First Report of Powdery Mildew in Eucalyptus Caused by Erysiphe neolycopersici in China.

Eucalyptus is one of the most fast-growing and widely planted hardwood trees in the tropical and subtropical regions (Grattapaglia and Kirst, 2008). In December 2021, powdery mildew diseases were observed on the Eucalyptus urophylla, E. urophylla × E. grandis, E. grandis × E. urophylla, and E. grandis trees growing in the Eucalyptus garden of the Guangxi University campus in Nanning (108°22'E, 22°48'N) of Guangxi Zhuang Autonomous Region, where is the main plantation area for Eucalyptus. The spread of this disease would bring potential challenges on the Eucalyptus plantation management in this region of China. The early symptoms of this disease in Eucalyptus were that the irregular white spots with surface-attached powder was observed on the leaves. At the late stages, this symptom was diffused to the whole leaves and even petioles and stems. It would finally cause significant defoliation, but barely lead to plant death in Eucalyptus. Microscopic observation showed that the mycelium was straight or flexuous, hyaline, thin-walled, septate, branched, and 3-7 µm wide (n = 50; average 4.86 µm). The appressorium was lobed and attached to one end of the mycelium alone, or paired attached to both ends of the mycelium. The conidiophore was straight or flexuous, unbranched, 54-100 × 6-10 µm (n = 40; average 75.47 µm × 8.22 µm). One to 3 conidium were connate on the conidiophores. Foot-cells were straight or flexuous at base, 5-8 µm wide (n = 40; average 6.53 µm). The conidium were ellipsoid or oval, and the size was 38-56 × 12-21 µm (n = 70; average 44.92 µm × 15.69 µm). The lobed or rod-shaped bud tube was produced at the conidium. According to the morphology, the fungus was identified as Erysiphe neolycopersici (Hsiao, et al. 2022). For the molecular characterization of the isolate, the sequences of the internal transcribed spacer (ITS), the 18S and 28S large subunit ribosomal DNA (SSU and LSU) (Scholin et al. 1994 , White et al. 1990), were sequenced and deposited in GenBank (OM422667, OM424285 and ON514159). The phylogenetic analysis showed that the ITS sequence showed 100% identity with sequences of E. neolycopersici (MW082786, MT370492, and JQ972700). The 28S rDNA sequence had the highest identity (99.69%) with that of E. neolycopersici (LC371327, LC371320, and OM368490). The SSU sequence had the highest identity (99.72%) with that of E. neolycopersici (LC516961). The pathogenicity test of the fungus was repeated thrice following the Koch's postulates. The diseased leaves were gently rubbed against 3 to 4 healthy mature leaves of more than five E. grandis seedlings (two-month-old). The inoculated and control plants were then cultured in the greenhouse (25 ℃, 16-h light/8-h dark and 70% humidity). Similar disease symptoms were observed on the inoculated leaves, but not on the control leaves seven days after inoculation. The isolates from three independent experiments were morphologically and genetically identical with the original isolate. As far as we know, this study is the first report of powdery mildew disease in Eucalyptus caused by E. neolycopersici in China.

Genome-wide characterization and analysis of Golden 2-Like transcription factors related to leaf chlorophyll synthesis in diploid and triploid Eucalyptus urophylla.

Golden 2-Like (GLK) transcription factors play a crucial role in chloroplast development and chlorophyll synthesis in many plant taxa. To date, no systematic analysis of GLK transcription factors in tree species has been conducted. In this study, 40 EgrGLK genes in the Eucalyptus grandis genome were identified and divided into seven groups based on the gene structure and motif composition. The EgrGLK genes were mapped to 11 chromosomes and the distribution of genes on chromosome was uneven. Phylogenetic analysis of GLK proteins between E. grandis and other species provided information for the high evolutionary conservation of GLK genes among different species. Prediction of cis-regulatory elements indicated that the EgrGLK genes were involved in development, light response, and hormone response. Based on the finding that the content of chlorophyll in mature leaves was the highest, and leaf chlorophyll content of triploid Eucalyptus urophylla was higher than that of the diploid control, EgrGLK expression pattern in leaves of triploid and diploid E. urophylla was examined by means of transcriptome analysis. Differential expression of EgrGLK genes in leaves of E. urophylla of different ploidies was consistent with the trend in chlorophyll content. To further explore the relationship between EgrGLK expression and chlorophyll synthesis, co-expression networks were generated, which indicated that EgrGLK genes may have a positive regulatory relationship with chlorophyll synthesis. In addition, three EgrGLK genes that may play an important role in chlorophyll synthesis were identified in the co-expression networks. And the prediction of miRNAs targeting EgrGLK genes showed that miRNAs might play an important role in the regulation of EgrGLK gene expression. This research provides valuable information for further functional characterization of GLK genes in Eucalyptus.

Induction and Characterization of Tetraploid Through Zygotic Chromosome Doubling in Eucalyptus urophylla.

Improvements in plant growth can bring great benefits to the forest industry. Eucalyptus urophylla is an important plantation species worldwide, and given that ploidy increases are often associated with plant phenotype changes, it was reasoned that its polyploidization may have good prospects and great significance toward its cultivation. In this study, the zygotic development period of E. urophylla was observed through paraffin sections, and a correlation between the development time of flower buds after pollination and the zygotic development period was established. On this basis, it was determined that the 25th day after pollination was the appropriate time for a high temperature to induce zygotic chromosome doubling. Then tetraploid E. urophylla was successfully obtained for the first time through zygotic chromosome doubling induced by high temperature, and the appropriate conditions were treating flower branches at 44°C for 6 h. The characterization of tetraploid E. urophylla was performed. Chromosome duplication brought about slower growing trees with thicker leaves, larger cells, higher net photosynthetic rates, and a higher content of certain secondary metabolites. Additionally, the molecular mechanisms for the variation in the tetraploid's characteristics were studied. The qRT-PCR results showed that genes mediating the tetraploid characteristics showed the same change trend as those of the characteristics, which verified that tetraploid trait variation was mainly caused by gene expression changes. Furthermore, although the tetraploid had no growth advantage compared with the diploid, it can provide important germplasm resources for future breeding, especially for the creation of triploids.