Taxonomic structure of tissue endophytic bacterial microbiome in clonal apple rootstock grown on sod-podzolic soils with variant properties

The taxonomic structure of tissue endophytic bacterial microbiome was comparatively studied in microplants (undifferentiated explant callus tissues, passage 25) and 5-year clonal apple rootstocks 57-490 and 54-118 cultured from corresponding tissues (passage 1) on sod-podzolic soils with variant granulometry, chemical, physical and physicochemical properties. Proteobacteria (91.6 %) predominated in vitro tissues among other endophytic bacterial phyla in rootstock 57-490, while Proteobacteria (52.5 %) and Firmicutes (47.4 %) — in rootstock 54-118. The endophytic Firmicutes ratio vs. in vitro tissues decreases (0.7-2.0 %) in roots and more severely (0-0.2 %) in leaves. Endophytic Actinobacteriota are revealed in 11.7 % in roots of the study rootstock in heavy loam soil, whilst in medium loam their ratio drops to 2.74.1 % in roots and 0.1-0.2 % in leaves. The phylogenetic diversity indices estimation for main endophytic bacterial phyla in apple rootstock tissue recovers their essentially lower diversity and evenness in culture endosphere (Shannon index 0.42-1.00) vs. open soil roots (1.34-2.08). The leaves Shannon index is typically low (0.06-0.13) indicating poor diversity and evenness of the main endophytic bacterial phyla.

Influence of Substrate Composition and Container Size on the Growth of Tissue Culture Propagated Apple Rootstock Plants

Substrate composition and container size are considered crucial for apple rootstock grown in a plug seedling system. This study investigated the effect of substrate material’s proportion and different container volumes on the growth of apple rootstock (M-9) plants propagated by tissue culture. In substrate composition, three different ratios of peat moss (PM): vermiculite (VL): perlite (PL) at 1:1:1 (S1), 1:2:3 (S2), 3:1:2 (S3) were used. For container size, plants were grown in 1000 mL (C1), 500 mL (C2), and 300 mL (C3) containers filled by 1:1:1 mixture of PM, VL and PL. In both cases, plants were treated eleven weeks in a green house. Our results demonstrate that the plant height, number of leaves, leaf area, shoot fresh weight and root fresh weight of apple rootstock were significantly higher in substrate composition S3 compared with S1 composition. However, chlorophyll content (SPAD) and photosynthesis rate were unaffected by variation of substrate composition. Furthermore, rootstock grown in C1 container showed plant height, number of leaves, leaf area, number of nodes, root length, shoot and root’s fresh and dry weight were significantly higher than those grown in C2 and C3 volume containers. The chlorophyll content and transpiration rate were not significantly affected by the different container volumes. These results suggest that the substrate ratio 3:1:2 of PM:VL:PL and container size 1000 mL were more favorable than other treatments for initial growth and development of the tissue culture propagated apple rootstock plants.

Apple gene MD13G1109800 is a member of Trihelix family transcription factors and expressed in response to abiotic stress

The Trihelix family of transcription factors plays an important role in the plant’s response to various abiotic stress types. In this work in apple Golden Delicious genome we identified apple gene MD13G1109800 as a member of Triheilx family in silico. Analysis of chromosomal localization showed that it is located on chromosome 13 and has four introns. The hypothetical protein encoded by it has a length of 365 amino acid residues, a molecular weight of 42097.23 Da, an isoelectric point pI = 6.21 and located in the nucleus. Analysis of the promoter region of the MD13G1109800 gene indicates that its product is a member of many signaling pathways triggered by both external and internal factors. The expression level of the MD13G1109800 gene increases under drought, low and high temperatures, as well as salinity in the MM-106 apple rootstock.

Morphological and Physio-Biochemical Responses of Watermelon Grafted onto Rootstocks of Wild Watermelon [Citrullus colocynthis (L.) Schrad] and Commercial Interspecific Cucurbita Hybrid to Drought Stress

This study aimed to assess the morphological and physio-biochemical responses of a commercial watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai) cv. ‘Crimson Sweet’ grafted onto a drought-tolerant rootstock of wild watermelon (bitter apple, Citrullus colocynthis (L.) Schrad, ‘Esfahan’) in comparison with an ungrafted ‘Crimson Sweet’ watermelon or one grafted onto a commercial interspecific Cucurbita hybrid (Cucurbita maxima Duch. × Cucurbita moschata Duch.) rootstock (‘Shintoza’) under water stress. The experiment was conducted in pots under a controlled environment in a greenhouse, and water stress was imposed by maintaining moisture level in pots at 100% (well water (WW)) or 50% (water deficit (WD)) of container capacity (CC). WD significantly decreased most of the morphological traits in ungrafted and grafted plants, while the decrease in growth traits was lower in grafted plants than ungrafted plants. The response of grafted plants onto wild watermelon rootstock (‘Esfahan’) for most of the affected parameters (shoot fresh and dry weight, vine length and internodal length) was, however, comparable to those grafted onto commercial Cucurbita hybrid rootstock (‘Shintoza’). Plants grafted onto bitter apple (wild watermelon) exhibited a relatively lower decrease in growth and biomass, besides showing higher antioxidant activity (e.g., guaiacol peroxidase) concomitant with the lower accumulation of malondialdehyde and electrolyte leakage in the leaf tissues in comparison with ungrafted plants. The overall growth performance, as well as those under water stress conditions in commercial rootstock-grafted watermelon, was related to its better plant water status (e.g., high relative water content) which was likely ascertained by its greater root efficiency. This suggests that watermelons grafted onto bitter apple rootstock and Cucurbita hybrid rootstock were constitutively more resistant to drought, with higher efficiency in mitigating oxidative stress than ungrafted treatment. The above findings demonstrated that bitter apple, a well-adapted desert species, can be used as an alternative rootstock to commercial rootstocks (e.g., ‘Shintoza’) for watermelon grafting under water stress conditions. In addition, bitter apple rootstock can be involved in rootstock breeding programs to improve drought tolerance in watermelon.

