Relationships Among the Rootstock, Crop Load, and Sugar Hormone Signaling of Apple Tree, and Their Effects on Biennial Bearing

Zonal Chemical Signal Pathways Mediating Floral Induction in Apple

Phytohormones that trigger or repress flower meristem development in apple buds are thought to be locally emitted from adjacent plant tissues, including leaves and fruitlets. The presence of fruitlets is known to inhibit adjacent buds from forming flowers and thus fruits. The resulting absence of fruitlets the following season restores flower-promoting signalling to the new buds. The cycle can lead to a biennial bearing behaviour of alternating crop loads in a branch or tree. The hormonal stimuli that elicit flowering is typically referred to as the floral induction (FI) phase in bud meristem development. To determine the metabolic pathways activated in FI, young trees of the cultivar 'Ruby Matilda' were subjected to zonal crop load treatments imposed to two leaders of bi-axis trees in the 2020/2021 season. Buds were collected over the expected FI phase, which is within 60 DAFB. Metabolomics profiling was undertaken to determine the differentially expressed pathways and key signalling molecules associated with FI in the leader and at tree level. Pronounced metabolic differences were observed in trees and leaders with high return bloom with significant increases in compounds belonging to the cytokinin, abscisic acid (ABA), phenylpropanoid and flavanol chemical classes. The presence of cytokinins, namely adenosine, inosine and related derivatives, as well as ABA phytohormones, provides further insight into the chemical intervention opportunities for future crop load management strategies via plant growth regulators.

Integrated soil-fruit-climate management system to improve apple production stability

Unstable agricultural systems, such as unreliable fruit production, threaten global food security and could negatively impact human nutrition. However, methods of maintaining a stable fruit supply have received little attention. Therefore, this study explored the impact of a systematic service model on the establishment of an integrated soil-fruit-climate management (ISM) approach and stabilization of apple yield based on a fixed-effects model using sample data from Fuji apple farmers in China for six consecutive years (2016-2021). The results showed that the systematic service model dramatically improved the adoption of ISM-based technology. By 2021, the rate of ISM technology adoption among farmers reached 84.4 %, especially pendulous branch-fruiting technology and Osmia pollination, which increased by 82.2 % and 37.8 %, respectively, compared to that in 2016. With the adoption of the ISM approach, apple yields and tree loadings stabilized over time. For example, the yield change rate and tree loading change rate were 14.8 % and 7.3 % lower, respectively, in 2021 than in 2016. We demonstrated through a fixed-effects analysis that the ISM approach exerts a fully mediated effect on the mechanism of action of service measure quantity disclosure, which positively impacted the rate of change in yield and tree loadings. This finding indicated that the six-in-one systematic service model of soil testing, pest and disease diagnosis, course training, meteorological monitoring, on-site guidance, and experimental demonstration established by the Science and Technology Backyard model resolved the information, goal, and hardware gaps that limited the adoption of this technology by farmers and promoted the construction of an integrated soil-fruit-climate management approach, which in turn has stabilized the apple production system.

Reducing the sink/source ratio of on-date palm plants during fruit growth has physiological and biochemical impacts on the shift in source-sink limitations

BACKGROUND

The present study examined the impact of reducing the 'sink' on the 'source' in On-palms with a bunch number greater than eight. The capacity of leaves and fruit, as well as assimilate loading and unloading in phloem, restrict plant growth and yield. The study evaluated yield components, as well as photosynthetic and hormonal feedback, resulting from source-sink relationships.

RESULTS

During the mid-Kimri, removing bunches from On-trees stabilized yield components and fruit size, suggesting that On-trees have a sink limitation. Bunch thinning boosted these indicators compared to normal trees with a bunch number between six and eight inclusive, indicating that On-trees had source limitations. In mid-Khalal, the treatments presented a type of source and sink limitation that is opposite to mid-Kimri. The thinning techniques addressed the source-sink limitation by adjusting the additional carbon allocation. This resulted in an increase of non-reducing sugars and starch in different organs, whereas reducing sugars decreased. These adjustments were made to reduce sucrose-phosphate synthase and sucrose synthase activity, raising invertase activity, lowering indole-3-acetic acid, zeatin, gibberellin, and abscisic acid hormone levels in fruits, as well as lowering trehalose production in organs. Levels of hormones, enzymes, and trehalose showed less variation during bunch thinning and source limitation compared to bunch removal and sink limitation.

