The Sw-5b NLR nucleotide-binding domain plays a role in oligomerization, and its self-association is important for activation of cell death signaling.

Plant and animal intracellular nucleotide-binding and leucine-rich repeat (NLR) receptors play important roles in sensing pathogens and activating defense signaling. However, the molecular mechanisms underlying the activation of host defense signaling by NLR proteins remain largely unknown. Many studies have determined that the coil-coil (CC) or Toll and interleukin-1 receptor/resistance protein (TIR) domain of NLR proteins and their dimerization/oligomerization are critical for activating downstream defense signaling. In this study, we demonstrated that, in tomato, the nucleotide-binding (NB) domain Sw-5b NLR alone can activate downstream defense signaling, leading to elicitor-independent cell death. Sw-5b NB domains can self-associate, and this self-association is crucial for activating cell death signaling. The self-association was strongly compromised after the introduction of a K568R mutation into the P-loop of the NB domain. Consequently, the NBK568R mutant induced cell death very weakly. The NBCΔ20 mutant lacking the C-terminal 20 amino acids can self-associate but cannot activate cell death signaling. The NBCΔ20 mutant also interfered with wild-type NB domain self-association, leading to compromised cell death induction. By contrast, the NBK568R mutant did not interfere with wild-type NB domain self-association and its ability to induce cell death. Structural modeling of Sw-5b suggests that NB domains associate with one another and likely participate in oligomerization. As Sw-5b-triggered cell death is dependent on helper NLR proteins, we propose that the Sw-5b NB domain acts as a nucleation point for the assembly of an oligomeric resistosome, probably by recruiting downstream helper partners, to trigger defense signaling.

Diversity and community structure of ectomycorrhizal fungi in Pinus thunbergii coastal forests bordering the Yellow Sea of China.

Ectomycorrhizas play a fundamental role in the function of forest ecosystems, being essential for plant nutrition absorption and soil quality. Many afforestation and reforestation programmes have begun to recover and maintain coastal forests in China, using pine species including Pinus thunbergii. We investigated the ectomycorrhizal colonization status of P. thunbergii in coastal pine forests of the Yellow Sea of China. We identified a total of 53 ectomycorrhizal fungal species in 74 soil samples collected from three sites and found that Thelephoraceae (10 spp.) and Russulaceae (8 spp.) were the most species-rich ectomycorrhizal fungal lineages. Russula sp. 1 was the most abundant species, accounting for 15.3% of the total ectomycorrhizal tips identified. Most of the remaining species were rare. At this small scale, host identity had no significant effect on the ectomycorrhizal fungal community composition (A = 0.036, P = 0.258), but sampling sites did (A = 0.135, P = 0.041). In addition, Na+ and K+ content and soil pH had significant effects on the ectomycorrhizal fungal community. The ectomycorrhizal fungal community associated with different host plants will become an important new direction for research, as ectomycorrhiza may have the potential to improve host capacity to establish in salt-stressed environments. This will provide a theoretical basis and technical support for saline soil reforestation and rehabilitation using pine species with compatible, native ectomycorrhizal fungi in Yellow Sea coastal areas.

Transcriptome analyses revealed molecular responses of Cynanchum auriculatum leaves to saline stress.

Cynanchum auriculatum is a traditional herbal medicine in China and can grow in saline soils. However, little is known in relation to the underlying molecular mechanisms. In the present study, C. auriculatum seedlings were exposed to 3.75‰ and 7.5‰ salinity. Next, transcriptome profiles of leaves were compared. Transcriptome sequencing showed 35,593 and 58,046 differentially expressed genes (DEGs) in treatments with 3.75‰ and 7.5‰, compared with the control, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of these DEGs enriched various defense-related biological pathways, including ROS scavenging, ion transportation, lipid metabolism and plant hormone signaling. Further analyses suggested that C. auriculatum up-regulated Na+/H+ exchanger and V-type proton ATPase to avoid accumulation of Na+. The flavonoid and phenylpropanoids biosynthesis pathways were activated, which might increase antioxidant capacity in response to saline stress. The auxin and ethylene signaling pathways were upregulated in response to saline treatments, both of which are important plant hormones. Overall, these results raised new insights to further investigate molecular mechanisms underlying resistance of C. auriculatum to saline stress.

Soil spore bank communities of ectomycorrhizal fungi in endangered Chinese Douglas-fir forests.

Chinese Douglas-fir (Pseudotsuga sinensis) is an endangered Pinaceae species found in several isolated regions of China. Although soil spore banks of ectomycorrhizal (ECM) fungi can play an important role in seedling establishment after disturbance, such as in the well-known North American relative (Pseudotsuga menziesii), we have no information about soil spore bank communities in relict forests of Chinese Douglas-fir. We conducted bioassays of 73 soil samples collected from three Chinese Douglas-fir forests, using North American Douglas-fir as bait seedlings, and identified 19 species of ECM fungi. The observed spore bank communities were significantly different from those found in ECM fungi on the roots of resident trees at the same sites (p = 0.02). The levels of potassium (K), nitrogen (N), organic matter, and the pH of soil were the dominant factors shaping spore bank community structure. A new Rhizopogon species was the most dominant species in the spore banks. Specifically, at a site on Sanqing Mountain, 22 of the 57 surviving bioassay seedlings (representing 21 of the 23 soil samples) were colonized by this species. ECM fungal richness significantly affected the growth of bioassay seedlings (R 2 = 0.20, p = 0.007). Growth was significantly improved in seedlings colonized by Rhizopogon or Meliniomyces species compared with uncolonized seedlings. Considering its specificity to Chinese Douglas-fir, predominance in the soil spore banks, and positive effect on host growth, this new Rhizopogon species could play critical roles in seedling establishment and forest regeneration of endangered Chinese Douglas-fir.