Physiological and transcriptional responses of the ectomycorrhizal fungus Cenococcum geophilum to salt stress.

Ectomycorrhizal (ECM) fungi improve the host plant's tolerance to abiotic and biotic stresses. Cenococcum geophilum (Cg) is among the most common ECM fungi worldwide and often grows in saline environments. However, the physiological and molecular mechanisms of salt tolerance in this fungus are largely unknown. In the present study, 12 isolates collected from different ecogeographic regions were used to investigate the mechanism of salt tolerance of Cg. The isolates were classified into four groups (salt-sensitive, moderately salt-tolerant, salt-tolerant, and halophilic) based on their in vitro mycelial growth under 0, 50, 125, 250, and 500 mM NaCl concentrations. Hence, the Na, Ca, P, and K concentrations of mycelia and the pH of the culture solution were determined. Compared with salt-tolerant isolates, treatment with 250 mM NaCl significantly increased the sodium concentration and decreased the potassium concentration of salt-sensitive isolates. RNA-sequencing and qRT-PCR analysis were conducted to identify differentially expressed genes (DEGs) involved in transmembrane transport and oxidoreductase activity pathways. The hydrogen peroxide concentration and activities of peroxidase and superoxide dismutase in mycelia were determined, and the accumulation and scavenging of reactive oxygen species in the salt-sensitive isolates were more active than those in the salt-tolerant isolates. The results supply functional validations to RNA-seq and qRT-PCR analysis. This study provides novel insights into the salt-stress response of Cg isolates and provides a foundation for elucidation of the salt-tolerance mechanism of ECM fungi.

Estimation of the most suitable nitrogen concentration for sporocarp formation in Laccaria japonica colonizing Pinus densiflora seedlings through in vitro mycelial culture.

Many ectomycorrhizal (ECM) fungi produce commercially valuable edible sporocarps. However, the effects of nitrogen (N) application on ECM fungal sporocarp formation remain poorly understood. In this study, we investigated the effect of application of various N concentrations (0, 5, 25, 50, 100, and 200 mg/L) on the growth of Laccaria japonica mycelia in vitro for 1 month. The results showed that L. japonica mycelial biomass was highest in the 50 mg/L treatment and was significantly inhibited at N concentrations higher than 200 mg/L. Next, we investigated the effects of N application on mycorrhizal colonization and sporocarp formation in L. japonica colonizing Pinus densiflora seedlings in pots. The seedlings were watered with nutrient solutions containing 0, 5, 25, 50, or 100 mg N/L. The biomass, photosynthetic rate, and mycorrhizal colonization rates of the seedlings were measured at 45 days (first appearance of primordia), 65 days (sporocarp appearance on the substrate surface), and 4 months after seedlings were transplanted. The numbers of primordia and sporocarps were recorded during the experimental period. Total carbon (C) and N content were determined in seedlings at 4 months after transplantation, and in L. japonica sporocarps. Both mycelial growth and sporocarp production reached their maximum at an N application concentration of 50 mg/L, suggesting that the most suitable N concentration for ECM fungal sporocarp formation can easily be estimated in vitro during mycelial growth. This finding may help determine the most suitable N conditions for increasing edible ECM fungus sporocarp production in natural forests.

A poplar B-box protein PtrBBX23 modulates the accumulation of anthocyanins and proanthocyanidins in response to high light.

Flavonoids, which modulate plant resistance to various stresses, can be induced by high light. B-box (BBX) transcription factors (TFs) play crucial roles in the transcriptional regulation of flavonoids biosynthesis, but limited information is available on the association of BBX proteins with high light. We present a detailed overview of 45 Populus trichocarpa BBX TFs. Phylogenetic relationships, gene structure, tissue-specific expression patterns and expression profiles were determined under 10 stress or phytohormone treatments to screen candidate BBX proteins associated with the flavonoid pathway. Sixteen candidate genes were identified, of which five were expressed predominantly in young leaves and roots, and BBX23 showed the most distinct response to high light. Overexpression of BBX23 in poplar activated expression of MYB TFs and structural genes in the flavonoid pathway, thereby promoting the accumulation of proanthocyanidins and anthocyanins. CRISPR/Cas9-generated knockout of BBX23 resulted in the opposite trend. Furthermore, the phenotype induced by BBX23 overexpression was enhanced under exposure to high light. BBX23 was capable of binding directly to the promoters of proanthocyanidin- and anthocyanin-specific genes, and its interaction with HY5 enhanced activation activity. We identified novel regulators of flavonoid biosynthesis in poplar, thereby enhancing our general understanding of the transcriptional regulatory mechanisms involved.

The effects of co-colonising ectomycorrhizal fungi on mycorrhizal colonisation and sporocarp formation in Laccaria japonica colonising seedlings of Pinus densiflora.