Detection of Above-Ground Physiological Indices of an Apple Rootstock Superior Line 12-2 with Improved Apple Replant Disease (ARD) Resistance

(1) Background: The cultivation of resistant rootstocks is an effective way to prevent ARD. (2) Methods: 12-2 (self-named), T337, and M26 were planted in replanted and sterilized soil. The aboveground physiological indices were determined. (3) Results: The plant heights and the stem thicknesses of T337 and M26 were significantly affected by ARD. Relative chlorophyll content (June–October), Pn (August–September), and Gs (August) of T337 and relative chlorophyll content (June–July, September), Pn (September–October), and Ci (September) of M26 were significantly affected by ARD. ARD had a significant effect on Fv/Fm (June), qP (June–July), and NPQ of T337 (June–October, except August) and Fv/Fm (June) and NPQ (June-October, except July) of M26. Additionally, ARD affected Rfd of M26 and T337 during August. SOD (August and October), POD (August–September), and CAT (July-August, October) activities and MDA (September–October) content of T338 as well as SOD (July–October), POD (June–October), and CAT (July-October) activities and MDA (July, September–October) content of M26 were significantly affected by ARD. ARD significantly reduced nitrogen (October), phosphorus (September–October), and zinc (July) contents of M26 and potassium (June) content of T337. The above physiological indices were not affected by ARD in 12-2. (4) Conclusions: 12-2 could be useful as an important rootstock to relieve ARD due to strong resistance.

Phytopythium vexans Associated with Apple and Pear Decline in the Saïss Plain of Morocco

An extensive survey conducted in the Saïss plain of Morocco during the 2017–2018 growing season revealed that 35 out of 50 apple and pear orchards were infested with a pathogen that causes the decline disease. Morphological and phylogenetic tree analyses using the cox II gene allowed us to identify the pathogen as Phytopythium vexans. Interestingly, no Phytophthora and Pythium species were isolated. The occurrence and prevalence of the disease varied between locations; the most infested locations were Meknes (100%), Imouzzer (83%), and Sefrou (80%). To fulfill Koch’s postulate, a greenhouse pathogenicity test was performed on the stem and collar of one-year-old healthy seedlings of apple rootstock M115. Symptoms similar to those observed in the field were reproduced in less than 4 months post-inoculation with root rot disease severity ranging from 70 to 100%. The survey results evidenced that apple rootstocks, soil type, and irrigation procedure may contribute significantly to the occurrence of the disease. The disease was most prevalent in drip water irrigation and sandy-clay soil on wild apple rootstock. Accordingly, a rational drip advanced watering system and good sanitation practices could eliminate water stagnation and help prevent the onset of this disease. It was concluded that Pp. vexans occurrence may be strongly influenced by irrigation mode and type of soil. Therefore, the obtained findings of this study could help to better understand the recurrence of this disease and to develop a reliable integrated strategy for its management.

The Arabidopsis pattern recognition receptor EFR enhances fire blight resistance in apple

Fire blight disease, caused by the bacterium Erwinia amylovora (E. amylovora), is responsible for substantial losses in cultivated apples worldwide. An important mechanism of plant immunity is based on the recognition of conserved microbial molecules, named pathogen-associated or microbe-associated molecular patterns (PAMPs or MAMPs), through pattern recognition receptors (PRRs), leading to pattern-triggered immunity (PTI). The interspecies transfer of PRRs represents a promising strategy to engineer broad-spectrum and durable disease resistance in crops. EFR, the Arabidopsis thaliana PRR for the PAMP elf18 derived from the elongation factor thermal unstable (EF-Tu) proved to be effective in improving bacterial resistance when expressed into Solanaceae and other plant species. In this study, we tested whether EFR can affect the interaction of apple with E. amylovora by its ectopic expression in the susceptible apple rootstock M.26. Stable EFR expression led to the activation of PAMP-triggered immune response in apple leaves upon treatment with supernatant of E. amylovora, as measured by the production of reactive oxygen species and the induction of known defense genes. The amount of tissue necrosis associated with E. amylovora infection was significantly reduced in the EFR transgenic rootstock compared to the wild-type. Our results show that the expression of EFR in apple rootstock may be a valuable biotechnology strategy to improve the resistance of apple to fire blight.