CONCLUSION

At Rutab, thinning types demonstrated the source limitation of On-trees. Bunch removal and bunch thinning by removing the source-sink limitation had the greatest effect on increasing yield components and fruit size, respectively. To improve the quality and quantity of fruit, it is important to use both thinning techniques simultaneously. © 2023 Society of Chemical Industry.

'Yunnan' quince rootstock promoted flower bud formation of 'Abbé Fetel' pear by altering hormone levels and PbAGL9 expression

Flower busd formation is an important plant growth process. It has been reported that dwarfing rootstocks can significantly affect the flower bud formation of scions. In this study, we found the dwarfing rootstock 'Yunnan' quince could significantly increase the flowering rate of 'Abbé Fetel' pear scions. The RNA-sequencing data revealed significant changes in the expression of genes related to hormone pathways. Furthermore, hormone analyses indicated that 'Yunnan' quince significantly decreased the GA₃ content and increased the cytokinin/auxin ratio in 'Abbé Fetel' pear apical buds. The hormone contents were consistent with the RNA-sequencing data. Moreover, we found the flower development-related genes PbAGL9 and PbCAL-A1 were significantly upregulated and PbTFL1 was significantly downregulated in 'Abbé Fetel'/'Yunnan' quince apical buds. To further clarify the relationship between hormones and flowering-related genes, a hormone response assay was carried out. We found the expression levels of PbCAl-A1, PbTFL1 and PbAGL9 were regulated by hormones including GA₃, CPPU and NAA. Y1H and dual-luciferase assays indicated that PbAGL9 significantly decreased the promoter activity of PbTFL1. In summary, 'Yunnan' quince upregulated PbCAL-A1 and PbAGL9, and downregulated PbTFL1 expression by decreasing the GA₃ content and increasing the cytokinin/auxin ratio in 'Abbé Fetel' pear apical buds. Additionally, 'Yunnan' quince down-regulate PbTFL1 by upregulating the expression of PbAGL9, and eventually promoted floral induction in 'Abbé Fetel' pear.

Transcriptome Analysis of the Effects of Grafting Interstocks on Apple Rootstocks and Scions

Apples are a major horticultural crop worldwide. Grafting techniques are widely utilized in apple production to keep the varieties pure. Interstocks are frequently used in Northern China to achieve intensive apple dwarfing cultivation. High-throughput sequencing was used to investigate differentially expressed genes in the phloem tissues of two different xenograft systems, M ('Gala'/'Mac 9'/Malus baccata (L.) Borkh.) and B ('Gala'/Malus baccata (L.) Borkh.). The results showed that dwarfing interstocks could significantly reduce the height and diameters of apple trees while have few effects on the growth of annual branches. The interstocks were found to regulate the expression of genes related to hormone metabolism and tree body control (GH3.9, PIN1, CKI1, ARP1, GA2ox1 and GA20ox1), these effects may attribute the dwarf characters for apple trees with interstocks. Besides, the interstocks reduce photosynthesis-related genes (MADH-ME4 and GAPC), promote carbon (C) metabolism gene expression (AATP1, GDH and PFK3), promote the expression of nitrogen (N)-metabolism-related genes (NRT2.7, NADH and GDH) in rootstocks, and improve the expression of genes related to secondary metabolism in scions (DX5, FPS1, TPS21 and SRG1). We also concluded that the interstocks acquired early blooming traits due to promotion of the expression of flowering genes in the scion (MOF1, FTIP7, AGL12 and AGL24). This study is a valuable resource regarding the molecular mechanisms of dwarf interstocks' influence on various biological processes and transplantation systems in both scions and rootstocks.

