Plant leucine-rich repeat (LRR) receptor-like kinases (RLKs) and LRR receptor-like proteins (RLPs) comprise a large family of cell surface receptors that play critical roles in signal perception and transduction. Both LRR-RLKs and LRR-RLPs rely on regulatory LRR-RLKs to initiate downstream signaling pathways. BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3 (BAK1/SERK3) and SUPPRESSOR OF BIR1-1 (SOBIR1) are important and extensively studied regulatory LRR-RLKs with distinct functions. Although the regulatory mechanism of BAK1 activation has been studied in detail, the activation mechanism of SOBIR1 remains poorly understood. Here, the crystal structures of the catalytically inactive kinase domain of SOBIR1 (SOBIR1-KD) from Arabidopsis thaliana were determined in complexes with AMP-PNP and Mg²⁺. The results show that SOBIR1-KD contains a uniquely long β3-αC loop and adopts an Src-like inactive conformation with an unusual architecture at the activation segment, which comprises three helices. Biochemical studies revealed that SOBIR1 is transphosphorylated by BAK1 following its autophosphorylation via an intermolecular mechanism, and the phosphorylation of Thr529 in the activation segment and the β3-αC loop are critical for SOBIR1 phosphorylation. Further functional analysis confirmed the importance of Thr529 and the β3-αC loop for the SOBIR1-induced cell death response in Nicotiana benthamiana. Taken together, these findings provide a structural basis for the regulatory mechanism of SOBIR1 and reveal the important elements and phosphorylation events in the special stepwise activation of SOBIR1-KD, the first such processes found in regulatory LRR-RLKs.

Varieties of Citrus are commercially important fruits that are cultivated worldwide and are valued for being highly nutritious and having an appealing flavor. Lignification of citrus fruit juice sacs is a serious physiological disorder that occurs during postharvest storage, for which the underlying transcriptional regulatory mechanisms remain unclear. In this study, we identified and isolated a candidate MYB transcription factor, CsMYB85, that is involved in the regulation of lignin biosynthesis in Citrus sinensis, which has homologs in Arabidopsis and other plants. We found that during juice sac lignification, CsMYB85 expression levels increase significantly, and therefore, suspected that this gene may control lignin biosynthesis during the lignification process. Our results indicated that CsMYB85 binds the CsMYB330 promoter, regulates its expression, and interacts with CsMYB308 in transgenic yeast and tobacco. A transient expression assay indicated that Cs4CL1 expression levels and lignin content significantly increased in fruit juice sacs overexpressing CsMYB85. At4CL1 expression levels and lignin content were also significantly increased in Arabidopsis overexpressing CsMYB85. We accordingly present convincing evidence for the participation of the CsMYB85 transcription factor in fruit juice sac lignification, and thereby provide new insights into the transcriptional regulation of this process in citrus fruits.

Human gut bacteria contribute significantly to human health and several studies have evaluated the effects of dietary fibers on human gut bacterial ecology. However, the relationship between different degrees of fiber polymerization and human gut bacteria is unknown. Here, we analyzed three fiber substrates with different degrees of polymerization, namely carboxymethylcellulose, β-glucans, and galactooligosaccharides. To probe the in vitro influence of the degree of polymerization of the fiber on human gut bacteria, we measured the pH, air pressure, and short-chain fatty acid content of fecal fermentation supplemented with these fiber substrates, and sequenced the 16S ribosomal RNA genes of the microbial community in the fiber-treated fermentations. The butyric acid concentration was shown to decline with decreasing degree of polymerization of the fiber. Illumina Miseq sequencing indicated that the degree of polymerization might have an influence on human gut microbial diversity and abundance. Principal coordinate analysis unveiled a relationship between the degree of fiber polymerization and the gut bacterial community. Specific microbiota operational taxonomic units (OTUs) within the genera Escherichia-Shigella, Fusobacterium, and Dorea were proportional to the degree of fiber significantly, whereas OTUs within the genera Bifidobacterium, Streptococcus, and Lactobacillus were inversely correlated with the degree of polymerization. Correlation analysis between the fiber degree of polymerization and gut bacteria may demonstrate the effect of fibers on gut microbiota, and subsequently, on human health.

Lung cancer is a common disease that is associated with poor prognosis. Fungal immunomodulatory protein from Nectria haematococca (FIP-nha) has potential as a lung cancer therapeutic; as such, illuminating its anti-tumor mechanism is expected to facilitate novel treatment options. Here, we showed that FIP-nha affects lung adenocarcinoma growth ex vivo and in vivo. Comparative quantitative proteomics showed that FIP-nha negatively regulates PI3K/Akt signaling and induces cell cycle arrest, autophagy, and apoptosis. We further demonstrated that FIP-nha suppresses Akt phosphorylation, leading to upregulation of p21 and p27 and downregulation of cyclin B1, cyclin D1, CDK2, and CDK4 expression, ultimately resulting in G1/S and G2/M cell cycle arrest. Meanwhile, FIP-nha-induced PI3K/Akt downregulation promotes A549 apoptosis by increasing the expression ratio of Bax/Bcl-2 and c-PARP and autophagy by decreasing the phosphorylation of mTOR. Thus, we comprehensively revealed the anti-tumor mechanism of FIP-nha, which inhibits tumor growth by modulating PI3K/Akt-regulated cell cycle arrest, autophagy, and apoptosis, and provided the basis for further application of fungal immunomodulatory proteins, especially FIP-nha.