Allophoma species (Pleosporales: Didymellaceae) associated with Thunbergia grandiflora in Guangxi Province, China

Abstract Background Thunbergia grandiflora belongs to the family Acanthaceae and is a widely distributed dicotyledonous plant in tropical and subtropical regions. Three isolates of Allophoma (Dothideomycetes, Pleosporales, Didymellaceae) were collected from leaves of T. grandiflora in Guangxi Province, China. New information Phylogenetic analyses of a combined ITS–LSU–rpb2–tub2 dataset indicate that one of our three strains represents an undescribed species with close affinity to A. minor and the other two strains clustered amongst other isolates of A. pterospermicola. Evidence from morphology and sequence analysis indicates that GUCC 2070.7 is a new species that we introduce here as A. thunbergiae. This is the first report about taxa of Allophoma from this host plant.


Introduction
Didymellaceae was established by De  with Didymella as the type genus. It is the largest family in the Pleosporales and accommodates more than 5400 taxon names (Crous et al. 2004), including saprobic, endophytic and pathogenic species (Aveskamp et al. 2008, Aveskamp et al. 2010, Marin-Felix et al. 2017. A great part of Didymellaceae species are reported as plant pathogens, which cause severe economic losses to many crops (Aveskamp et al. 2008). Recently, Didymellaceae was revised, based on morphological and phylogenetic analyses of ex-type sequences of LSU, ITS, rpb2 and tub2 loci, resulting in 19 genera (Chen et al. 2015, Currently, 37 genera are accepted (Wijayawardene et al. 2017, Wijayawardene et al. 2020, Valenzuela-Lopez et al. 2018, Hou et al. 2020a, Hou et al. 2020b. Allophoma is presently accepted with 14 species (Hongsanan et al. 2020, Hou et al. 2020a, Wijayawardene et al. 2020) and two of them were firstly obtained from Guizhou and Guangxi Provinces, China , Marin-Felix et al. 2019). The genus includes several important plant-pathogenic taxa, for example, Allophoma labilis (basionym: Phoma labilis), which often cause leaf necrosis, canker and stem lesions or stem rot, resulting in a negative effect on the health of plants (Zimowska 2011, Garibaldi et al. 2012, Nagarjun and Suryanarayana 2016, O'Neill and Mayne 2016, Babaahmadi et al. 2018,Jayasiri et al. 2019. Allophoma is characterised by superficial or immersed pycnidial conidiomata with ostioles, a 2−5-layered pseudo-parenchymatous wall, phialidic conidiogenous cells and aseptate variously-shaped, mostly guttulate conidia. The size of the pycnidia, conidiogenous cells and conidia are used to distinguish amongst different species in Allophoma (Chen et al. 2015).
In recent years, most species of fungi have been described from Asia, mostly China (Cheek et al. 2020). Our research group investigates the fungi on medicinal plants in south-western China, which has, thus far, resulted in the discovery of several new taxa (Long et al. 2019, Zhang et al. 2020a, An et al. 2021. Here, we studied diseased leaves of Thunbergia grandiflora collected from the Medicinal Botanical Garden in Nanning City, Guangxi Province, China. Following isolation, purification, morphological examination and phylogenetic analyses, a new species and one known species were discovered.

Isolation and morphological study
The samples were collected in 2017 at the Medicinal Botanical Garden, Nanning, Guangxi, China. Single spore isolates were obtained on oatmeal agar (OA), malt extract agar (MEA) and potato dextrose agar (PDA), followed by incubation at 25 °C. Colony diameters were measured after 1 week (Boerema et al. 2004). The colour of colonies of inoculated Petri dishes was determined following Rayner (1970). Morphological structures were examined and photographed using a Nikon Eclipse 80i microscope. Micro-morphological descriptions and measurements of mature conidiomata, conidia and conidiogenous cells on OA or MEA and PDA cultures were based on Aveskamp et al. (2010) Sequences that were used for phylogenetic analysis. The accession numbers in bold are those generated in this study. Ex-type strains are marked by an asterisk (*).

DNA isolation, PCR and sequencing
Fungal mycelia were scraped off the surface of the pure culture plate with a sterile scalpel. Total genomic DNA was extracted using the A BIOMIGA Fungus Genomic DNA Extraction Kit (GD2416, BIOMIGA, San Diego, California, USA). Four loci of each fungal strains were amplified, including the internal transcribed spacer (ITS) region with primers V9G (De Hoog and Van den Ended 1998) and ITS4 (White et al. 1990); the large subunit (LSU) of the ribosomal RNA gene with primers LR0R (Hopple 1994), LR5 and LR7 (Vilgalys and Hester 1990); the second-largest subunit of the RNA polymerase II (rpb2) wih primers RPB2-5F2 (Sung et al. 2007) and fRPB2-7cR (Liu et al. 1999); and β-tubulin (tub2) with primers Btub2Fd and Btub4Rd ). DNA amplifications were performed in 25-μl reaction volumes, containing 2.5 μl 10 × PCR buffer, 1 μl of each primer (10 μM and final extension at 72°C for 10 min. The amplification products were sent to SinoGenoMax (Beijing) for sequencing. The newly-generated DNA sequences were submitted to GenBank (accession numbers in Table 1). The DNA base differences on four loci amongst our strains and ex-type or representative strains of relative Allophoma taxa are shown in Table 2

Sequence alignment and phylogenetic analyses
The related DNA sequences for phylogenetic analyses in this study were downloaded from GenBank (Table 1) Table 2.
DNA base differences amongst our strains and related species in four gene regions.

Description
Pathogenic on the leaf spot of Thunbergia grandiflora. Lesions initially on the upper leaf surface, scattered, distinct, irregular, the maximum length of the spot more than 10-15 mm, the edge of the spots yellow, the centre of necrotic section brown, on the lower leaf surface similar. Sexual morph: Undetermined. Asexual morph (Fig. 1):

Etymology
In reference to the host (Thunbergia grandiflora), from which the fungus was isolated.
Culture characteristics: Colonies on PDA, 46−50 mm diameter after 1 week, regular at margin, densely covered by floccose aerial mycelia, grey, with a white concentric ring near the margin; reverse pale black, with a white concentric ring near the margin. Colonies on MEA, 52−58 mm diameter after 1 week, regular at margin, dull green, aerial mycelia floccose, aerial mycelia sparsely, grey near the centre; reverse changing towards margin from the centre greyish-brown to brown. Colonies on OA 34−47 mm diameter after 1 week, irregular at margin, covered by floccose aerial mycelia, mycelia sparse in some furrowed zone, reverse buff to pale olivaceous.

Analysis
Phylogenetic analyses (

Discussion
Phoma sensu lato was previously a large genus with phoma-like species (De Gruyter et al. 2012), but was recently characterised using molecular data, resulting in many species that were transferred to new genera, such as Allophoma (Chen et al. 2015). In this study, our isolates from Thunbergia (GUCC 2070.3, GUCC 2070.6 and GUCC 2070.7) represent species of Allophoma (Didymellaceae). One of these isolates, GUCC 2070.7, was retrieved close to A. minor in our phylogenetic tree (Fig. 3). In Table 3 Fig. 3, Table 2). Thunbergia grandiflora, native to China, is here reported as a host for Allophoma species for the first time.    Boerema (1993)