Metabolic Pathways for Observed Impacts of Crop Load on Floral Induction in Apple

The triggers of biennial bearing are thought to coincide with embryonic development in apple and occurs within the first 70 days after full bloom (DAFB). Strong evidence suggests hormonal signals are perceived by vegetative apple spur buds to induce flowering. The hormonal response is typically referred to as the floral induction (FI) phase in bud meristem development. To determine the metabolic pathways activated in FI, young trees of the biennial bearing cultivar 'Nicoter' and the less susceptible cultivar 'Rosy Glow' were forced into an alternate cropping cycle over five years and an inverse relationship of crop load and return bloom was established. Buds were collected over a four-week duration within 70 DAFB from trees that had maintained a four-year biennial bearing cycle. Metabolomics profiling was undertaken to determine the differentially expressed pathways and key signalling molecules associated with biennial bearing. Marked metabolic differences were observed in trees with high and low crop load treatments. Significant effects were detected in members of the phenylpropanoid pathway comprising hydroxycinnamates, salicylates, salicylic acid biosynthetic pathway intermediates and flavanols. This study identifies plant hormones associated with FI in apples using functional metabolomics analysis.

Metabolic Response of Malus domestica Borkh cv. Rubin Apple to Canopy Training Treatments in Intensive Orchards

In this study, we used apple tree (Malus domestica Borkh.) cv. Rubin grafts on dwarfing P60 rootstock. Our planting scheme was single rows with 1.25 m between trees and 3.5 m between rows. The aim of this study was to determine the impact of canopy training treatments, as a stress factor, on metabolic response to obtain key information on how to improve physiological behavior and the management of the growth and development of apple trees. The results indicated that all applied canopy training treatments significantly increased the total phenol and total starch contents in apple tree leaves. The total starch increased from 1.5- to almost 3-fold in all treatments, especially during the 2017 harvesting season, compared to the control. The fructose, sorbitol, and ratio of chlorophyll a to b in leaves also significantly increased. Higher precipitation levels induced changes in the accumulation of secondary metabolites in apple tree leaves and fruits during the 2017 harvesting season. The total phenol content significantly increased in apple tree leaves in all treatments, but the fructose content decreased. We observed the same tendencies in total phenolic content and glucose concentration in apple fruits. Therefore, the defense reaction might be a preferred option for apple tree cultivation and the optimization of its growth and development.

Contrasting effects of genotype and root size on the fungal and bacterial communities associated with apple rootstocks

The endophytic microbiome of plants is believed to have a significant impact on its physiology and disease resistance, however, the role of host genotype in determining the composition of the endophytic microbiome of apple root systems remains an open question that has important implications for defining breeding objectives. In the current study, the bacterial and fungal microbiota associated with four different apple rootstocks planted in April, 2018 in the same soil environment and harvested in May, 2019 were evaluated to determine the role of genotype on the composition of both the bacterial and fungal communities. Results demonstrated a clear impact of genotype and root size on microbial composition and diversity. The fungal community was more affected by plant genotype whereas the bacterial community was shaped by root size. Fungal and bacterial abundance was equal between different-sized roots however, significantly higher microbial counts were detected in rhizosphere samples compared to root endosphere samples. This study provides information that can be used to develop a comprehensive and readily applicable understanding of the impact of genotype and environmental factors on the establishment of plant microbiome, as well as its potential function and impact on host physiology.

Inheritance of Yield Components and Morphological Traits in Avocado cv. Hass From "Criollo" "Elite Trees" via Half-Sib Seedling Rootstocks

