Macadamia nut plantings in China are expanding year by year. In order to breed and promote superior varieties, this study analyzed the effects of different rootstocks and scions on the survival rate of grafted seedlings, and then selected the best substrate composition for plant growth. The results showed that the survival rate of the HAES788 variety as rootstock and Guire No. 1 as scion was the highest, reaching 96%. The optimal grafting time in December was better than that in March. Furthermore, among 16 substrate formulations, T12, T13, T15, and T16 had advantages of agglomerated soil and more well-developed root systems compared to the CK made of loess. The plant height, stem diameter, leaf length, leaf width, and dry weight of the aboveground and underground parts of the grafted seedlings planted in these substrate formulations were significantly higher than those plants planted in the CK. In addition, the substrate formulations T12, T13, T15, and T16 significantly improved the organic matter, total nitrogen, and total potassium content of the substrate soils, but little improvement was observed for total phosphorus content after 13 months. Overall, macadamia grafting times are best in December, with HAES788 and Guire No. 1 being the best rootstock and scion. The optimal substrate formulations are T12, T13, T15, and T16. This study provides a solid foundation for the production of high-quality macadamia plants.

Macadamia nuts are one of the most important economic food items in the world. Pericarp thickness and flavonoid composition are the key quality traits of Macadamia nuts, but the underlying mechanism of pericarp formation is still unknown. In this study, three varieties with significantly different pericarp thicknesses, namely, A38, Guire No.1, and HAES 900, at the same stage of maturity, were used for transcriptome analysis, and the results showed that there were significant differences in their gene expression profile. A total of 3837 new genes were discovered, of which 1532 were functionally annotated. The GO, COG, and KEGG analysis showed that the main categories in which there were significant differences were flavonoid biosynthesis, phenylpropanoid biosynthesis, and the cutin, suberine, and wax biosynthesis pathways. Furthermore, 63 MiMYB transcription factors were identified, and 56 R2R3-MYB transcription factors were clustered into different subgroups compared with those in Arabidopsis R2R3-MYB. Among them, the S4, S6, and S7 subgroups were involved in flavonoid biosynthesis and pericarp formation. A total of 14 MiMYBs’ gene expression were verified by RT-qPCR analysis. These results provide fundamental knowledge of the pericarp formation regulatory mechanism in macadamia nuts.

Macadamia integrifolia is a vital cash crop. The shells of its nuts serve multiple purposes in both agricultural practices and waste management initiatives. In this research, transcriptome analysis was carried out on three macadamia nut varieties with significantly different shell thicknesses, namely ‘A38’, ‘Guire No.1’ (‘GR1’), and HAES842 (‘842’), at the same stage of maturity. The results revealed remarkable differences in their gene expression profiles. A total of 4311 novel genes were identified, among which 1631 were functionally annotated. Analyses using Gene Ontology (GO), Clusters of Orthologous Groups (COGs), and the Kyoto Encyclopedia of Genes and Genomes (KEGG) indicated that the main categories of differentially expressed genes (DEGs) were associated with plant–pathogen interactions. Additionally, 10 members of the heat shock protein 90 (MiHSP90) family were identified and classified into subgroups A, B, and C by comparing them with the HSP90 gene family members of Arabidopsis and rice. Among these, the MiHSP90.1, MiHSP90.2, and MiHSP90.9 proteins were differentially highly expressed in the three macadamia nut varieties. These findings provide fundamental insights into the regulatory mechanisms underlying shell formation in macadamia nuts.

Macadamia nut plantings in China are expanding year by year. In order to further promote the superior varieties, this study analyzed the effects of different varieties of rootstock and scion on the survival rate as well as the effects of 18 different ratios of substrate composition on seedling growth were analyzed. The results showed that: The survival rate of HAES788 varieties as rootstock and Guire No.1 as scion was the highest, reaching 96%. The optimal grafting time in December was better than that in March. Substrate formulations 12, 13, 15 and 16 had agglomerated soil and well-developed root systems compared to the controls. Plant height, stem diameter, leaf length, leaf width, and dry weight of aboveground and underground parts were significantly higher than those of the control. In addition, the substrate formulations 12, 13, 15 and 16 significantly improved the organic matter, total nitrogen, and total potassium content of the substrate soils, but little on total phosphorus content. All in all, macadamia grafting times are best in December, with HAES788 and Guire No.1 being the best rootstock and scion. Substrate formulations 12, 13, 15 and 16 are the optimal formula. This study provides a solid foundation to produce high-quality macadamia seedlings.

BHLH transcription factors are central to plant development and to the metabolism of fatty acids and flavonoids. Through the transcriptomic, metabolomic, and phytohormone/elemental profiling of two macadamia varieties with divergent fatty acid content, ‘Guire No. 1’ (GR1) and ‘A4’, we identified 142 MibHLH transcription factors, classified into 21 subfamilies, of which 14 show collinearity with Arabidopsis AtbHLH members. MibHLH46 was markedly differentially expressed, and its expression strongly correlated with that of fatty acid biosynthesis genes. Promoter analysis further revealed abundant MYC and G-box elements in fatty acid and flavonoid biosynthetic genes. Comparative metabolomics showed that ‘GR1’ accumulates higher levels of several mineral elements (Fe, Ca, Mg) and abscisic acid (ABA) than ‘A4’. These transcriptomic trends were confirmed by qRT-PCR, which validated the up-regulation of genes related to fatty acid oxidation, α-linolenic acid metabolism, and the flavonoid pathway in ‘GR1’. We propose that MibHLH46, as a candidate regulator, contributes to kernel fatty acid and flavonoid metabolism by transcriptionally regulating key biosynthetic genes, providing a basis for identifying regulatory genes in macadamia lipid metabolism.