Indian jujube 'Cuimi'.

The Crosstalk of the Salicylic Acid and Jasmonic Acid Signaling Pathways Contributed to Different Resistance to Phytoplasma Infection Between the Two Genotypes in Chinese Jujube

Jujube witches’ broom disease (JWB), one of the most serious phytoplasma diseases, usually results in the destruction of Chinese jujube (Ziziphus jujuba Mill.). Although most jujube cultivars are sensitive to JWB, we found a few genotypes that are highly resistant to JWB. However, the molecular mechanism of phytoplasma resistance has seldom been studied. Here, we used Chinese jujube “T13,” which has strong resistance to JWB, and a typical susceptible cultivar, “Pozao” (“PZ”), as materials to perform comparative transcriptome, hormone, and regulation analyses. After phytoplasma infection, the differential expression genes (DEGs) were detected at all three growth phases (S1, S2, and S3) in “PZ,” but DEGs were detected only at the first growth phase in “T13.” Meanwhile, no phytoplasma was detected, and the symptoms especially witches’ broom caused by JWB were not observed at the last two growth phases (S2 and S3) in “T13.” Protein–protein interaction analysis also showed that the key genes were mainly involved in hormone and reactive oxygen species (ROS) signaling. In addition, during the recovered growth phase in “T13” from S1 to S2, the level of hydrogen peroxide (H₂O₂) was significantly increased and then decreased from S2 to S3. Moreover, jasmonic acid (JA) was significantly accumulated in “PZ” diseased plants, especially at the S2 phase and at the S2 phase in “T13,” while the content of salicylic acid (SA) decreased significantly at the S2 phase of “T13” compared to that in “PZ.” The changes in H₂O₂ and JA or SA were consistent with the changes in their key synthesis genes in the transcriptome data. Finally, exogenous application of an SA inhibitor [1-aminobenzotriazole (ABT)] rescued witches’ broom symptoms, while the contents of both JA and MeJA increased after ABT treatment compared to the control, demonstrating that exogenous application of an SA inhibitor rescued the symptoms of jujube after phytoplasma infection by decreasing the contents of SA and increasing the contents of JA and MeJA. Collectively, our study provides a new perspective on the transcriptional changes of Chinese jujube in response to JWB and novel insights that the crosstalk of JA and SA signaling communicated together to contribute to “T13” JWB resistance.

Pre-symptomatic modified phytohormone profile is associated with lower phytoplasma titres in an Arabidopsis seor1ko line

The proteins AtSEOR1 and AtSEOR2 occur as conjugates in the form of filaments in sieve elements of Arabidopsis thaliana. A reduced phytoplasma titre found in infected defective-mutant Atseor1ko plants in previous work raised the speculation that non-conjugated SEOR2 is involved in the phytohormone-mediated suppression of Chrysanthemum Yellows (CY)-phytoplasma infection transmitted by Euscelidius variegatus (Ev). This early and long-lasting SEOR2 impact was revealed in Atseor1ko plants by the lack of detectable phytoplasmas at an early stage of infection (symptomless plants) and a lower phytoplasma titre at a later stage (fully symptomatic plants). The high insect survival rate on Atseor1ko line and the proof of phytoplasma infection at the end of the acquisition access period confirmed the high transmission efficiency of CY-phytoplasma by the vectors. Transmission electron microscopy analysis ruled out a direct role of SE filament proteins in physical phytoplasma containment. Time-correlated HPLC–MS/MS-based phytohormone analyses revealed increased jasmonate levels in midribs of Atseor1ko plants at an early stage of infection and appreciably enhanced levels of indole acetic acid and abscisic acid at the early and late stages. Effects of Ev-probing on phytohormone levels was not found. The results suggest that SEOR2 interferes with phytohormonal pathways in Arabidopsis midrib tissues in order to establish early defensive responses to phytoplasma infection.

Quantification of Phytohormones by HPLC-MS/MS Including Phytoplasma-Infected Plants

There is strong evidence that phytohormones such as abscisic acid, auxin, salicylic acid, and jasmonates might play a role in defense of the host plants during phytoplasma infections. However, these compounds are usually present at low concentration in complex matrixes, requiring a sensitive and selective method to analyze and quantify them. Here, we present a HPLC-MS/MS method to quantify phytohormones in different infected and noninfected plant tissues.

