Phloem Metabolites of Prunus Sp. Rather than Infection with Candidatus Phytoplasma Prunorum Influence Feeding Behavior of Cacopsylla pruni Nymphs

Communication between undamaged plants can elicit changes in volatile emissions from neighbouring plants, thereby altering their susceptibility to aphids

Plant volatiles play an important role in intra- and interspecific plant communication, inducing direct and indirect defenses against insect pests. However, it remains unknown whether volatile interactions between undamaged cultivars alter host plant volatile emissions and their perception by insect pests. Here, we tested the effects of exposure of a spring barley, Hordeum vulgare L., cultivar, Salome, to volatiles from other cultivars: Fairytale and Anakin. We found that exposing Salome to Fairytale induced a significantly higher emission of trans-β-ocimene and two unidentified compounds compared when exposed to Anakin. Aphids were repelled at a higher concentration of trans-β-ocimene. Salome exposure to Fairytale had significant repulsive effects on aphid olfactory preference, yet not when Salome was exposed to Anakin. We demonstrate that volatile interactions between specific undamaged plants can induce changes in volatile emission by receiver plants enhancing certain compounds, which can disrupt aphid olfactory preferences. Our results highlight the significant roles of volatiles in plant-plant interactions, affecting plant-insect interactions in suppressing insect pests. This has important implications for crop protection and sustainable agriculture.

Phloem Sap Composition: What Have We Learnt from Metabolomics?

Phloem sap transport is essential for plant nutrition and development since it mediates redistribution of nutrients, metabolites and signaling molecules. However, its biochemical composition is not so well-known because phloem sap sampling is difficult and does not always allow extensive chemical analysis. In the past years, efforts have been devoted to metabolomics analyses of phloem sap using either liquid chromatography or gas chromatography coupled with mass spectrometry. Phloem sap metabolomics is of importance to understand how metabolites can be exchanged between plant organs and how metabolite allocation may impact plant growth and development. Here, we provide an overview of our current knowledge of phloem sap metabolome and physiological information obtained therefrom. Although metabolomics analyses of phloem sap are still not numerous, they show that metabolites present in sap are not just sugars and amino acids but that many more metabolic pathways are represented. They further suggest that metabolite exchange between source and sink organs is a general phenomenon, offering opportunities for metabolic cycles at the whole-plant scale. Such cycles reflect metabolic interdependence of plant organs and shoot-root coordination of plant growth and development.

Altered volatile emission of pear trees under elevated atmospheric CO2 levels has no relevance to pear psyllid host choice

The impact of climate change drivers on cultivated plants and pest insects has come into research focus. One of the most significant drivers is atmospheric carbon dioxide, which is converted into primary plant metabolites by photosynthesis. Increased atmospheric CO2 concentrations therefore affect plant chemistry. The chemical composition of non-volatile and volatile organic compounds of plants is used by insects to locate and identify suitable host plants for feeding and reproduction. We investigated whether elevated CO2 concentrations in the atmosphere affect the plant-pest interaction in a fruit crop of high economic importance in Europe. Therefore, potted pear trees were cultivated under specified CO2 conditions in a Free-Air Carbon dioxide Enrichment (FACE) facility at Geisenheim University in Germany for up to 14 weeks, beginning from bud swelling. We compared emitted volatiles from these pear trees cultivated for 7 and 14 weeks under two different CO2 levels (ambient: ca. 400 ppm and elevated: ca. 450 ppm CO2) and their impact on pest insect behavior. In total, we detected and analyzed 76 VOCs from pear trees. While we did not detect an overall change in VOC compositions, the relative release of single compounds changed in response to CO2 increase. Differences in VOC release were inconsistent over time (phenology stages) and between study years, indicating interactions with other climate parameters, such as temperature. Even though insect-plant interaction can rely on specific volatile compounds and specific mixtures of compounds, respectively, the changes of VOC patterns in our field study did not impact the host choice behavior of C. pyri females. In olfactometer trials, 64% and 60% of the females preferred the odor of pear trees cultivated under elevated CO2 for 7 and 14 weeks, respectively, over the odor from pear trees cultivated under ambient CO2. In binary-choice oviposition assays, C. pyri females laid most eggs on pears during April 2020; on average, 51.9 (± 51.3) eggs were laid on pears cultivated under eCO2 and 60.3 (± 48.7) eggs on aCO2.

