Gene expression and biochemical changes of carbohydrate metabolism in in vitro micro-propagated apple plantlets infected by 'Candidatus Phytoplasma mali'

Transcriptomic Profiling of Sugarcane White Leaf (SCWL) Canes during Maturation Phase

Sugarcane white leaf (SCWL) disease, caused by Candidatus Phytoplasma sacchari, results in the most damage to sugarcane plantations. Some SCWL canes can grow unnoticed through the maturation phase, subsequently resulting in an overall low sugar yield, or they can be used accidentally as seed canes. In this work, 12-month-old SCWL and asymptomatic canes growing in the same field were investigated. An abundance of phytoplasma in SCWL canes affected growth and sugar content as well as alterations of transcriptomic profiles corresponding to several pathways that responded to the infection. Suppression of photosynthesis, porphyrin and chlorophyll metabolism, coupled with an increase in the expression of chlorophyllase, contributed to the reduction in chlorophyll levels and photosynthesis. Blockage of sucrose transport plausibly occurred due to the expression of sugar transporters in leaves but suppression in stalks, resulting in low sugar content in canes. Increased expression of genes associated with MAPK cascades, plant hormone signaling transduction, callose plug formation, the phenylpropanoid pathway, and calcium cascades positively promoted defense mechanisms against phytoplasma colonization by an accumulation of lignin and calcium in response to plant immunity. Significant downregulation of CPK plausibly results in a reduction in antioxidant enzymes and likely facilitates pathogen invasion, while expression of sesquiterpene biosynthesis possibly attracts the insect vectors for transmission, thereby enabling the spread of phytoplasma. Moreover, downregulation of flavonoid biosynthesis potentially intensifies the symptoms of SCWL upon challenge by phytoplasma. These SCWL sugarcane transcriptomic profiles describe the first comprehensive sugarcane-phytoplasma interaction during the harvesting stage. Understanding molecular mechanisms will allow for sustainable management and the prevention of SCWL disease-a crucial benefit to the sugar industry.

Detection of apple proliferation disease in Malus × domestica by near infrared reflectance analysis of leaves

In this study near infrared spectroscopical analysis of dried and ground leaves was performed and combined with a multivariate data analysis to distinguish 'Candidatus Phytoplasma mali' infected from non-infected apple trees (Malus × domestica). The bacterium is the causative agent of Apple Proliferation, one of the most threatening diseases in commercial apple growing regions. In a two-year study, leaves were sampled from three apple orchards, at different sampling events throughout the vegetation period. The spectral data were analyzed with a principal component analysis and classification models were developed. The model performance for the differentiation of Apple Proliferation diseased from non-infected trees increased throughout the vegetation period and gained best results in autumn. Even with asymptomatic leaves from infected trees a correct classification was possible indicating that the spectral-based method provides reliable results even if samples without visible symptoms are analyzed. The wavelength regions that contributed to the differentiation of infected and non-infected trees could be mainly assigned to a reduction of carbohydrates and N-containing organic compounds. Wet chemical analyses confirmed that N-containing compounds are reduced in leaves from infected trees. The results of our study provide a valuable indication that spectral analysis is a promising technique for Apple Proliferation detection in future smart farming approaches.

Surveying Potential Vectors of Apple Proliferation Phytoplasma: Faunistic Analysis and Infection Status of Selected Auchenorrhyncha Species

Apple proliferation (AP) is one of the economically most important diseases in European apple cultivation. The disease is caused by the cell-wall-less bacterium ' Candidatus Phytoplasma mali', which is transmitted by Cacopsylla picta (Foerster) and Cacopsylla melanoneura (Foerster) (Hemiptera: Psylloidea). In South Tyrol (Italy), severe outbreaks were documented since the 1990s. Infestation rates of AP do not always correlate with the population densities of the confirmed vectors, implying the presence of other, so far unknown, hemipterian vectors. By elucidating the species community of Auchenorrhyncha (Insecta: Hemiptera) at a regional scale, more than 31,000 specimens were captured in South Tyrolean apple orchards. The occurrence of 95 species was confirmed, whereas fourteen species are new records for this territory. Based on the faunistical data, more than 3600 individuals out of 25 species were analyzed using quantitative PCR to assess the presence of AP phytoplasma. The pathogen was sporadically detected in some individuals of different species, for example in Stictocephala bisonia Kopp and Yonk (Hemiptera: Membracidae). However, the concentration of phytoplasma was much lower than in infected C. picta and C. melanoneura captured in the same region, confirming the role of the latter mentioned psyllids as the main insect vectors of AP- phytoplasma in South Tyrol.