Silencing of CrNPR1 and CrNPR3 Alters Plant Susceptibility to Periwinkle Leaf Yellowing Phytoplasma

Phytoplasmas are prokaryotic plant pathogens that cause considerable loss in many economically important crops, and an increasing number of phytoplasma diseases are being reported on new hosts. Knowledge of plant defense mechanisms against such pathogens should help to improve strategies for controlling these diseases. Salicylic acid (SA)-mediated defense may play an important role in defense against phytoplasmas. Here, we report that SA accumulated in Madagascar periwinkle (Catharanthus roseus) infected with periwinkle leaf yellowing (PLY) phytoplasma. CrPR1a expression was induced in both symptomatic and non-symptomatic tissues of plants exhibiting PLY. NPR1 plays a central role in SA signaling, and two NPR1 homologs, CrNPR1 and CrNPR3, were identified from a periwinkle transcriptome database. Similar to CrPR1a, CrNPR1 expression was also induced in both symptomatic and non-symptomatic tissues of plants exhibiting PLY. Silencing of CrNPR1, but not CrNPR3, significantly repressed CrPR1a induction in Tobacco rattle virus-infected periwinkle plants. In addition, symptoms of PLY progressed fastest in CrNPR1-silenced plants and slowest in CrNPR3-silenced plants. Consistently, expression of CrNPR1, but not CrNPR3, was induced by phytoplasma infection as well as SA treatment. This study highlights the importance of NPR1- and SA-mediated defense against phytoplasma in periwinkle and offers insight into plant-phytoplasma interactions to improve disease control strategies.

Transcriptome profiling analysis revealed co-regulation of multiple pathways in jujube during infection by 'Candidatus Phytoplasma ziziphi'

BACKGROUND

Jujube witches' broom (JWB), caused by a phytoplasma, devastates jujube tree (Ziziphus jujuba) growth and production in Asia. Although host responses to phytoplasmas are studied at the phenotypic, physiological, biochemical and molecular levels, it remains unclear how a host plant responds at the molecular level during the primary stage of infection.

METHODS

To understand the response of the jujube tree to JWB infection, leaves were sampled at different times during the phytoplasma infection. Transcriptomic analyses at six stages were performed to reveal how phytoplasma infection affects Chinese jujube gene expression through the determination of the key differentially expressed genes (DEGs), and their related pathways. Quantitative real-time PCR was applied to validate 10 differentially expressed genes at different JWB phytoplasma infection stages.

RESULTS

A total of 25,067 unigenes were mapped to jujube genome reference sequences. In the first infection stage (0-2 weeks after grafting (WAG), a total of 582 jujube genes were differentially regulated but no visible symptoms appeared. Quite a few DEGs related to abscisic acid (ABA) and cytokinin (CTK) were down-regulated, while some related to jasmonic acid (JA) and salicylic acid (SA) were up-regulated, Genes related to plant-pathogen interaction were also differentially expressed. In the second infection stage (37-39WAG), witches' broom symptoms were visible, and a total of 4373 DEGs were identified. Genes involved in biosynthesis and signal transduction of ABA, brassinosteroid (BR), CTK, ethylene (ET), and auxin (IAA), GA, JA and SA, plant-pathogen interaction, flavonoid biosynthesis genes were significantly regulated, suggesting that jujube trees activated defense factors related to SA, JA, ABA and secondary metabolites to defend against phytoplasma infection. By the third infection stage (48-52WAG), serious symptoms occurred and 3386 DEGs were identified. Most DEGs involved in biosynthesis and signal transduction of JA, SA and GA were up-regulated, while those relating to ABA were down-regulated. Genes involved in plant-pathogen interaction were up- or down-regulated, while phenylpropanoid and flavonoid biosynthesis genes were significantly up-regulated. Meanwhile, DEGs involved in photosynthesis, chlorophyll and peroxisome biosynthesis, and carbohydrate metabolism were down-regulated. These results suggested that phytoplasma infection had completely destroyed jujube trees' defense system and had disturbed chlorophyll synthesis and photosynthetic activity in the infected leaves at the late stage, resulting in yellow leaves and other JWB symptoms.

DISCUSSION

The results in this report suggested that phytohormone biosynthesis and signal transduction, photosynthesis, and secondary metabolism all played important roles in the battle between colonization and defense in the interaction between Ca. Phytoplasma ziziphi and jujube.

An effector of apple proliferation phytoplasma targets TCP transcription factors-a generalized virulence strategy of phytoplasma?

The plant pathogen Candidatus Phytoplasma mali (P. mali) is the causative agent of apple proliferation, a disease of increasing importance in apple-growing areas within Europe. Despite its economic importance, little is known about the molecular mechanisms of disease manifestation within apple trees. In this study, we identified two TCP (TEOSINTE BRANCHED/CYCLOIDEA/PROLIFERATING CELL FACTOR) transcription factors of Malus x domestica as binding partners of the P. mali SAP11-like effector ATP_00189. Phytohormone analyses revealed an effect of P. mali infection on jasmonates, salicylic acid and abscisic acid levels, showing that P. mali affects phytohormonal levels in apple trees, which is in line with the functions of the effector assumed from its binding to TCP transcription factors. To our knowledge, this is the first characterization of the molecular targets of a P. mali effector and thus provides the basis to better understand symptom development and disease progress during apple proliferation. As SAP11 homologues are found in several Phytoplasma species infecting a broad range of different plants, SAP11-like proteins seem to be key players in phytoplasmal infection.

