Chromosome-scale genome assembly and insights into the metabolome and gene regulation of leaf color transition in an important oak species, Quercus dentata

Quercus dentata Thunb., a dominant forest tree species in northern China, has significant ecological and ornamental value due to its adaptability and beautiful autumn coloration, with color changes from green to yellow into red resulting from the autumnal shifts in leaf pigmentation. However, the key genes and molecular regulatory mechanisms for leaf color transition remain to be investigated. First, we presented a high-quality chromosome-scale assembly for Q. dentata. This 893.54 Mb sized genome (contig N50 = 4.21 Mb, scaffold N50 = 75.55 Mb; 2n = 24) harbors 31 584 protein-coding genes. Second, our metabolome analyses uncovered pelargonidin-3-O-glucoside, cyanidin-3-O-arabinoside, and cyanidin-3-O-glucoside as the main pigments involved in leaf color transition. Third, gene co-expression further identified the MYB-bHLH-WD40 (MBW) transcription activation complex as central to anthocyanin biosynthesis regulation. Notably, transcription factor (TF) QdNAC (QD08G038820) was highly co-expressed with this MBW complex and may regulate anthocyanin accumulation and chlorophyll degradation during leaf senescence through direct interaction with another TF, QdMYB (QD01G020890), as revealed by our further protein-protein and DNA-protein interaction assays. Our high-quality genome assembly, metabolome, and transcriptome resources further enrich Quercus genomics and will facilitate upcoming exploration of ornamental values and environmental adaptability in this important genus.

[Ectomycorrhizal fungi enhance salt tolerance of Quercus mongolica by regulating ion balance]

We explored the effects of salt stress on the growth of Quercus mongolica and the effects of ectomycorrhizal fungi (ECMF) on the ion balance of Q. mongolica. After inoculating four kinds of ECMFs (Gomphidius visci-dus, Suillus leteus, Suillus grevillea, Boletus edulis) on Q. mongolica seedlings, we treated the annual non-mycorrhizated and mycorrhizated seedlings with NaCl stress (0, 100, 200, 300 mmol·L^-1) for 36 days, and then analyzed the mycorrhizal characteristics, growth, leaf injury symptoms, leaf electrolyte permeability, water content, and ion contents in roots, stems, and leaves. The results showed that the four ECMFs could establish a symbiotic system with Q. mongolica, and that root system of mycorrhizal seedlings was stronger than that of non-mycorrhizal seedlings. Under salt stress, the growth of Q. mongolica seedlings was inhibited, with the symptoms of scorched leaves. The damage to leaf plasma membrane and the degree of water loss were aggravated with the increases of salt stress. Under low salt stress (100 mmol·L^-1), Q. mongolica preferentially accumulated Na⁺ in roots and stems. Under medium-high salt stress (200-300 mmol·L^-1), roots became the primary organ for accumulating Na⁺. ECMF regulated ion balance in plant by increasing the Na⁺ level in roots and reducing the Na⁺ accumulation in stems and leaves, enhancing the absorption of K⁺ and Ca²⁺ to increase the K⁺/Na⁺ and Ca²⁺/Na⁺. The four ECMFs had different mitigation effects on salt poisoning of Q. mongolica. G. viscidus had the strongest effect, followed by S. leteus, while S. grevillei and B. edulis had relatively little effect.

Transcriptome analysis identifies key gene LiMYB305 involved in monoterpene biosynthesis in Lilium 'Siberia'

Lilium is a popular cut flower that is highly favored by consumers due to its snowy white color and strong fragrance, which originates from the release of monoterpenes. However, the underlying molecular mechanism of monoterpene synthesis remains poorly understood. In this study, the content of three main monoterpenes (linalool, ocimene, and myrcene) was examined in Lilium 'Siberia', and RNA sequencing of the 11 stages of flower development was conducted. The biosynthesis of the three monoterpenes increased with flower development, reaching their peak levels at the full flowering stage. Transcriptome data revealed 257,140 unigenes, with an average size of 794 bp, from which 43,934 differentially expressed genes were identified and enriched in the KEGG pathways partly involved in plant hormone signal transduction and monoterpenoid biosynthesis. Furthermore, the essential factor LiMYB305 was identified by WGCNA after the release of the flower fragrance. The transient silencing of LiMYB305 in petals using VIGS technology showed that the mRNA expression levels of LiLiS, LiOcS, and LiMyS were significantly downregulated and that the release of linalool, ocimene, and myrcene had decreased significantly. Y1H, LUC, and EMSA experiments revealed that LiMYB305 directly bound and activated the LiOcS promoter to increase the synthesis of monoterpenes. Taken together, these results provide insight into the molecular mechanism of monoterpene synthesis and provide valuable information to investigate the formation of the flower fragrance in Lilium.

Lilac (Syringa oblata) genome provides insights into its evolution and molecular mechanism of petal color change

Color change during flower opening is common; however, little is understood on the biochemical and molecular basis related. Lilac (Syringa oblata), a well-known woody ornamental plant with obvious petal color changes, is an ideal model. Here, we presented chromosome-scale genome assembly for lilac, resolved the flavonoids metabolism, and identified key genes and potential regulatory networks related to petal color change. The genome assembly is 1.05 Gb anchored onto 23 chromosomes, with a BUSCO score of 96.6%. Whole-genome duplication (WGD) event shared within Oleaceae was revealed. Metabolome quantification identified delphinidin-3-O-rutinoside (Dp3Ru) and cyanidin-3-O-rutinoside (Cy3Ru) as the major pigments; gene co-expression networks indicated WRKY an essential regulation factor at the early flowering stage, ERF more important in the color transition period (from violet to light nearly white), while the MBW complex participated in the entire process. Our results provide a foundation for functional study and molecular breeding in lilac.

