Morpho-anatomical and microbiological analysis of kiwifruit roots with KVDS symptoms

Italy, one of the largest producers of kiwifruit in the world, has lost 10% of its production in recent years because of the spread of the kiwifruit vine decline syndrome (KVDS). Although the aetiology of KVDS has not been characterized, root rot symptoms are often associated with water stagnation and root asphyxia. To investigate causal factors and potential solutions to counter this syndrome, an experimental trial was undertaken in a kiwifruit orchard affected by KVDS in Latina (central Italy) in 2020. Root samples from healthy plants were collected and compared with samples taken from plants affected by KVDS. Macroscopically, the roots affected by KVDS were rotting, showing a loss of rhizodermis and cortical parenchyma. Microscopic analysis revealed damage to the root system with tissue breakdown and decomposition, flaking of the rhizodermis, cortical area with a clear loss of cell turgor, initial decay of the stele and evident detachment of the cortex from the central conducting tissues. Light microscopy, morphological and molecular analyses were carried out on the rhizodermis of roots showing decay and death symptoms. Total DNA extracted from the pure fungal colonies was amplified by PCR with ITS primers, amplicons directly sequenced, and the obtained nucleotide sequences were compared with those present in the GenBank database (NCBI) through BLAST analysis. Genomic analysis allowed the identification of three abundant fungi namely Ilyonectria vredenhoekensis, Fusarium oxysporum and Paraphaeosphaeria michotii. Further investigation is required to determine the role of these fungi in KVDS, whether they are species favoured by water stagnation and root asphyxia; their abundance and presence in other regions, orchards, and kiwifruit species; if they compromise roots functionality individually or conjunction with other microbial pathogens or abiotic factors; and if they contribute to plant death associated with KVDS

Moria del kiwi: alterazione della struttura anatomica e morfologica delle radici di actinidia sottoposte a condizioni di asfissia del suolo

produzione negli ultimi anni a causa della diffusione della sindrome del declino dell’actinidia (KVDS/moria). Sintomi simili al KVDS sono stati osservati in diversi ambienti e vengono spesso associati a ristagno idrico e asfissia radicale, con conseguente marciume radicale. Nell'ambito del progetto Zespri “Water and soil management of G3 in Italy”, nel 2020 è stata avviata la sperimentazione in actinidieti colpiti da moria a Latina (Lazio, - ET0 732 mm) al fine di indagare sulle possibili cause e suggerire delle soluzioni per contrastare questa fisiopatia. Sono stati raccolti campioni di radici da piante sane e confrontati con campioni raccolti da piante affette da KVDS. Per l’analisi microscopica, le radici sono state fissate in formalina al 10%, disidratate e incluse in paraffina. Ogni singolo campione è stato sezionato in sezioni dello spessore di 5 μm e colorate con diverse metodiche. Macroscopicamente, le radici affette da KVDS sono risultate marcescenti, mostrando una perdita di rizoderma e parenchima corticale. L’analisi microscopica ha rilevato danneggiamenti del sistema radicale con rottura e decomposizione tissutale, sfaldamento di rizoderma, area corticale con evidente perdita di turgore cellulare, disfacimento iniziale della stele ed evidente distacco della corteccia dai tessuti conduttori centrali. Nel campione di controllo, le radici hanno presentato un rizoderma con spessore di 13 μm e una dimensione media di cellule del parenchima di 44,5 μm, a differenza del campione KVDS, in cui lo spessore del rizoderma, quasi assente, è stato di 8,3 μm e la dimensione media delle cellule di 34,7 μm. Dall’analisi dei gas tellurici è emerso che, conseguentemente all'insorgenza del ristagno idrico nel suolo, nei suoli attorno alle piante colpite da KVDS, il potenziale redox, parametro inversamente correlato alla concentrazione di ossigeno, è risultato essere significativamente più basso (+331 vs. +368 mV; media 0-90 cm di profondità), mentre sono state riscontrate concentrazioni più elevate di CO2 (7467 vs. 5870 ppm; media 0-90 cm di profondità), un indicatore di condizioni anossiche del suolo. Per migliorare le qualità fisica del suolo e assicurare una crescita ottimale delle radici di actinidia, sarà applicata una gestione innovativa del suolo volta ad aumentarne la sostanza organica e ridurre la compattazione, facilitando il movimento orizzontale e verticale dell'acqua nel terreno per fornire alle radici di actinidia l'ossigeno necessario per mitigare gli effetti di microrganismi potenzialmente patogeni, molti dei quali proliferano in ambienti anaerobi. Sarà, infine, ottimizzata la gestione della chioma e dell’apparato radicale per bilanciare il rapporto tra radici e foglie e migliorare la capacità delle piante di riprendersi da questo declino fisiologico

Dynamic interactions between host plant and its symbiontic endophytic bacteria: the olive tree as a holobiont