Forest trees are colonised by different species of ectomycorrhizal (ECM) fungi that interact competitively or mutualistically with one another. Most ECM fungi can produce sporocarps. To date, the effects of co-colonising fungal species on sporocarp formation in ECM fungi remain unknown. In this study, we examined host plant growth, mycorrhizal colonisation, and sporocarp formation when roots of Pinus densiflora are colonised by Laccaria japonica and three other ECM fungal species (Cenococcum geophilum, Pisolithus sp., and Suillus luteus). Sporocarp numbers were recorded throughout the experimental period. The biomass, photosynthetic rate, and mycorrhizal colonisation rate of the seedlings were also measured at 45 days, 62 days, and 1 year after seedlings were transplanted. Results indicated that C. geophilum and S. luteus may negatively impact mycorrhizal colonisation and sporocarp formation in L. japonica. Sporocarp formation in L. japonica was positively correlated with conspecific mycorrhizal colonisation but negatively correlated with the biomass of seedlings of P. densiflora. The co-occurring ECM fungi largely competed with L. japonica, resulting in various effects on mycorrhizal colonisation and sporocarp formation in L. japonica. A variety of mechanisms may be involved in the competitive interactions among the different ECM fungal species, including abilities to more rapidly colonise root tips, acquire soil nutrients, or produce antibiotics. These mechanisms need to be confirmed in further studies.

Fine mapping of a locus presumably involved in hybrid inviability (HIs-1) between flowering cherry cultivar Cerasus × yedoensis 'Somei-yoshino' and its wild relative C. spachiana.

Flowering cherry is an extremely renowned ornamental tree, consisting of a variety of species and cultivars. Because cherry species have no strict barriers for interspecific hybridization before fertilization, identification of the gene underlying post-zygotic hybrid inviability will help breeders identify specimens for breeding and help us understand speciation mechanisms. In this study, we mapped the genetic linkages and physical genome sequences for a presumed hybrid inviability locus (HIs-1) that we observed in the seedlings crossed between Cerasus × yedoensis 'Somei-yoshino' and its wild relative C. spachiana. By the surveying linkage maps of 'Somei-yoshino' and C. spachiana, we identified correlation with seedling inviability only in linkage group 4 (LG4) of the 'Somei-yoshino' map. When we produced a finer-scaled map of HIs-1 in LG4, we found that HIs-1 is located between two microsatellite (SSR) markers with a 3.8 cM span. Using eight SSR markers based on peach genome sequences, we further refined the candidate region for HIs-1. This region was located between Pp04C001 and Pp04C007 markers, spanning 240 Kb of the peach genome, in which 45 transcribed genes had been estimated. From these candidate genes, it will be feasible to identify molecular mechanisms involved in cherry hybrid inviability.

An Improved and Simplified Propagation System for Pollen-Free Sugi (Cryptomeria japonica) via Somatic Embryogenesis.

Sugi (Japanese cedar, Cryptomeria japonica) is the most important forestry tree species in Japan, covering 44% of the total artificial forest area. Large amounts of pollen released from these forests each spring cause allergic reactions in approximately 40% of the population, which are a serious social and public health problem in Japan. As a countermeasure, there is an urgent need to reforest using male-sterile plants (MSPs; pollen-free plants); however, the production of MSPs via conventional methods is inefficient, time consuming, and requires considerable resources in terms of labor and space. In the present paper, we described an improved and simplified methodology for the efficient propagation of pollen-free Japanese cedar, combining the use of genetic markers (marker-assisted selection or marker-aided selection) for the early selection of male-sterile genotypes and the use of somatic embryogenesis (SE) for the clonal mass propagation of seedlings. We describe all the stages involved in the production process of somatic seedlings. Our results demonstrated that this methodology easily and efficiently produces MSPs with a discrimination rate of 100% in a short period of time. Production of 243.6 ± 163.6 cotyledonary embryos per plate, somatic embryo germination, and plantlet conversion frequencies of 87.1 ± 11.9% and 84.8 ± 12.6%, respectively, and a 77.6 ± 12.1% survival rate after ex vitro acclimatization was achieved. Moreover, we also describe an easy method for the collection of somatic embryos prior to germination, as well as an efficient and practical method for their storage at 5°C. Finally, a representative schedule for the propagation of pollen-free sugi somatic seedlings is presented as a reference for practical uses. This methodology will definitively help to accelerate the production of C. japonica MSPs across Japan.

Marker-Assisted Selection for Pollen-Free Somatic Plants of Sugi (Japanese Cedar, Cryptomeria japonica): A Simple and Effective Methodology for Selecting Male-Sterile Mutants With ms1-1 and ms1-2.

Pollen allergy caused by sugi (Japanese cedar, Cryptomeria japonica) is a serious problem in Japan. One of the measures against pollinosis is the use of male-sterile plants (MSPs; pollen-free plants). In this context, the development of a novel technique for the efficient production of sugi MSPs, which combines marker-assisted selection (MAS) with somatic embryogenesis (SE), was recently reported by our research group. To improve the efficiency of MSP production, in this paper we report improved MAS for male-sterile individuals from embryogenic cells, cotyledonary embryos, and somatic plants of sugi using a newly developed marker in the form of the causative mutation of MS1 itself, selecting individuals with ms1-1 and ms1-2 male-sterile mutations. We also describe simplified methods for extracting DNA from different plant materials and for MAS using LAMP diagnostics. Finally, we show that MAS can be efficiently performed using the one-step indel genotyping (ING) marker developed in this study and using InstaGene for DNA extraction. The combination of SE and 100% accurate marker selection during the embryogenic cell stage enables the mass production of MS1 male-sterile sugi seedlings.