Linking omics and ecology to dissect interactions between the apple proliferation phytoplasma and its psyllid vector Cacopsylla melanoneura

Phytoplasmas are bacterial plant pathogens that are detrimental to many plants and cause devastating effects on crops. They are not viable outside their host plants and depend on specific insect vectors for their transmission. So far, research has largely focused on plant-pathogen interactions, while the complex interactions between phytoplasmas and insect vectors are far less understood. Here, we used next-generation sequencing to investigate how transcriptional profiles of the vector psyllid Cacopsylla melanoneura (Hemiptera, Psyllidae) are altered during infection by the bacterium Candidatus Phytoplasma mali (P. mali), which causes the economically important apple proliferation disease. This first de novo transcriptome assembly of an apple proliferation vector revealed that mainly genes involved in small GTPase mediated signal transduction, nervous system development, adhesion, reproduction, actin-filament based and rhythmic processes are significantly altered upon P. mali infection. Furthermore, the presence of P. mali is accompanied by significant changes in carbohydrate and polyol levels, as revealed by metabolomics analysis. Taken together, our results suggest that infection with P. mali impacts on the insect vector physiology, which in turn likely affects the ability of the vector to transmit phytoplasma.

The phytopathogen ‘Candidatus Phytoplasma mali’ alters apple tree phloem composition and affects oviposition behavior of its vector Cacopsylla picta

Apple proliferation disease is caused by the phloem-dwelling bacterium ‘Candidatus Phytoplasma mali’, inducing morphological changes in its host plant apple, such as witches’ broom formation. Furthermore, it triggers physiological alterations like emission of volatile organic compounds or phytohormone levels in the plant. In our study, we assessed phytoplasma-induced changes in the phloem by sampling phloem sap from infected and non-infected apple plants. In infected plants, the soluble sugar content increased and the composition of phloem metabolites differed significantly between non-infected and infected plants. Sugar and sugar alcohol levels increased in diseased plants, while organic and amino acid content remained constant. As ‘Ca. P. mali’ is vectored by the phloem-feeding insect Cacopsylla picta (Foerster, 1848), we assessed whether the insect–plant interaction was affected by ‘Ca. P. mali’ infection of the common host plant Malus domestica Borkh. Binary-choice oviposition bioassays between infected and non-infected apple leaves revealed C. picta’s preference for non-infected leaves. It is assumed and discussed that the changes in vector behavior are attributable to plant-mediated effects of the phytoplasma infection.

Revealing the 1H NMR metabolome of mirasol chili peppers (Capsicum annuum) infected by Candidatus Phytoplasma trifolii

The infection of Capsicum annuum cv. mirasol by Candidatus Phytoplasma trifolii (16SrVI) causes devastating crop losses in northern Mexico. This study addresses the metabolomics profiling of mirasol chili peppers (Capsicum annuum cv. mirasol) infected by Candidatus Phytoplasma trifolii. For this study, 25 diseased fruits and 25 healthy fruits were used. Principal component analysis (PCA) and orthogonal projections to latent structures discriminant analysis (OPLS-DA) revealed dramatic changes in the content of 42 metabolites which were identified in diseased and healthy mirasol chili peppers. The endogenous levels of fructose, glucose and formic acid were substantially decreased in the diseased chili peppers. In the same group of samples, high concentrations of alanine, asparagine, fumaric acid, sucrose and threonine were observed. The content of Choline didńt present a significant difference. This evidence supports the fact that Candidatus Phytoplasma trifolii infection reduces de CO₂ fixation into carbohydrates, decreases invertase activity, and inhibits glycolysis in the diseased plant tissues. The levels of ascorbic acid, capsaicin and dihydrocapsaicinin in diseased fruits were dramatically decreased, suggesting that Candidatus Phytoplasma trifolii can reduce the pungency and the nutraceutical value of mirasol chili peppers.