Beneficial bacteria-plant interactions play an important role in plants, as bacterial endophytes do not cause apparent damage and contribute to host plant protection and survival. The first part of this study was aimed to characterise the bacterial communities present in soil, leaf surface (phyllosphere) and xylem sap (XS) of olive trees. The XS was extracted from olive shoots using Sholander pressure chambers. The experimental orchard has been divided in two plots that have been managed for 18 years with two different systems: a) ‘sustainable management’ (Smng), with no-tillage, fertigation and internal C-inputs (spontaneous weeds and pruning residues), and b) an adjacent rainfed ‘conventional management’ (Cmng), that included soil tillage and mineral fertilization. A metagenomic approach was used to detect microorganisms, in order to estimate bacterial diversity and abundance, and to identify the bacterial taxa of the three analysed compartments in plants subjected to the two systems. The abundance and diversity of bacterial taxa in soil was higher compared to those found in the phyllosphere and XS. Bacterial communities generally came from the soil and reached the aerial plant parts through the XS. The application of different agronomic practices strongly influenced the composition of soil, phyllosphere and XS bacterial communities. As an example, the Smng system caused an improvement in the abundance of soil N-cycling bacteria and the presence of plant protective bacterial species in the phyllosphere. In the second part of the study, in order to examine the chemical changes due to XS bacterial colonization under the two different management systems, XS metabolome was analyzed in two sampling times (ST1: May; ST2: October) by ultra-high performance liquid chromatography (UHPLC) coupled to a hybrid quadrupole-time-of-flight mass spectrometer (QTOF-MS). The discriminating compounds were 94 at ST1 and 119 at ST2, and 35 of them were in common between the two sampling times. The majority of the discriminating metabolites (73 on 94 at ST1, and 109 on 119 at ST2) were found at higher concentration in the XS of Smng plants, compared to that of Cmng ones. Most of the discriminating metabolites found in the XS (about 80%, both at ST1 and ST2) were involved in plant secondary metabolism, mainly for plant chemical defense, growth regulation and signal transduction. The most prevailing class of compounds included terpenoids, phytohormones, alkaloids, sterols/steroids, retinols/retinoids, tocopherols and carotenoids. The potential agronomic benefits of the specific bacterial taxa detected under the Smng system could improve plant growth protection and provide a higher crop quality in olive plants and similar fruit species. For the first time, we have demonstrated that the endophytes and the XS metabolome of a tree crop significantly respond to the agronomic practices adopted. On this basis, we can assert that the host plant and its microbiota can be considered as a halobiont, and that this ecological unit is not constant but depends on external conditions. From a practical point of view, the results of our study encourage the use of a set of sustainable agricultural practices in a productive orchard, in order to enhance plant physiological status, increase yield quantity/quality, safeguard the environment and ameliorate human health

The agro-ecosystemic benefits of sustainable management in an Italian olive grove

Biotic and abiotic stresses are the main causes of decreased productivity and yield losses for crop species. Conventional agricultural management of fruit orchards (excessive soil tillage, empirical fertilization, burning of pruning material) generates a progressive reduction in organic matter and general soil impoverishment, contributing to agrosystem vulnerability and the appearance of plant disease. The well-being of the plant and the biotic complexity of the whole orchard system are the first “obstacles” to pathogen diffusion. In this study, we experimented with some agronomic methods aimed at increasing microbiological soil fertility and soil water storage capacity, applying sustainable agronomic management (soil cover by spontaneous vegetation, light and annual pruning and reuse of pruning residues within the orchard, irrigation with treated wastewater). We compared the results of conventional and sustainable fruit orchard managements. The experiment was carried out in a mature olive grove (‘Maiatica di Ferrandina’) and the two experimental plots (conventional and sustainable) were followed for more than 10 years. Adoption of sustainable agricultural practices increased the soil organic matter content by 1.1% in the 0-40 cm soil layer and the water infiltration rate 10-fold with respect to conventional management. Soil microbiota in the sustainable plot showed higher biomass and biodiversity. The findings demonstrated that the application of sustainable agricultural practices in fruit orchards has positive results in terms of soil fertility and biodiversity, with benefits to the whole agroecosystem stability

Physical structure and chemical quality of soils in G3 kiwifruit orchards differentially managed

In the last years, a new physiopathy that hits kiwifruit plants (Kiwifruit Vines decline Syndrome; KVDS) is emerging. We hypothesize that soil compaction and asphyxia could have a priming effect in the emergence of KVDS. On this basis, we characterized soils from three areas of a kiwifruit orchard located in Latina (Central Italy): one with plants showing severe symptoms of KVDS (Mfield), another with plants having intermediate symptoms (Ifield), and the last with healthy plants (Cfield). Soils were characterized physically showing a gradient of compaction, clay/silt content and water content, with the highest values in M and the lowest in C, while not significant differences were found regarding the content of chemical parameters (e.g., organic matter, acidity, and macro- and micro-nutrients). From the analysis of telluric gases it emerged that, following the onset of waterlogging in the soil, the redox potential, a parameter that indicates reducing conditions and so inversely related to oxygen concentration, was found to be significantly lower in Mfield. Higher concentrations of CO2 and CH4, two indicators of anoxic soil conditions were found in Mfield. The microscope analysis of the soils showed that Mfield soils had fewer macropores, whose number is directly related to the oxygen content. To improve the physical qualities of the soil and ensure an optimal growth of kiwifruit roots, an innovative management of the soil is necessary, including the application of external organic matter up to a depth of 40 cm, the use of decompacting cover crops (e.g., Rafanus spp.), reducing soil compaction and in order to provide the oxygen necessary to mitigate the effects of potentially pathogenic microorganisms, many of which proliferate in anaerobic environments. An adequate soil management is also aimed at facilitating the horizontal and vertical movements of soil water. Our activity will be oriented to the optimization of root and canopy management to balance the relationship between roots and leaves and improve the ability of plants to recover from this physiological decline. Particularly in kiwifruit, where waterlogged and compacted soils are a serious issue, soil physicochemical quality has a key role not only in terms of fertility and productivity, but it also important to assure plant growth and production

