Boron is an essential micronutrient for plant growth as it participates in cell wall integrity. The growth and development of Acacia melanoxylon stem can be adversely affected by a lack of boron. To explore the mechanism of boron deficiency in A. melanoxylon stem, the changes in morphological attributes, physiological, endogenous hormone levels, and the cell structure and component contents were examined. In addition, the molecular mechanism of shortened internodes resulting from boron deficiency was elucidated through transcriptome analysis. The results showed that boron deficiency resulted in decreased height, shortened internodes, and reduced root length and surface area, corresponding with decreased boron content in the roots, stems, and leaves of A. melanoxylon. In shortened internodes of stems, oxidative damage, and disordered hormone homeostasis were induced, the cell wall was thickened, hemicellulose and water-soluble pectin contents decreased, while the cellulose content increased under boron deficiency. Furthermore, plenty of genes associated with cell wall metabolism and structural components, including GAUTs, CESAs, IRXs, EXPs, TBLs, and XTHs were downregulated under boron deficiency. Alterations of gene expression in hormone signaling pathways comprising IAA, GA, CTK, ET, ABA, and JA were observed under boron deficiency. TFs, homologous to HD1s, NAC10, NAC73, MYB46s, MYB58, and ERF92s were found to interact with genes related to cell wall metabolism, and the structural components were identified. We established a regulatory mechanism network of boron deficiency-induced shortened internodes in A. melanoxylon based on the above results. This research provides a theoretical basis for understanding the response mechanism of woody plants to boron deficiency.

Acacia melanoxylon is a fast-growing macrophanerophyte with strong adaptability whose leaf enables heteromorphic development. Light is one of the essential environmental factors that induces the development of the heteroblastic leaf of A. melanoxylon, but its mechanism is unclear. In this study, the seedlings of A. melanoxylon clones were treated with weak light (shading net with 40% of regular light transmittance) and normal light (control) conditions for 90 d and a follow-up observation. The results show that the seedlings’ growth and biomass accumulation were inhibited under weak light. After 60 days of treatment, phyllodes were raised under the control condition while the remaining compound was raised under weak light. The balance of root, stem, and leaf biomass changed to 15:11:74 under weak light, while it was 40:15:45 under control conditions. After comparing the anatomical structures of the compound leaves and phyllode, they were shown to have their own strategies for staying hydrated, while phyllodes were more able to control water loss and adapt to intense light. The compound leaves exhibited elevated levels of K, Cu, Ca, and Mg, increased antioxidant enzyme activity and proline content, and higher concentrations of chlorophyll a, carotenoids, ABA, CTK, and GA. However, they displayed a relatively limited photosynthetic capacity. Phyllodes exhibited higher levels of Fe, cellulose, lignin, IAA content, and high photosynthetic capacity with a higher maximum net photosynthetic rate, light compensation point, dark respiration rate, and water use efficiency. The comparative analysis of compound leaves and phyllodes provides a basis for understanding the diverse survival strategies that heteroblastic plants employ to adapt to environmental changes.

Aquilaria crassna Pierre ex Lecomte is a principal species renowned for its production of agarwood. However, the active components of agarwood are not universally in compliance with the standards set by the Chinese Pharmacopoeia. We have identified an elite A. crassna tree with agarotetrol and alcohol extract levels that exceed these standards and have successfully established a stable in vitro micropropagation system using stem segments from this elite tree. The effects of auxins and minerals on axillary-bud induction, shoot multiplication, and rooting were investigated. The most effective medium for axillary-bud induction was a half-strength Murashige and Skoog (1/2MS) medium supplemented with 0.50 mg/L 6-benzylaminopurine (6-BA), achieving an induction rate of 53.33% with minimal hyperhydricity. The optimal shoot proliferation medium was an MS medium with 0.40 mg/L 6-BA, yielding a propagation coefficient of 2.96 without hyperhydricity. The best rooting medium comprised quarter-strength MS (1/4MS) macroelements and 1/2MS microelements with 0.10 mg/L naphthaleneacetic acid (NAA), resulting in an 82.54% rooting rate. Substrate effects on transplant survival and growth were also evaluated, and peat soil was identified as the best substrate, achieving a survival rate of 96.67%. This study introduces a straightforward and efficient in vitro micropropagation system utilizing mature A. crassna as explants. It holds significant importance for the consistent production of agarwood that complies with the standards of the Chinese Pharmacopoeia and provides a model for the targeted breeding of medicinal plants.