Plant Hormones in Phytoplasma Infected Plants

Phytoplasmas are bacterial plant pathogens that need a plant host and an insect vector for their spread and survival. In plants, the physiological responses that phytoplasmas trigger result in symptom development through effects on hormonal, nutritional, and stress signaling pathways, and the interactions between these. In this review, recent advances on the involvement of plant hormones together with their known and deduced roles in plants infected with phytoplasmas are discussed. Several studies have directly, or in many cases indirectly, addressed plant hormone systems in phytoplasma-infected plants. These have provided accumulating evidence that phytoplasmas extensively affect plant hormone pathways. Phytoplasmas thus, with disturbing complex plant hormone networks, suppress plant immunity and modify plant structure, while optimizing their nutrient acquisition and facilitating their colonization of the plants, and their dissemination among plants by their insect vectors.

In vitro tissue culture of apple and other Malus species: recent advances and applications

Studies on the tissue culture of apple have allowed for molecular, biotechnological and applied breeding research to advance. In the past 8 years, over 100 papers advancing basic biology, genetic transformation and cryobiology have emerged. Apple (Malus × domestica Borkh.; Rosaceae) is an important fruit crop grown mainly in temperate regions of the world. In vitro tissue culture is a biotechnological technique that has been used to genetically improve cultivars (scions) and rootstocks. This updated review presents a synthesis of findings related to the tissue culture of apple and other Malus spp. between 2010 and 2018. Increasingly complex molecular studies that are examining the apple genome, for example, in a bid to identify the cause of epigenetic mutations and the role of transposable elements in this process would benefit from genetically stable source material, which can be produced in vitro. Several notable or curious in vitro culture methods have been reported to improve shoot regeneration and induce the production of tetraploids in apple cultivars and rootstocks. Existing studies have revealed the molecular mechanism underlying the inhibition of adventitious roots by cytokinin. The use of the plant growth correction factor allows hypothetical shoot production from leaf-derived thin cell layers relative to conventional leaf explants to be determined. This updated review will allow novices and established researchers to advance apple and Malus biotechnology and breeding programs.

Photosynthetic responses to phytoplasma infection in Chinese jujube

Phytoplasma is one of the most devastating plant pathogens. Jujube witches' broom (JWB) is a typical and highly fatal phytoplasma disease of Chinese jujube (Ziziphus jujuba Mill.), which is widely cultivated in Asia. To further elucidate the mechanism of plant-phytoplasma interaction, we first compared the effects of phytoplasma infection on photosynthetic pigments and activities between a JWB-resistant cultivar (Xingguang) and a susceptible cultivar (Pozao). Total chlorophyll and carotenoid levels were significantly decreased in the susceptible cultivar at later stages of infection, but were remarkably increased in the resistant cultivar at the earlier stages. Compared to uninfected plant, a significant decrease in the main photochemical parameters (Fv/Fm, ΦPSII and qP) was recorded at the initial stages of infection in the resistant cultivar, but occurred at later stages in the susceptible cultivar. Meanwhile, the qRT-PCR results of four key photosynthesis-related genes (ZjGluTR, ZjCBP, ZjRubisco and ZjRCA2) demonstrated that the expression patterns were similar in uninfected cultivars, but up-regulated in resistant cultivar and down-regulated in the susceptible one at 12 wks after grafting inoculation. Collectively, our data indicated that the resistant cultivar 'Xingguang' undergoes a decrease in initial stage (inhibiting phytoplasma multiplication) and then a rapid enhancement of photosynthetic activity (helping jujube recovery) in response to phytoplasma infection, while the susceptible cultivar 'Pozao' displays a later decrease in photosynthetic activity. The novel photosynthetic response pattern of the resistant cultivar may contribute to its stronger immunity to phytoplasma infection, which provides new insights into plant-phytoplasma interactions.

Physiological responses of the hybrid larch (Larix × eurolepis Henry) to cadmium exposure and distribution of cadmium in plantlets

