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N-(2-Chloro-4-pyridyl)-N'-phenylurea (CPPU) is a highly active cytokinin-like plant growth regulator that promotes chlorophyll biosynthesis, cell division, and cell expansion. It also increases fruit set and accelerates fruit enlargement. However, there has been no report about the effect of CPPU on fruit development and its physiological mechanism in macadamia. In this study, we investigated the effect of CPPU treatment at early fruit development via foliar spray or raceme soaking at 20 mg·L-1 on fruit set and related physiology in macadamia. Changes in carbohydrate contents and endogenous hormones in leaves, bearing shoots and fruit were also examined. Results showed that CPPU significantly reduced young fruit drop and delayed the wave of fruit drop by 1-2 weeks. The treatment significantly decreased the contents of total soluble sugars and starch in the leaves, but increased them in the bearing shoots and total soluble sugars in the husk (pericarp) and seeds. These findings suggested that CPPU promoted carbohydrate mobilization from the leaves to the fruit. In addition, CPPU increased the contents of indole-3-acetic acid (IAA), gibberellin acid (GA3), and zeatin riboside (ZR) and decreased the abscisic acid (ABA) in the husk. Therefore, CPPU treatment reduced the early fruit drop by increasing carbohydrate availability and by modifying the balance among endogenous hormones.

Global fruit and nut yields are affected by shortfalls in pollinator populations, and pollen limitation is most prevalent among tropical, bee-pollinated and self-incompatible plants. Macadamia is a subtropical, bee-pollinated crop in which some cultivars have been found to be highly outcrossing. We aimed to determine the extent of outcrossing and its effects on nut quality across a wide range of international macadamia cultivars in three countries. We sampled fruit from 19 macadamia cultivars across 23 sites in Australia, Brazil and South Africa. We used genotype-by-sequencing and MassARRAY methods to assign paternity to individual fruit and we assessed pollen-parent effects on nut quality. Macadamia was highly outcrossing, producing 80–100% of fruit by cross-pollination, at 17 of the 23 sites. Mixed mating (41–72% outcrossing) was identified at five sites, and low outcrossing (10%) was identified in one cultivar at one site where it was isolated from other flowering macadamia trees. Outcrossed fruit often had significantly better quality than selfed fruit, with 1.61–3.39 g higher nut-in-shell mass, 0.53–1.55 g higher kernel mass, 3.3–6.4% higher kernel recovery, and 3.0–3.5% higher oil concentration. The differences in kernel recovery equated to differences in value of USD 433–841 per ton of nut-in-shell at prices of USD 3000 per ton. In summary, macadamia cultivars were mostly highly outcrossing, and outcrossed nuts often had higher quality than selfed nuts. Growers should consider interplanting different cultivars more closely and distributing bee hives more widely to maximise cross-pollination, produce high yields, and optimise nut quality.

The consumption of macadamia nuts has increased due to their cardioprotective and antioxidant properties. However, this rise is consistent with an increase in the cases of macadamia nut allergy, leading to severe reactions. Although two Macadamia integrifolia allergens (Mac i 1 and Mac i 2) have been identified in Australian and Japanese patients, the allergenic sensitization patterns in Western European populations, particularly in Spain, remain unclear. For this purpose, seven patients with macadamia nut allergy were recruited in Spain. Macadamia nut protein extracts were prepared and, together with hazelnut and walnut extracts, were used in Western blot and inhibition assays. IgE-reactive proteins were identified using MALDI-TOF/TOF mass spectrometry (MS). Immunoblotting assays revealed various IgE-binding proteins in macadamia nut extracts. Mass spectrometry identified three new allergens: an oleosin, a pectin acetylesterase, and an aspartyl protease. Cross-reactivity studies showed that hazelnut extract but not walnut extract inhibited macadamia nut oleosin-specific IgE binding. This suggests that oleosin could be used as marker for macadamia–hazelnut cross-reactivity. The results show an allergenic profile in the Spanish cohort different from that previously detected in Australian and Japanese populations. The distinct sensitization profiles observed highlight the potential influence of dietary habits and environmental factors exposure on allergenicity.

Macadamia nuts are, economically, the second most important crop in the state of Hawai’i. A recent decline in yield and acreage has been attributed to insect damage and diseases such as Macadamia Quick Decline (MQD) caused by Phytophthora tropicalis and P. heveae. To develop an improved methodology for the diagnosis and treatment of MQD, investigations were undertaken to better understand the pathosystem of the disease. These investigations included sampling from multiple locations from sectioned trees utilizing two methods of tissue collection and isolations using two baiting techniques. The collection of tissue from the cambium and phloem of trees after scraping away the bark and in locations of recent or current sap exudation using a narrow diameter steel awl proved to be an efficient means for the molecular detection of the MQD pathogens from infected trees exhibiting MQD symptoms. In addition, a more efficient and cost-effective baiting method using apple puree was developed.