A high-quality genome assembly for the lilac (Syringa oblata) provides insight into the evolution of this shrub and potential mechanisms underlying petal color change.

Ectomycorrhizal Inoculation Enhances the Salt Tolerance of Quercus mongolica Seedlings

Salt stress harms the growth and development of plants, and the degree of soil salinization in North China is becoming increasingly severe. Ectomycorrhiza (ECM) is a symbiotic system formed by fungi and plants that can improve the growth and salt tolerance of plants. No studies to date have examined the salt tolerance of Quercus mongolica, a typical ectomycorrhizal tree species of temperate forests in the northern hemisphere. Here, we inoculated Q. mongolica with two ectomycorrhizal fungi (Gomphidius viscidus; Suillus luteus) under NaCl stress to characterize the effects of ECM. The results showed that the symbiotic relationship of Q. mongolica with G. viscidus was more stable than that with S. luteus. The cross-sectional area of roots increased after inoculation with the two types of ectomycorrhizal fungi. Compared with the control group, plant height, soluble sugar content, and soluble protein content of leaves were 1.62, 2.41, and 2.04 times higher in the G. viscidus group, respectively. Chlorophyll (Chl) content, stomatal conductance (Gs), and intracellular CO2 concentration (Ci) were significantly higher in Q. mongolica inoculated with ectomycorrhizal fungi than in the control, but differences in the net photosynthetic rate (Pn), transpiration rate (Tr), and photosystem II maximum photochemical efficiency (Fv/Fm) were lower. The relative conductivity of Q. mongolica inoculated with the two ectomycorrhizal fungi was consistently lower than that of non-mycorrhizal seedlings, with the effect of G. viscidus more pronounced than that of S. luteus. The malondialdehyde (MDA) content showed a similar pattern. Peroxidase (POD) and catylase (CAT) levels in mycorrhizal seedlings were generally higher than those of non-mycorrhizal seedlings under normal conditions, and were significantly higher than those of non-mycorrhizal seedlings on the 36th and 48th day after salt treatment, respectively. Overall, the results indicated that the salt tolerance of Q. mongolica seedlings was improved by ectomycorrhizal inoculation.

First Report of Anthracnose Caused by Colletotrichum gloeosporioides on Pistachio (Pistacia vera) in China

In the 1990s, pistachio (Pistacia vera L. 'Kerman' and 'Peters') was introduced in China. They are found in many orchards in Beijing and Gansu and Hebei provinces, northern China. In 2009, a new disease was observed on leaves, stems, and fruits in pistachio orchards in Gansu Province. Disease incidence in 8- to 12-year-old orchards was 30%. Yield losses reached 25%. Symptoms began as discrete, sunken, black spots, approximately 10 mm in diameter, followed by circular lesions that eventually coalesced with tissue death recorded and orange fructifications developed on lesions. Pieces of diseased leaves, stems, and fruits were surfaced sterilized and placed on 2% potato dextrose agar (PDA) at 25°C. A fungus was consistently isolated. After 10 days, cultures on PDA showed aerial, white mycelium that turned gray to grayish black with a salmon-to-orange conidial mass at 25°C and a 12-h photoperiod. Brown, 80 to 120 μm long setae were observed in the acervulus. Conidia were hyaline, fusiform to nearly straight, and averaged 12 to 18 × 3 to 5 μm. On the basis of morphological characteristics, the fungus was identified as Colletotrichum gloeosporioides (Penz.) Sacc. (2). On PDA, 0.5 μg/ml of benomyl was applied for the sensitivity test (3). Benomyl completely inhibited the growth of the fungus. Mycelial DNA was extracted, PCR amplified using ITS1 and ITS4 primers for the ribosomal DNA internal transcribed spacers 1 and 2, and sequenced. The DNA sequence was recorded in GenBank as No. HQ631378. The DNA sequence was blasted showing 99% identity with Accession Nos. GQ144454 and GU004376, for C. gloeosporioides. Pathogenicity tests were conducted under greenhouse conditions at 25°C. Three replicates of 2-year-old 'Kerman' plants were inoculated with mycelial PDA plugs placed on 0.5-cm² stem wounds and then wrapped with Parafilm. Controls were inoculated with PDA plugs without the fungus. After 3 weeks, stem cankers were observed on inoculated plants. Control plants remained healthy. Pathogenicity was also tested on injured leaves and fruits. A 10-μl drop of a spore suspension of 10⁴ conidia/ml was applied on 'Kerman' and 'Peters' leaves and 'Kerman' fruits and placed on plates with a wet filter paper at 25°C. Small, black lesions were observed at 2 days after inoculation. At 7 days, necrotic lesions covered the entire surface. C. gloeosporioides was reisolated from necrotic lesions. Controls did not develop symptoms. C. acutatum has been reported on pistachio in Australia (1), but to our knowledge, this is the first report of anthracnose caused by C. gloeosporioides on pistachio. References: (1) G. J. Ash and V. M. Lanoiselet. Australas. Plant Pathol. 30:365, 2001. (2) J. Y. Lu. Plant Pathogenic Mycology. China Agricultural Press, Beijing, 2001. (3) N. A. R. Peres et al. Plant Dis. 86:620, 2002.