Grafting induces precocity and maintains clonal integrity in fruit tree crops. However, the complex rootstock × scion interaction often precludes understanding how the tree phenotype is shaped, limiting the potential to select optimum rootstocks. Therefore, it is necessary to assess (1) how seedling progenies inherit trait variation from elite 'plus trees', and (2) whether such family superiority may be transferred after grafting to the clonal scion. To bridge this gap, we quantified additive genetic parameters (i.e., narrow sense heritability-h ², and genetic-estimated breeding values-GEBVs) across landraces, "criollo", "plus trees" of the super-food fruit tree crop avocado (Persea americana Mill.), and their open-pollinated (OP) half-sib seedling families. Specifically, we used a genomic best linear unbiased prediction (G-BLUP) model to merge phenotypic characterization of 17 morpho-agronomic traits with genetic screening of 13 highly polymorphic SSR markers in a diverse panel of 104 avocado "criollo" "plus trees." Estimated additive genetic parameters were validated at a 5-year-old common garden trial (i.e., provenance test), in which 22 OP half-sib seedlings from 82 elite "plus trees" served as rootstocks for the cv. Hass clone. Heritability (h ²) scores in the "criollo" "plus trees" ranged from 0.28 to 0.51. The highest h ² values were observed for ribbed petiole and adaxial veins with 0.47 (CI 95%0.2-0.8) and 0.51 (CI 0.2-0.8), respectively. The h ² scores for the agronomic traits ranged from 0.34 (CI 0.2-0.6) to 0.39 (CI 0.2-0.6) for seed weight, fruit weight, and total volume, respectively. When inspecting yield variation across 5-year-old grafted avocado cv. Hass trees with elite OP half-sib seedling rootstocks, the traits total number of fruits and fruits' weight, respectively, exhibited h ² scores of 0.36 (± 0.23) and 0.11 (± 0.09). Our results indicate that elite "criollo" "plus trees" may serve as promissory donors of seedling rootstocks for avocado cv. Hass orchards due to the inheritance of their outstanding trait values. This reinforces the feasibility to leverage natural variation from "plus trees" via OP half-sib seedling rootstock families. By jointly estimating half-sib family effects and rootstock-mediated heritability, this study promises boosting seedling rootstock breeding programs, while better discerning the consequences of grafting in fruit tree crops.

Inheritance of Rootstock Effects in Avocado (Persea americana Mill.) cv. Hass

Grafting is typically utilized to merge adapted seedling rootstocks with highly productive clonal scions. This process implies the interaction of multiple genomes to produce a unique tree phenotype. However, the interconnection of both genotypes obscures individual contributions to phenotypic variation (rootstock-mediated heritability), hampering tree breeding. Therefore, our goal was to quantify the inheritance of seedling rootstock effects on scion traits using avocado (Persea americana Mill.) cv. Hass as a model fruit tree. We characterized 240 diverse rootstocks from 8 avocado cv. Hass orchards with similar management in three regions of the province of Antioquia, northwest Andes of Colombia, using 13 microsatellite markers simple sequence repeats (SSRs). Parallel to this, we recorded 20 phenotypic traits (including morphological, biomass/reproductive, and fruit yield and quality traits) in the scions for 3 years (2015-2017). Relatedness among rootstocks was inferred through the genetic markers and inputted in a "genetic prediction" model to calculate narrow-sense heritabilities (h 2) on scion traits. We used three different randomization tests to highlight traits with consistently significant heritability estimates. This strategy allowed us to capture five traits with significant heritability values that ranged from 0.33 to 0.45 and model fits (r) that oscillated between 0.58 and 0.73 across orchards. The results showed significance in the rootstock effects for four complex harvest and quality traits (i.e., total number of fruits, number of fruits with exportation quality, and number of fruits discarded because of low weight or thrips damage), whereas the only morphological trait that had a significant heritability value was overall trunk height (an emergent property of the rootstock-scion interaction). These findings suggest the inheritance of rootstock effects, beyond root phenotype, on a surprisingly wide spectrum of scion traits in "Hass" avocado. They also reinforce the utility of polymorphic SSRs for relatedness reconstruction and genetic prediction of complex traits. This research is, up to date, the most cohesive evidence of narrow-sense inheritance of rootstock effects in a tropical fruit tree crop. Ultimately, our work highlights the importance of considering the rootstock-scion interaction to broaden the genetic basis of fruit tree breeding programs while enhancing our understanding of the consequences of grafting.