Background The large Gondwanan plant family Proteaceae is an early-diverging eudicot lineage renowned for its morphological, taxonomic and ecological diversity. Macadamia is the most economically important Proteaceae crop and represents an ancient rainforest-restricted lineage. The family is a focus for studies of adaptive radiation due to remarkable species diversification in Mediterranean-climate biodiversity hotspots, and numerous evolutionary transitions between biomes. Despite a long history of research, comparative analyses in the Proteaceae and macadamia breeding programs are restricted by a paucity of genetic information. To address this, we sequenced the genome and transcriptome of the widely grown Macadamia integrifolia cultivar 741. Results Over 95 gigabases of DNA and RNA-seq sequence data were de novo assembled and annotated. The draft assembly has a total length of 518 Mb and spans approximately 79% of the estimated genome size. Following annotation, 35,337 protein-coding genes were predicted of which over 90% were expressed in at least one of the leaf, shoot or flower tissues examined. Gene family comparisons with five other eudicot species revealed 13,689 clusters containing macadamia genes and 1005 macadamia-specific clusters, and provides evidence for linage-specific expansion of gene families involved in pathogen recognition, plant defense and monoterpene synthesis. Cyanogenesis is an important defense strategy in the Proteaceae, and a detailed analysis of macadamia gene homologues potentially involved in cyanogenic glycoside biosynthesis revealed several highly expressed candidate genes. Conclusions The gene space of macadamia provides a foundation for comparative genomics, gene discovery and the acceleration of molecular-assisted breeding. This study presents the first available genomic resources for the large basal eudicot family Proteaceae, access to most macadamia genes and opportunities to uncover the genetic basis of traits of importance for adaptation and crop improvement.

Macadamia is a true dicotyledonous plant that thrives in a mild, humid, low wind environment. It is cultivated and traded internationally due to its high-quality nuts thus, has significant development prospects and scientific research value. However, information on the genetic resources of Macadamia spp. remains scanty. The mitochondria (mt) genomes of three economically important Macadamia species, Macadamia integrifolia, M. ternifolia and M. tetraphylla, were assembled through the Illumina sequencing platform. The results showed that each species has 71 genes, including 42 protein-coding genes, 26 tRNAs, and 3 rRNAs. Repeated sequence analysis, RNA editing site prediction, and analysis of genes migrating from chloroplast (cp) to mt were performed in the mt genomes of the three Macadamia species. Phylogenetic analysis based on the mt genome of the three Macadamia species and 35 other species was conducted to reveal the evolution and taxonomic status of Macadamia. Furthermore, the characteristics of the plant mt genome, including genome size and GC content, were studied through comparison with 36 other plant species. The final non-synonymous (Ka) and synonymous (Ks) substitution analysis showed that most of the protein-coding genes in the mt genome underwent negative selections, indicating their importance in the mt genome. The findings of this study provide a better understanding of the Macadamia genome and will inform future research on the genus.

Peptides with xanthine oxidase (XOD)-inhibitory activities are potential dietary interventions for the treatment of hyperuricemia and gout. In this study, Macadamia integrifolia antimicrobial protein 2 (MiAMP2) was chosen for in silico proteolysis with pepsin, trypsin and chymotrypsin through ExPASy PeptideCutter, and the obtained peptides were further screened by water solubility, ADMET prediction and molecular docking. Four novel peptides, namely RPLY, PGPR, HGGR and GPY, were supposed to be non-toxic and have XOD-inhibitory potential. Molecular docking analysis showed that these peptides were able to bind to the active sites (such as Leu873, Phe914 and Thr1010) of XOD through interactions mainly including conventional hydrogen bond, alkyl and pi-interaction. These peptides were further synthesized to verify their in vitro XOD-inhibitory activities, and the IC50 values of PGPR, GPY and HGGR were shown to be 24.84 ± 0.02, 30.44 ± 0.33 and 24.89 ± 0.19 mM, respectively. Kinetic experiments demonstrated that PGPR and HGGR are mixed-type inhibitors, and GPY is a competitive inhibitor toward XOD with the inhibitory constant (Ki) values ranging from 1.09 to 8.98. Our results suggested that MiAMP2 is an important source of bioactive peptides for the management of hyperuricemia.

