Flower bud formation in ‘Fuji’ apple (Malus domestica Borkh.) is difficult, which severely constrains commercial production. Spermidine (Spd) plays an important role in floral induction, but the mechanism of its action is incompletely understood. To investigate the effect of Spd on flowering, 6-year-old ‘Fuji’ apple trees were treated with 1 × 10⁻⁵ mol L⁻¹ Spd to study the responses of polyamines [putrescine (Put), Spd and spermine (Spm)], hormones [gibberellins (GA₃) and abscisic acid (ABA)], and polyamine-, hormone- and flowering-related genes. Spd application promoted flowering during floral induction by increasing MdGA2ox2 (gibberellin 2-oxidase) through GA₃ reduction and increasing MdNCED1 and MdNCED3 (9-cis-epoxycarotenoid dioxygenase) through ABA enrichment during 60 to 80 days after full bloom. The flowering rate as well as the expressions of flower-related genes, except for MdLEY (LEAFY), also increased, thereby promoting flowering. In addition, spraying with Spd significantly increased the contents of endogenous polyamines except for Spm in terminal buds by increasing the expressions of polyamine-associated genes. We hypothesize that the contribution of Spd to flowering is related to crosstalk among polyamines, hormone signals, and related gene expressions, which suggests that Spd participates in the apple floral induction process.

Malus hupehensis belongs to the Malus genus (Rosaceae) and is an indigenous wild crabapple of China. This species has received more and more attention, due to its important medicinal, and excellent ornamental and economical, values. In this study, the whole chloroplast (cp) genome of Malus hupehensis, using a Hiseq X Ten sequencing platform, is reported. The M. hupehensis cp genome is 160,065 bp in size, containing a large single copy region (LSC) of 88,166 bp and a small single copy region (SSC) of 19,193 bp, separated by a pair of inverted repeats (IRs) of 26,353 bp. It contains 112 genes, including 78 protein-coding genes (PCGs), 30 transfer RNA genes (tRNAs), and four ribosomal RNA genes (rRNAs). The overall nucleotide composition is 36.6% CG. A total of 96 simple sequence repeats (SSRs) were identified, most of them were found to be mononucleotide repeats composed of A/T. In addition, a total of 49 long repeats were identified, including 24 forward repeats, 21 palindromic repeats, and four reverse repeats. Comparisons of the IR boundaries of nine Malus complete chloroplast genomes presented slight variations at IR/SC boundaries regions. A phylogenetic analysis, based on 26 chloroplast genomes using the maximum likelihood (ML) method, indicates that M. hupehensis clustered closer ties with M. baccata, M. micromalus, and M. prunifolia than with M. tschonoskii. The availability of the complete chloroplast genome using genomics methods is reported here and provides reliable genetic information for future exploration on the taxonomy and phylogenetic evolution of the Malus and related species.

Arbuscular mycorrhizal fungi (AMF) and plants form a symbiotic relationship that promotes plant growth and development. However, the regulatory mechanisms through which AMF promote plant growth and development are largely unexplored. In this study, the apple rootstock M26 was assessed physiologically, transcriptionally and metabolically when grown with and without AMF inoculation. AMF significantly promoted the number of lateral root (LR) increase and shoot elongation. Root transcriptomic and metabolic data showed that AMF promoted lateral root development mainly by affecting glucose metabolism, fatty acid metabolism, and hormone metabolism. Shoot transcriptomic and metabolic data showed that AMF promoted shoot elongation mainly by affecting hormone metabolism and the expression of genes associated with cell morphogenesis. To investigate whether shoot elongation is caused by root development, we analyzed the root/shoot dry weight ratio. There was a correlation between shoot growth and root development, but analysis of root and shoot metabolites showed that the regulation of AMF on plant shoot metabolites is independent of root growth. Our study bridged the gap in the field of growth and development related to AMF.

The roots of most land plants are in symbiosis with arbuscular mycorrhizal (AM) fungi. The fungus promotes nutrient uptake from the soil while receiving plant photosynthate as lipids and sugars. Nutrient exchange must be regulated by both partners, but the mechanisms underlying the regulation of lipid supplement from the plant to the AM fungus remain elusive. Here, we conducted a molecular study on the role of increased abscisic acid (ABA) levels during AM fungus infection in the roots of apple (Malus spp.). AM fungus induced the expression of two ABA synthesis genes, MdNCED3.1 and 3.2, in apple roots and increased the ABA content, which promoted the growth of the AM fungus. The effect of ABA on symbiosis was confirmed in transgenic apple roots either overexpressing or silencing MdNCED3.1 or MdNCED3.2. Transcriptome analysis and transgenic manipulation revealed that the transcription factor MdABF2 played a key role in the ABA-mediated formation of symbiosis during AM infection and that MdABF2 could regulate the expression levels of genes related to fatty acid (FA) synthesis (e.g., MdKASIII) and translocation (such as MdSTR2) in apple roots. Activation of these genes boosted the levels of available fatty acids in the roots and increased the AM fungal colonization and arbuscule development in the roots. These results reveal a molecular pathway in which ABA signaling positively regulates fatty acid synthesis and transport, thereby enhancing lipid supply to AM fungi and promoting AM symbiosis.

Prunus davidiana is a Chinese wild peach species belonging to the family Rosaceae. In this study, we first assembled the complete chloroplast (cp) genome of P. davidiana by Illumina paired-end reads data, then carried out phylogenetic analysis. The P. davidiana cp genome was 158,055 bp, containing a large single copy region (LSC) of 86,248 bp and a small single copy region (SSC) of 19,047 bp separated by a pair of inverted repeats (IRs) of 26,380 bp. The cp genome contained 131 genes, including 86 protein-coding genes, 37 transfer RNA (tRNA) genes and 8 ribosomal RNA (rRNA) genes. The result of the phylogenetic analysis showed that P. davidiana is closely related to P. mongolica.