Cacopsyllapruni (Hemiptera, Psyllidae) in an apricot orchard is more attracted to white sticky traps dependent on host phenology

The colour preference of the plum psyllid, Cacopsyllapruni (Hemiptera, Psyllidae), is yet poorly studied. This species is the only known vector of the 'Candidatus Phytoplasma prunorum', the agent of European stone fruit yellows (ESFY), a devastating disease of several cultivated Prunus species in Europe. As ESFY is still uncurable, vector control, thus vector monitoring, is pivotal to protect these trees. Cacopsyllapruni is a univoltine, host-shelter-alternating species; overwintered adults migrate from conifer to wild or cultivated Prunus species (family Rosaceae) in late winter-early spring. To select the most effective colour indicating the arrivals of the immigrants, yellow, fluorescent yellow, white, red and transparent sticky traps were deployed in an apricot orchard in Hungary. The two most abundant species in sticky traps were C.pruni and C.melanoneura. Catches of white traps were significantly biased towards C.pruni as compared to C.melanoneura specimens. Moreover, white sticky traps were better at catching plum psyllids than the other colours. Attraction to white was strongest when immigrants from shelter plants kept arriving in the orchard, coinciding with the blooming principal phenophase of apricot trees. When the host flowering growth stage was over, catches of C.pruni in white traps declined sharply to the level of yellow traps that was highest during this post-blooming period. We recommended white sticky traps for promptly monitoring C.pruni in apricot orchards because it is more potent and more selective than yellow ones during the critically important early flowering interval.

Cellular and Metabolite Changes in the Secondary Phloem of Chinese Fir (Cuninghamia lanceolata (Lamb.) Hook.) during Dormancy Release

Wood in the cold temperate zone is the product of the alternation of the growing season and the dormant period of trees, but our knowledge of the process of dormancy release in trees remains limited. Chinese fir (Cuninghamia lanceolata (Lamb.) Hook.) was used to investigate cellular and metabolite changes in the secondary phloem tissue during dormancy release. The sampling dates were 2 March, 28 March, and 13 April. The microsections of wood-forming tissue were prepared using the paraffin embedding technique to observe the formation of cambium cells; metabolites in secondary phloem cells were extracted using a methanol/chloroform organic solvent system. The results showed that the secondary phloem consists of phloem fibers, sieve cells and phloem parenchyma. The cells were regularly arranged in continuous tangential bands and were in the order of Phloem fiber-Sieve cell-Phloem parenchyma-Sieve cell-Phloem parenchyma-Sieve cell-Phloem parenchyma-Sieve cell-Sieve cell-Phloem parenchyma-. The Chinese fir cambium was in dormancy on 2 March and 28 March, while on 13 April, it was already in the active stage and two layers of xylem cells with several layers of phloem cells were newly formed. The width of the cambium zone increased from 18.7 ± 5.7 μm to 76.5 ± 3.0 μm and the average radial diameter of sieve cells expanded from 15.4 ± 7.5 μm to 21.5 ± 7.4 μm after dormancy release. The cambium zone width and the average radial diameter of sieve cells before and after dormancy release were significantly different (p < 0.01). The phloem parenchyma cells without resin were squeezed and deformed by the sieve cells, and the width of the phloem during the active period was 197.0 ± 8.5 μm, which was larger than that during the dormant period. Ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based metabolomics was employed to analyze the secondary phloem of Chinese fir on 28 March and 13 April. Thirty-nine differential metabolites during dormancy release were detected. The results showed that the composition of Chinese fir metabolites was different before and after dormancy release. The relative increase in pyruvic acid and ascorbic acid contents proved that the rate of energy metabolism in Chinese fir increased substantially after dormancy release. Changes in cell development and the composition of metabolites revealed that the dormancy release of Chinese fir was at early April and the formation period of phloem tissue is earlier than xylem tissue.

