HISTONE DEACETYLASE 9 transduces heat signal in plant cells.

Heat stress limits plant growth, development, and crop yield, but how plant cells precisely sense and transduce heat stress signals remains elusive. Here, we identified a conserved heat stress response mechanism to elucidate how heat stress signal is transmitted from the cytoplasm into the nucleus for epigenetic modifiers. We demonstrate that HISTONE DEACETYLASE 9 (HDA9) transduces heat signals from the cytoplasm to the nucleus to play a positive regulatory role in heat responses in Arabidopsis. Heat specifically induces HDA9 accumulation in the nucleus. Under heat stress, the phosphatase PP2AB'ß directly interacts with and dephosphorylates HDA9 to protect HDA9 from 26S proteasome-mediated degradation, leading to the translocation of nonphosphorylated HDA9 to the nucleus. This heat-induced enrichment of HDA9 in the nucleus depends on the nucleoporin HOS1. In the nucleus, HDA9 binds and deacetylates the target genes related to signaling transduction and plant development to repress gene expression in a transcription factor YIN YANG 1-dependent and -independent manner, resulting in rebalance of plant development and heat response. Therefore, we uncover an HDA9-mediated positive regulatory module in the heat shock signal transduction pathway. More important, this cytoplasm-to-nucleus translocation of HDA9 in response to heat stress is conserved in wheat and rice, which confers the mechanism significant implication potential for crop breeding to cope with global climate warming.

First Report of Leaf spot Caused by Xylaria arbuscula on Flue-cured Tobacco in China.

Flue-cured tobacco (Nicotiana tabacum L.) is a leafy, annual, solanaceous plant grown commercially for its leaves in China. Around 70% of tobacco production in China occurs in southwest China. In summer of 2019, leaf spot symptoms were observed on ten to twenty percent of tobacco plants in a 2 ha commercial field of Bijie (27.32° N, 105.29° E), Guizhou province, China. The leaf spots were white with dark-brown in edges, irregularly round and oval, and diseased tissue dropped out leaving the leaves ragged in appearance (Fig. 1A, 1B). One diseased leaf from each of five plants was sampled. From five leaves, a total of 15 small (5 mm × 5 mm) pieces of leaf tissue were cut from the edge of the lesions after surface sterilization and placed on potato dextrose agar (PDA) medium. Five fungal colonies that were similar in appearance were isolated and one was purified, BEZ22, was selected arbitrarily for identification. Mycelia of the pathogen was initally white and dense, and then black carbonized mycelia appeared from the center of the colony 7 days' after incubation. Mycelia was white, sparse and radiated when incubated on OA (oatmeal agar) (Fig. 1E, 1F, 1G, 1H). Genomic DNA of the isolate was extracted. The internal transcribed spacers (ITS) with primers ITS1/ITS4 (White et al. 1990), actin (ACT) gene with primers ACT-512F/ACT-738R (Hsieh et al. 2005), beta-tubulin (TUB2) with primers T1/T22 (O'Donnell & Cigelnik 1997) and RNA polymerase II second largest subunit gene (RPB2) with primers fRPB2-5F/ fRPB2-7cR (Liu et al. 1999) were amplified and sequenced, respectively. The generated sequences were deposited in GenBank with accession numbers MT804353 (ITS), MT809582 (ACT), MT799790 (TUB2) and MT799789 (RPB2). Using BLASTN searches, the sequences of each gene above were aligned with the voucher specimum, Xylaria arbuscula 89041211. The number of nucleotides that were similar for ITS (GU300090) was 550/551 (99%); for ACT (GQ421286), 266/266 bp (100%); for TUB2 (GQ478226), 1501/1501 bp (100%); and for RPB2 (GQ844805), 1135/1135 bp (100%), respectively (Fig. 2). A phylogenetic tree was constructed based on these four sequences with a final alignment of 3456 characters (ITS 551, ACT 266, TUB2 1501 and RPB2 1138). Thus, based on morphological and phylogenetic analyses, the isolate BEZ22 was identified as Xylaria arbuscula. To verify pathogenicity, six tobacco plants at seedling stage (5-6 leaves) without visible disease were inoculated using mycelial plugs (5 mm in diameter). Leaves inoculated with PDA only plugs served as controls. After inoculation, all tobacco plants were maintained in a greenhouse with 85% relative humidity at 25 oC under a 12/12 h light/dark cycle. Five days after inoculation, typical early symptoms were observed on the inoculated leaves, and not on the control leaves. Koch's postulates were fulfilled by re-isolation of the pathogen from diseased leaves. Xylaria arbuscula has also been reported as a pathogen of Macadamia in Hawaii (Wenhsiung et al. 2009) and sugarcane in Indonesia (Maryono et al. 2020). However, to our best knowledge, this is the first report of X. arbuscula causing leaf spot on tobacco in China. This leaf spot has the potential to cause serious damage to tobacco in this region that could result in reduced production, consequently disease management of this pathogen should be considered.