Physical structure and chemical quality of waterlogged soils in a kiwifruit orchard

In the last years, kiwifruit vines have been affected by the kiwifruit vine decline syndrome (KVDS), which is damaging the Italian kiwifruit industry. We hypothesize that soil compaction and asphyxia could have a priming effect in the emergence of KVDS. On this basis, we characterized soils from three areas of a kiwifruit orchard in Latina (Lazio region, Italy): one with vines showing severe symptoms of KVDS (Kfield), another with vines having intermediate symptoms (Ifield), and the last with healthy vines (Cfield) as control. Soils were characterized physically showing a gradient of compaction, clay/silt content and water content, with the highest values in Kfield and the lowest in Cfield, while soil chemical properties were not significantly different. The soil gas redox potential after the onset of waterlogging was significantly lower in Kfield than in the other treatments. This parameter indicates reducing soil conditions and it is negatively correlated to oxygen concentration. Higher CO2 and CH4 concentrations, two indicators of anoxic soil conditions, were found in Kfield, compared to Cfield. The microscope analysis of the soils showed that Kfield soils had fewer macropores than Cfield, whose number is positively correlated to the oxygen content. Implementation of soil and water management strategies could improve kiwifruit roots growth and vine productivity, and also help reduce symptoms of KVDS in impacted vineyards

Preliminary assesment of ABA concentration in roots of drip irrigated peach trees

Abscisic acid (ABA) is a phytohormone able to regulate the stomatal behaviour, transpiration and photosynthesis. ABA synthesis is usually associated to drought conditions. Based on the evidence that drip irrigation wets only 15-20% of soil surface and that 80% is dry during summer, this study tests the hypothesis that in a well drip irrigated tree the ABA concentration in roots located in the inter-row (non-irrigated) is higher than that measured in roots under the emitter line (irrigated row). Abscisic acid was determined by a competitive enzyme-linked immunosorbent assay in roots collected from a well irrigated tree (x 3) in summer at row and inter-row positions in a nectarine orchard grown in Southern Italy. Results show that leaf water potential was optimal (0.3 MPa at pre-dawn). However, ABA concentration was 3.74 pmol g-1 FW in roots from inter-row while it was 2.58 pmol g-1 FW in that growing along the row. Possible effects of the increased ABA levels at a part of the root-zone on leaf gas exchanges are discussed

Soil management type differentially modulates the metabolomic profile of olive xylem sap

In conventional olive growing, frequent soil tillage strongly reduces the complexity and diversity of the agro-ecosystem. Here, a metabolomic analysis was carried out on the xylem sap (XS) of olive plants (Olea europaea L.) from a grove located in Southern Italy (Basilicata region). The orchard has been divided in two plots that have been managed for 18 years with two different systems: a) ‘sustainable management’ (Smng), with no-tillage, fertigation and internal C-inputs (spontaneous weeds and pruning residues), and b) an adjacent rainfed ‘conventional management’ (Cmng), that included soil tillage and mineral fertilization. The XS was extracted from olive shoots in two sampling times (ST1: May; ST2: October) using a Sholander pressure chamber, and its metabolome analyzed by ultra-high performance liquid chromatography (UHPLC) coupled to a hybrid quadrupole-time-of-flight mass spectrometer (QTOF-MS). The discriminating compounds were 94 at ST1 and 119 at ST2, and 35 of them were in common between the two sampling times. The majority of the discriminating metabolites (73 on 94 at ST1, and 109 on 119 at ST2) were found at higher concentration in the XS of Smng plants, compared to that of Cmng ones. Most of the discriminating metabolites found in XS (about 80%, both at ST1 and ST2) were involved in plant secondary metabolism, mainly for plant chemical defense, growth regulation and signal transduction. The most prevailing class of compounds included terpenoids, phytohormones, alkaloids, sterols/steroids, retinols/retinoids, tocopherols and carotenoids. For the first time, we have demonstrated that the XS of a tree crop significantly responds to a shift of soil management. Generally, the plants of the Smng plot showed an up-regulated secondary metabolism. The results of our study encourage the use of a set of sustainable agricultural practices in a productive orchard, in order to enhance plant physiological status, increase yield quantity/quality, safeguard the environment and ameliorate human health