Influence of ontogenetic and migration stage on feeding behavior of Cacopsylla picta on 'Candidatus Phytoplasma mali' infected and non-infected apple plants

The summer apple psyllid Cacopsylla picta (Foerster) is the vector of 'Candidatus Phytoplasma mali', the causal agent of apple proliferation disease (AP). During its phloem-feeding activities it transmits this biotrophic bacterium from infected to healthy apple trees (Malus domestica Borkh.) causing high economic losses. During its life cycle, C.picta performs two host switches: In summer, the new adult generation (emigrants) hatch on apples before they emigrate to their overwintering host conifers. The following spring, the overwintered adult generation (remigrants) remigrate into apple orchards for mating and oviposition. The preimaginal stages (nymphs) develop on apple. It is known that phytopathogen-induced changes in plant physiology can affect insect-plant-interactions. In 12 h recordings of electrical penetration graphs (EPG) it was assessed whether 'Ca. P. mali' infection of the plant affected probing and feeding behavior of the vector C.picta. Its life stage and the infection status of the host plant (and the interaction between these factors) significantly affected the first occurrence, duration and frequency of probing and feeding phases. On 'Ca. P. mali' infected plants, the phloem salivation phase occurred later than on non-infected plants. Even though all life stages fed both on phloem and xylem, significant differences were found in the frequency and duration of phloem and xylem ingestion phases. Nymphs spent the shortest time non-probing, earlier started the first leaf penetration and longer ingested xylem compared with adults. Further, phloem phases differed between migratory stages; remigrants had higher numbers of phloem ingestion events and spent longer ingesting phloem than emigrants. For emigrants, however, phloem contact was very rarely observed during our recordings. The impact of our findings for understanding the multitrophic interactions between host plant, pathogen and behavior of vector insects are discussed with regard to the epidemiology of AP and pest control strategies of the vector.

Specialized 16SrX phytoplasmas induce diverse morphological and physiological changes in their respective fruit crops

The host-pathogen combinations-Malus domestica (apple)/`Candidatus Phytoplasma mali´, Prunus persica (peach)/`Ca. P. prunorum´ and Pyrus communis (pear)/`Ca. P. pyri´ show different courses of diseases although the phytoplasma strains belong to the same 16SrX group. While infected apple trees can survive for decades, peach and pear trees die within weeks to few years. To this date, neither morphological nor physiological differences caused by phytoplasmas have been studied in these host plants. In this study, phytoplasma-induced morphological changes of the vascular system as well as physiological changes of the phloem sap and leaf phytohormones were analysed and compared with non-infected plants. Unlike peach and pear, infected apple trees showed substantial reductions in leaf and vascular area, affecting phloem mass flow. In contrast, in infected pear mass flow and physicochemical characteristics of phloem sap increased. Additionally, an increased callose deposition was detected in pear and peach leaves but not in apple trees in response to phytoplasma infection. The phytohormone levels in pear were not affected by an infection, while in apple and peach trees concentrations of defence- and stress-related phytohormones were increased. Compared with peach and pear trees, data from apple suggest that the long-lasting morphological adaptations in the vascular system, which likely cause reduced sap flow, triggers the ability of apple trees to survive phytoplasma infection. Some phytohormone-mediated defences might support the tolerance.

Involvement of SUT1 and SUT2 Sugar Transporters in the Impairment of Sugar Transport and Changes in Phloem Exudate Contents in Phytoplasma-Infected Plants

Phytoplasmas inhabit phloem sieve elements and cause abnormal growth and altered sugar partitioning. However, how they interact with phloem functions is not clearly known. The phloem responses were investigated in tomatoes infected by “Candidatus Phytoplasma solani” at the beginning of the symptomatic stage, the first symptoms appearing in the newly emerged leaf at the stem apex. Antisense lines impaired in the phloem sucrose transporters SUT1 and SUT2 were included. In symptomatic sink leaves, leaf curling was associated with higher starch accumulation and the expression of defense genes. The analysis of leaf midribs of symptomatic leaves indicated that transcript levels for genes acting in the glycolysis and peroxisome metabolism differed from these in noninfected plants. The phytoplasma also multiplied in the three lower source leaves, even if it was not associated with the symptoms. In these leaves, the rate of phloem sucrose exudation was lower for infected plants. Metabolite profiling of phloem sap-enriched exudates revealed that glycolate and aspartate levels were affected by the infection. Their levels were also affected in the noninfected SUT1- and SUT2-antisense lines. The findings suggest the role of sugar transporters in the responses to infection and describe the consequences of impaired sugar transport on the primary metabolism.

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.