Leaf Spot Caused by Epicoccum latusicollum on Tobacco in China.

Tobacco (Nicotiana tabacum L.) is a leafy, annual, solanaceous plant grown commercially for its leaves in China. In continuing research on foliar diseases of tobacco in Guizhou province in August 2019, diseased leaves of tobacco that had sandy beige, elliptical or irregular shaped lesions, with brown in edge, and surrounded by yellow halos on 40% of leaves on 5% plants were obtained (cv. Yunyan 87) in Zhenan (28.55° N, 107.43° E), Guizhou, China (Fig. 1A, 1B). Diseased leaf segments were surface sterilized and plated on potato dextrose agar (PDA). Isolate (T41) was selected for identification. The colonies had white aerial hyphae, with orange-red on the underside when cultured on PDA (Fig. 1G, 1H). The colonies had woolly aerial hyphae, white to grey eventually, and produced pycnidia on oatmeal agar (OA) (Boerema et al. 2004) (Fig. 1I, 1J). Pycnidia were dark, spherical or flat spherical, and 69.2-178.0 µm in diameter. Conidia were oval mostly, aseptate, usually guttulate, and the size was 5.0 - 6.5 µm × 3.2 - 5.4 µm (Fig. 1K, 1L). The rDNA internal transcribed spacer region (ITS) with primers ITS1f/ITS4 (White et al. 1990; Gardes and Bruns 1993), 28S ribosomal RNA gene (LSU) with primers LROR/LR7 (Rehner and Samuels 1994), beta-tubulin gene (TUB2) with primers Btub2Fd/Btub4Rd (Woudenberg et al. 2009) and RNA polymerase II second largest subunit gene (RPB2) with primers RPB2-5F2/fRPB2-7cR (Liu et al. 1999) of T41 were sequenced (GenBank accession numbers were MN704804, MN710367, MN718012 and MN718013, respectively). Maximum Likelihood (ML) analyses and Bayesian Inferences (BI) analyses based on concatenated these four sequences were conducted with RAxML v. 7.2.6 and MrBayes v. 3.2.1, respectively, which showed that T41 comprised a clade with Epicoccum latusicollum strains (CGMCC 3.18346 and LC 8153) (ML/BI = 100/1) (Fig. 2). Based on morphological and multi-gene molecular data, isolate T41 was identified as E. latusicollum described as a new taxon by Chen et al. (2017). To verify pathogenicity, tobacco plants at seedling stage (7-8 leaves) without visible disease were inoculated using conidial suspension (106 spores/ml), following Guo et al. (2020). All inoculated plants were maintained in a greenhouse with relative humidity ranging from 50% to 85% at 28 °C under a 12/12 h light/dark cycle. Seven days after incubation, typical symptoms were observed on inoculated leaves but not on control leaves (Fig. 1C, 1D, 1E, 1F). Koch's postulates were fulfilled by re-isolation of E. latusicollum from diseased leaves. E. latusicollum has been reported to cause black root on yam in China (Han et al. 2019). Meanwhile, there are many plants could be caused leaf spot by this genus, such as Lablab purpureus (Mahadevakumar et al. 2014) and Bletilla striata (Zhou et al. 2018). However, to the best of our knowledge, this is the first report of E. latusicollum causing leaf spot on tobacco in China. Because considerable loss occurred due to infection from E. latusicollum on tobacco leaves, this pathogen is worthy of further study and disease management practices need to be developed to prevent further losses.