Suppression of aggressive strains of 'Candidatus phytoplasma mali' by mild strains in Catharanthus roseus and Nicotiana occidentalis and indication of similar action in apple trees

To study antagonistic interactions of 'Candidatus Phytoplasma mali' strains, graft inoculation of Catharanthus roseus and Nicotiana occidentalis was performed with mild strains 1/93Vin and 1/93Tab as suppressors and three aggressive strains as challengers. Inoculation of the suppressors was carried out in either the cross-protection modus prior to grafting of the challengers or by co-inoculating suppressors and challengers. Monitoring using multiplex real-time polymerase chain reaction assays revealed that, in long-term cross-protection trials with C. roseus, suppressor 1/93Vin was present in all root and randomly collected stem samples over the entire observation period. In contrast, the challengers were never detected in such stem samples and rarely in the roots. Following simultaneous inoculation, the suppressor successively colonized all stem and root regions whereas detection of challenger AT steadily decreased. However, this strain remained detectable in up to 13 and 27% of stem and root samples, respectively. The cross-protection trials with N. occidentalis yielded results similar to that of the cross-protection experiments with C. roseus. Comparison of the symptomatology of infected apple trees with the presence of putatively suppressive strains indicated that suppression of severe strains also occurs in apple. Phylogenetic analysis using a variable fragment of AAA+ ATPase gene AP460 of 'Ca. P. mali' revealed that suppressors 1/93Vin and 1/93Tab, together with several other mild strains maintained in apple, cluster distantly from obviously nonsuppressive strains that were predominantly highly virulent.

Acquired Tolerance in Apricot Plants that Stably Recovered from European Stone Fruit Yellows

European stone fruit yellows (ESFY) is one of the most destructive phytoplasma diseases of plum, apricot, and peach in Europe. Conventional preventive defense strategies have been ineffective. Because apricot cultivars with innate-constitutive resistance against ESFY are not available, the aim of this more than 20-year-long study was to seek acquired resistance or tolerance. In the first experiment, we surveyed an orchard with seven apricot cultivars for 12 years in an area of northern Italy with a high rate of natural occurrence of ESFY. Of the diseased plants, a few (8.7%) became completely symptomless but retained the phytoplasma, as confirmed by polymerase chain reaction (PCR). In the second experiment, we grafted buds from two stably recovered plants and from two nonrecovered plants onto 'Rubira' peach. Over the next 9 years in an orchard with a high rate of natural infection, 93.0% of the "nonrecovered clones" became diseased but only 1.5% of the plants grafted with the two "recovered clones" developed ESFY symptoms. According to PCR analyses, all of the exposed test plants were ESFY-infected, whether they were derived from recovered or nonrecovered mothers. This could indicate that epigenetic changes occurred in recovered plants due to a graft-transmissible memory. Based on the results attained from the two described experiments, we propose that an acquired tolerance that occurred in stably recovered apricot trees was graft transmitted from two tolerant apricot clones. In contrast, we did not demonstrate a cross-protection process based on protectant avirulent phytoplasma strains that suppress severe strains.

Effects of an innovative strategy to contain grapevine Bois noir: field treatment with resistance inducers

Grapevine Bois noir (BN) is a phytoplasma disease that is widespread in most viticultural regions of the world, and it can result in heavy reductions to yields and grape juice quality. At present, there is no effective strategy to reduce the incidence of BN-infected grapevines. However, phytoplasma-infected plants can recover through spontaneous or induced symptom remission. Five elicitors (chitosan, two glutathione-plus-oligosaccharine formulations, benzothiadiazole, and phosetyl-Al) were applied weekly to the canopy of BN-infected 'Chardonnay' grapevines from early May to late July. The best and most constant recovery inductions were obtained with benzothiadiazole and the two glutathione-plus-oligosaccharine formulations. The plants that recovered naturally or following the elicitors showed qualitative and quantitative parameters of production no different from healthy plants. In another vineyard, diseased plants showed reduced shoot length and production compared with healthy plants, and there were no negative effects on these parameters for grapevines sprayed with a glutathione-plus-oligosaccharine formulation. The application of resistance inducers promoted the recovery of BN-infected grapevines with no adverse effects on the plants. Therefore, grapevine can be used as a model species to test this innovative strategy to contain phytoplasma diseases.