Phytoextraction of Cd is a growing biotechnology although we currently know few Cd hyperaccumulators, i.e., plant species able to accumulate at least 0.1 mg Cd g(-1) dry weight in aerial organs. Owing their deep root system and high biomass, trees are more and more preferred to herbaceous species for phytoextraction. Assuming that conifers could be relevant models under cold climates, we investigated cadmium tolerance of the hybrid larch Larix × eurolepis Henry (Larix decidua × Larix kaempferi) and the efficiency of this species to store this metal. In vitro grown larches were chosen in order to reduce time of exposure and to more rapidly evaluate their potential efficiency to accumulate Cd. One-month-old plantlets were exposed for 2 and 4 weeks to 250 and 500 μM Cd. Results showed that they tolerated a 4-week exposure to 250 μM Cd, whereas the content of photosynthetic pigment strongly dropped in plantlets growing in the presence of 500 μM Cd. In the presence of 250 μM Cd, shoot growth slightly decreased but photosynthetic pigment and total soluble carbohydrate contents were not modified and no lipid peroxidation was detected. In addition, these plantlets accumulated proline, particularly in shoots (two to three times more than control). In roots, Cd concentration in the intracellular fraction was always higher than in the cell wall fraction contrary to shoots where Cd concentration in the cell wall fraction increased with time and Cd concentration in the medium. In shoots, Cd concentration was lower than in roots with a ratio of 0.2 after 4 weeks of exposure but stayed around 0.2 mg g(-1) dry weight, thus a value higher than the threshold requested for Cd hyperaccumulators. Hybrid larch would thus be a relevant candidate for field test of Cd phytoextraction.

Maturation of nematode-induced galls in Medicago truncatula is related to water status and primary metabolism modifications

Root-knot nematodes are obligatory plant parasitic worms that establish and maintain an intimate relationship with their host plants. During a compatible interaction, these nematodes induce the redifferentiation of root cells into multinucleate and hypertrophied giant cells (GCs). These metabolically active feeding cells constitute the exclusive source of nutrients for the nematode. We analyzed the modifications of water status, ionic content and accumulation of metabolites in development and mature galls induced by Meloidogyne incognita and in uninfected roots of Medicago truncatula plants. Water potential and osmotic pressure are significantly modified in mature galls compared to developing galls and control roots. Ionic content is significantly modified in galls compared to roots. Principal component analyses of metabolite content showed that mature gall metabolism is significantly modified compared to developing gall metabolism. The most striking differences were the three-fold increase of trehalose content associated to the five-fold diminution in glucose concentration in mature galls. Gene expression analysis showed that trehalose accumulation was, at least, partially linked to a significantly lower expression of the trehalase gene in mature galls. Our results point to significant modifications of gall physiology during maturation.

Metabolomic analysis reveals the potential metabolites and pathogenesis involved in mulberry yellow dwarf disease

To analyse the molecular mechanisms of phytoplasma pathogenicity, the comprehensive metabolomic changes of mulberry leaf and phloem sap in response to phytoplasma infection were examined using gas chromatography-mass spectrometry. The metabolic profiles obtained revealed that the metabolite compositions of leaf and phloem sap were different, and phytoplasma infection has a greater impact on the metabolome of phloem sap than of leaf. Phytoplasma infection brought about the content changes in various metabolites, such as carbohydrates, amino acids, organic acids, etc. Meanwhile, the results of biochemical analysis showed that the degradation of starch was repressed, and the starch content was increased in the infected leaves. In addition, we found that phytoplasma infection changed the levels of abscisic acid and cytokinin and break phytohormone balance. Interestingly, our data showed that the contents of H2O2 and superoxide were increased in the infected leaves, but not in the phloem saps. Based on the results, the expression levels of the genes involved in the metabolism of some changed metabolites were examined, and the potential molecular mechanisms of these changes were discussed. It can be concluded that both the leaf and phloem saps have a complicated metabolic response to phytoplasma infection, but their response mechanisms were different.

Purple top symptoms are associated with reduction of leaf cell death in phytoplasma-infected plants

Plants exhibit a wide variety of disease symptoms in response to pathogen attack. In general, most plant symptoms are recognized as harmful effects on host plants, and little is known about positive aspects of symptoms for infected plants. Herein, we report the beneficial role of purple top symptoms, which are characteristic of phytoplasma-infected plants. First, by using plant mutants defective in anthocyanin biosynthesis, we demonstrated that anthocyanin accumulation is directly responsible for the purple top symptoms, and is associated with reduction of leaf cell death caused by phytoplasma infection. Furthermore, we revealed that phytoplasma infection led to significant activation of the anthocyanin biosynthetic pathway and dramatic accumulation of sucrose by about 1000-fold, which can activate the anthocyanin biosynthetic pathway. This is the first study to demonstrate the role and mechanism of the purple top symptoms in plant-phytoplasma interactions.