Acacia melanoxylon is highly valued for its commercial applications, with the heartwood exhibiting a range of colors from dark to light among its various clones. The underlying mechanisms contributing to this color variation, however, have not been fully elucidated. In an effort to understand the factors that influence the development of dark heartwood, a comparative analysis was conducted on the microstructure, substance composition, differential gene expression, and metabolite profiles in the sapwood (SW), transition zone (TZ), and heartwood (HW) of two distinct clones, SR14 and SR25. A microscopic examination revealed that heartwood color variations are associated with an increased substance content within the ray parenchyma cells. A substance analysis indicated that the levels of starches, sugars, and lignin were more abundant in SP compared to HW, while the concentrations of phenols, flavonoids, and terpenoids were found to be higher in HW than in SP. Notably, the dark heartwood of the SR25 clone exhibited greater quantities of phenols and flavonoids compared to the SR14 clone, suggesting that these compounds are pivotal to the color distinction of the heartwood. An integrated analysis of transcriptome and metabolomics data uncovered a significant accumulation of sinapyl alcohol, sinapoyl aldehyde, hesperetin, 2′, 3, 4, 4′, 6′-peptahydroxychalcone 4′-O-glucoside, homoeriodictyol, and (2S)-liquiritigenin in the heartwood of SR25, which correlates with the up-regulated expression of CCRs (evm.TU.Chr3.1751, evm.TU.Chr4.654_667, evm.TU.Chr4.675, evm.TU.Chr4.699, and evm.TU.Chr4.704), COMTs (evm.TU.Chr13.3082, evm.TU.Chr13.3086, and evm.TU.Chr7.1411), CADs (evm.TU.Chr10.2175, evm.TU.Chr1.3453, and evm.TU.Chr8.1600), and HCTs (evm.TU.Chr4.1122, evm.TU.Chr4.1123, evm.TU.Chr8.1758, and evm.TU.Chr9.2960) in the TZ of A. melanoxylon. Furthermore, a marked differential expression of transcription factors (TFs), including MYBs, AP2/ERFs, bHLHs, bZIPs, C2H2s, and WRKYs, were observed to be closely linked to the phenols and flavonoids metabolites, highlighting the potential role of multiple TFs in regulating the biosynthesis of these metabolites and, consequently, influencing the color variation in the heartwood. This study facilitates molecular breeding for the accumulation of metabolites influencing the heartwood color in A. melanoxylon, and offers new insights into the molecular mechanisms underlying heartwood formation in woody plants.

The research of functional genes in Acacia melanoxylon, a precious and fast-growing timber species with wide adaptability, has been greatly limited due to the absence of reliable and suitable reference genes. To fill this gap, five different algorithms (comparative ΔCt, NormFinder, geNorm, BestKeeper, and RankAggreg) were employed to assess the expression stability of ten candidate genes under nine different experimental sets and their three combined groups. The results showed that PP2a and RPL4 maintained stable expression in all 144 samples and a group of different tissues or organs. PAT10 and TIP41 were the best-performing genes in different clonal varieties, pinnate compound leaves at different growth states, salt, and indole acetic acid sets. PP2a and PAT10 were the top two choices for gibberellin and abiotic stress groups. PP2a and UBI11 exhibited stable expression in drought treatment. UBI3 combined with OTUD6B, RPL4, or PP2a were identified as the optimal reference genes in the heat, ethephon, or exogenous hormone groups, respectively. The reliability of the selected reference genes was further confirmed by evaluating the expression patterns of AmWRKY6 and AmWRKY33 genes. This study provides the first comprehensive evaluation of reference gene stability in A. melanoxylon and promotes future research on the gene expression analysis of the species.

This study evaluated the effects of red LED (RL), blue LED (BL), and white LED (WL) on the growth, physiological responses, and hormonal regulation of Aquilaria crassna tissue-cultured plantlets. Morphological assessment revealed that both RL and BL treatments reduced growth variation, with RL significantly promoting shoot elongation and secondary root development. Compared to WL, RL also enhanced the rooting rate and aboveground biomass. Analysis of hormones and physiological indicators indicated that RL and BL treatments decreased abscisic acid (ABA), cytokinin (CTK), and malondialdehyde (MDA) contents, while increasing indole-3-acetic acid (IAA), gibberellic acid (GA), soluble sugar levels, and superoxide dismutase (SOD) and catalase (CAT) activities, thereby altering hormone balance and antioxidant system stability. Correlation analysis revealed that light quality was significantly negatively correlated with ABA content, while root development was closely associated with hormonal balance and antioxidant capacity. A comprehensive evaluation using the entropy-weighted TOPSIS method ranked RL as the most favorable light condition for overall growth and development, with a closeness coefficient of 0.71. These findings provide a scientific basis for optimizing light quality management to improve the efficiency and quality of A. crassna tissue culture systems.