Meloidogyne is the most damaging plant parasitic nematode genus affecting vegetable crops worldwide. The induction of plant defense mechanisms against Meloidogyne in tomato by some Trichoderma spp. strains has been proven in pot experiments, but there is no information for tomato bearing the Mi-1.2 resistance gene or for other important fruiting vegetable crops. Moreover, Trichoderma is mostly applied for managing fungal plant pathogens, but there is little information on its effect on nematode-antagonistic fungi naturally occurring in soils. Thus, several experiments were conducted to determine (i) the ability of two commercial formulates of Trichoderma asperellum (T34) and Trichoderma harzianum (T22) to induce systemic resistance in tomato and cucumber against an avirulent Meloidogyne incognita population in splitroot experiments; (ii) the effect of combining T34 with tomato carrying the Mi-1.2 resistance gene to an avirulent M. incognita population in sterilized soil; and (iii) the effect of combining T34 with tomato carrying the Mi-1.2 resistance gene to a virulent M. incognita population in two suppressive soils in which Pochonia chlamydosporia is naturally present, and the effect of T34 on the level of P. chlamydosporia egg parasitism. Both Trichoderma formulates induced resistance to M. incognita in tomato but not in cucumber. In tomato, the number of egg masses and eggs per plant were reduced by 71 and 54% by T34, respectively. T22 reduced 48% of the number of eggs per plant but not the number of egg masses. T34 reduced the number of eggs per plant of the virulent M. incognita population in both resistant and susceptible tomato cultivars irrespective of the suppressive soil, and its effect was additive with the Mi-1.2 resistance gene. The percentage of fungal egg parasitism by P. chlamydosporia was not affected by the isolate T34 of T. asperellum. Frontiers in Microbiology | www.frontiersin.org 1 January 2020 | Volume 10 | Article 3042 Pocurull et al. Additive Effect Trichoderma-Mi1.2 Against RKN Conflict of Interest:The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Four rotation sequences consisting of ungrafted tomato cv. Durinta-melon cv. Paloma or tomato grafted onto the resistant rootstock 'Aligator'-melon grafted onto the resistant Cucumis metuliferus accession BGV11135, and in reverse order, were conducted from 2015 to 2017 in a plastic greenhouse infested or not with Meloidogyne incognita to determine the plant tolerance (T), the minimum relative crop yield (m) and fruit quality. The relationship between M. incognita densities in soil at transplanting (Pi) of each crop and the crop yield was assessed and T and m were estimated by the Seinhorst's damage model. In addition, the volume and the number of nuclei of single giant cells and the number of giant cells, its volume and the number of nuclei per feeding site in susceptible tomato and melon were compared to those in the resistant tomato and C. metuliferus 15 days after nematode inoculation in pot test. The relationship between the Pi and the relative crop yield fitted the Seinhorst's damage model in both ungrafted and grafted tomato and melon, but not for all years and cropping seasons. The estimated T for ungrafted and grafted tomato did not differ but m was lower in the former (34%) than the latter (67%). Sodium concentration in fruits from ungrafted but not from grafted tomato increased with nematode densities in spring 2015 and 2016. The estimated ungrafted melon T did not differ from the grafted melon cultivated in spring, but it did when it was cultivated in summer. The relative crop yield of ungrafted melon was lower (2%) than the grafted cultivated in spring (62%) and summer (20%). Sodium concentration in melon fruits from ungrafted plants increased with nematode densities. No variations in fruit quality from grafted melon cultivated in spring were found, although less dry matter and soluble solid content at highest nematode densities were registered when it was cultivated in summer. Lower number of giant cells per feeding site was observed in both susceptible tomato germplasms compared to the resistant ones but they were more voluminous and held higher number of nuclei per giant cell and per feeding site.