Despite Macadamia ’s global economic importance as a nutrient-dense nut crop, the structural and evolutionary characteristics of its chloroplast genomes (plastomes) remain underexplored. Here, we present the first comprehensive plastomic analysis of 185 cultivated Macadamia accessions from a Chinese germplasm collection. These plastomes exhibited conserved quadripartite structures (159,195–159,726 bp), comprising a large single-copy (LSC, 87,651–88,107 bp), a small single-copy (SSC, 18,743–18,819 bp), and inverted repeats (IRs, 26,378–26,422 bp). All plastomes maintained 115 functional genes (81 protein-coding, 30 tRNA, 4 rRNA) and were classified into 23 unique haplotypes. We identified 573 SNPs, 577 long repeats, and five hypervariable regions ( trnS – trnG-exon1 , ndhD – psaC , trnH – psbA , petA – psbL , and psbC – psbZ ), with nucleotide diversity (Pi) values > 0.01. Phylogenetic reconstruction based on whole plastomes resolved maternal lineages and revealed distinct clustering patterns between ancestral and modern cultivars, with first-generation hybrids demonstrating significant plastomic uniformity. Notably, three accessions (‘Bailahe1’, ‘JingG6’, ‘Jing40’) showed ambiguous phylogenetic placement, warranting further investigation. Our findings provide critical plastomic resources for cultivar authentication, evolutionary studies, and the strategic expansion of genetic diversity in Macadamia breeding programs, particularly valuable given the crop’s recent domestication history and narrow genetic base.

Self-fertility is a commercially valuable trait in crop species, enabling fruit set without reliance on pollinators or external pollinisers. In macadamia, most cultivars are self-infertile, though some can produce nuts with self-pollen. The mechanisms underlying this variation remain unclear. This study investigates herkogamy, dichogamy and in vivo pollen tube growth to investigate self-fertility. The traits were measured on cultivars from self-fertile and self-infertile groups. Herkogamy was assessed by pistil length (PL), stamen length (SmL), and stigma-anther distance (SAD). Significant interactions were observed between fertility groups and cultivars (nested within groups), with PL ranging from 11.0 to 14.5 mm, SmL from 6.5 to 9.0 mm, and SAD from 6.6 to 7.9 mm. The non-significant differences in SAD, together with the presence of approach herkogamy (stigma positioned above anthers) in both the self-fertile and self-infertile groups, demonstrated that spatial separation does not explain fertility differences. Temporal separation was assessed via pollen viability and stigma receptivity across six floral stages, where all cultivars exhibited protandry (dichogamy). The overlap in male and female reproductive maturity in all cultivars confirmed that temporal differences do not account for self-fertility. Fluorescence microscopy revealed inhibited pollen tube growth within styles of incompatible pollinations, indicated stylar self-incompatibility. Pollen tube progression ranged from 0 to 73.3% pistils where pollen tube reached the lower style, with significant general, specific, and reciprocal effects. We concluded that self-fertility in macadamia is not governed by spatial or temporal reproductive differences, but instead reflects a homomorphic stylar self-incompatibility mechanism, providing a foundation for future molecular and genetic investigations.

Abstract Macadamia is cultivated globally, and its nuts contain abundant monounsaturated fatty acids. Oil bodies, storage units of lipids in plant seeds, exhibit favourable stability, emulsification, and weak gelation properties. Macadamia oil body (MOB) emulsion gels were prepared by using κ-carrageenan (κ-Car) and konjac glucomannan (KGM) of different ratios (1:0, 1:0.5, 1:1, 0.5:1, 0:1) and different composite polysaccharide contents (0, 0.1, 0.2, 0.3, 0.4 wt%) through microgel particle compaction (particulate gels) and natural gelation (filled gels), respectively. Characteristics of MOB emulsion gels were investigated by confocal laser scanning microscopy, rheology, storage stability, and oxidative stability. MOB emulsion particulate gels exhibited enhanced G′ and apparent viscosity with the incorporation of microgel particles (MGPs) or an elevated KGM proportion. However, MGPs caused a deterioration of storage stability (30/60/90 days), while KGM improved storage stability. MOB emulsion-filled gels presented lower G′ and apparent viscosity than particulate gels with equivalent composite polysaccharide content and complex ratio (except 1:0). Nevertheless, filled gels exhibited superior storage stability and antioxidant properties than particulate gels. Moreover, KGM can lead to a decline in G′, apparent viscosity, and storage stability of filled gels. Alternatively, the antioxidant properties of MOB emulsion gels could be significantly enhanced by loading resveratrol. Graphical Abstract Graphical Abstract