A rotation sequence of ungrafted and grafted tomato-melon-pepper-watermelon on resistant rootstocks ‘Brigeor’, Cucumis metuliferus, ‘Oscos’ and Citrullus amarus, respectively, was carried out in a plastic greenhouse, ending with a susceptible or resistant tomato crop. The rotation was conducted in plots infested by an avirulent (Avi) or a partially virulent (Vi) Meloidogyne incognita population to the Mi1.2 gene. At the beginning of the study, the reproduction index (RI, relative reproduction in the resistant respect susceptible tomato) of Avi and Vi populations was 1.3% and 21.6%, respectively. Soil nematode density at transplanting (Pi) and at the end (Pf) of each crop, disease severity and crop yield were determined. Moreover, the putative virulence selection and fitness cost were determined at the end of each crop in pot tests. In addition, a histopathological study was carried out 15 days after nematode inoculation in pot test. The volume and number of nuclei per giant cell (GC) and the number of GC, their volume and the number of nuclei per feeding site in susceptible watermelon and pepper were compared with C. amarus and resistant pepper. At the beginning of the study, the Pi of Avi and Vi plots did not differ between susceptible and resistant germplasm. At the end of the rotation, the Pf of Avi was 1.2 the Pi in susceptible and 0.06 in resistant, the cumulative yield of grafted crops was 1.82 times higher than that of the ungrafted susceptible ones, and the RI in resistant tomato less than 10% irrespective of the rotation sequence. Concerning the Vi, Pf was below the detection level at the end of the rotation in resistant and 3 times Pi in the susceptible. The cumulative yield of grafted crops was 2.83 times higher than that of the ungrafted and the RI in resistant tomato was 7.6%, losing the population’s virulence. In the histopathological study, no differences in number of GC per feeding site were observed in watermelon compared to C. amarus, but they were more voluminous and contained higher number of nuclei per GC and per feeding site. Regarding pepper, Avi population did not penetrate resistant rootstock.
Plant-parasitic nematodes are a significant cause of yield losses and food security issues. Specifically, nematodes of the genus Meloidogyne can cause significant production losses in horticultural crops around the world. Understanding the mechanisms of the ever-changing physiology of plant roots by imaging the galls induced by nematodes could provide a great insight into their control. However, infected roots are unsuitable for light microscopy investigation due to the opacity of plant tissues. Thus, samples must be cleared to visualize the interior of whole plants in order to make them transparent using clearing agents. This work aims to identify which clearing protocol and microscopy system is the most appropriate to obtain 3D images of tomato cv. Durinta and eggplant cv. Cristal samples infected with Meloidogyne incognita to visualize and study the root–nematode interaction. To that extent, two clearing solutions (BABB and ECi), combined with three different dehydration solvents (ethanol, methanol and 1-propanol), are tested. In addition, the advantages and disadvantages of alternative imaging techniques to confocal microscopy are analyzed by employing an experimental custom-made setup that combines two microscopic techniques, light sheet fluorescence microscopy and optical projection tomography, on a single instrument.
Proximal remote sensing devices are novel tools that enable the study of plant health status through the measurement of specific characteristics, including the color or spectrum of light reflected or transmitted by the leaves or the canopy. The aim of this study is to compare the RGB and multispectral data collected during five years (2016–2020) of four fruiting vegetables (melon, tomato, eggplant, and peppers) with trial treatments of non-grafted and grafted onto resistant rootstocks cultivated in a Meloidogyne incognita (a root-knot nematode) infested soil in a greenhouse. The proximal remote sensing of plant health status data collected was divided into three levels. Firstly, leaf level pigments were measured using two different handheld sensors (SPAD and Dualex). Secondly, canopy vigor and biomass were assessed using vegetation indices derived from RGB images and the Normalized Difference Vegetation Index (NDVI) measured with a portable spectroradiometer (Greenseeker). Third, we assessed plant level water stress, as a consequence of the root damage by nematodes, using stomatal conductance measured with a porometer and indirectly using plant temperature with an infrared thermometer, and also the stable carbon isotope composition of leaf dry matter.. It was found that the interaction between treatments and crops (ANOVA) was statistically different for only four of seventeen parameters: flavonoid (p < 0.05), NBI (p < 0.05), NDVI (p < 0.05) and the RGB CSI (Crop Senescence Index) (p < 0.05). Concerning the effect of treatments across all crops, differences existed only in two parameters, which were flavonoid (p < 0.05) and CSI (p < 0.001). Grafted plants contained fewer flavonoids (x¯ = 1.37) and showed lower CSI (x¯ = 11.65) than non-grafted plants (x¯ = 1.98 and x¯ = 17.28, respectively, p < 0.05 and p < 0.05) when combining all five years and four crops. We conclude that the grafted plants were less stressed and more protected against nematode attack. Leaf flavonoids content and the CSI index were robust indicators of root-knot nematode impacts across multiple crop types.
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