Hydrogen sulfide causes excision of a genomic island in Pseudomonas syringae pv. phaseolicola

© 2017, The Author(s). Hydrogen sulfide (H2S) is known to be an important signalling molecule in both animals and plants, despite its toxic nature. In plants it has been seen to control stomatal apertures, so altering the ability of bacteria to invade plant tissues. Bacteria are known to generate H2S as well as being exposed to plant-generated H2S. During their interaction with plants pathogenic bacteria are known to undergo alterations to their genomic complement. For example Pseudomonas syringae pv. phaseolicola (Pph) strain 1302A undergoes loss of a section of DNA known as a genomic island (PPHGI-1) when exposed to the plants resistance response. Loss of PPHGI-1 from Pph 1302A enables the pathogen to overcome the plants resistance response and cause disease. Here, with the use of H2S donor molecules, changes induced in Pph 1302A genome, as demonstrated by excision of PPHGI-1, were investigated. Pph 1302A cells were found to be resistant to low concentrations of H2S. However, at sub-lethal H2S concentrations an increase in the expression of the PPHGI-1 encoded integrase gene (xerC), which is responsible for island excision, and a subsequent increase in the presence of the circular form of PPHGI-1 were detected. This suggests that H2S is able to initiate excision of PPHGI-1 from the Pph genome. Therefore, H2S that may emanate from the plant has an effect on the genome structure of invading bacteria and their ability to cause disease in plants. Modulation of such plant signals may be a way to increase plant defence responses for crops in the future

Diversity of the Endophytic Fungal spp. in Selected Rice (Oryza sativa L) Varieties of Sri Lanka and their Hydrolytic Enzyme Producing Abilities

Rice (Oryza sativa L.) is the staple food in Sri Lanka and with increases in population, the demand for rice has also increased requiring higher yields. The use of endophytic fungal (EF) assemblages has been successful in increasing the yields in a number of crops including rice. However, the existent knowledge on the diversity among the species of EF assemblages associated with different rice varieties is hardly sufficient for this purpose. Therefore, this study was aimed at identifying and assessing the diversity of EF present in three newly improved rice varieties At 362, Bg 352, Bw 367 and one traditional variety i.e. Suwandel grown in different geographical locations/climatic zones of Sri Lanka. Healthy plant samples of each rice variety was collected during the Maha and Yala seasons in 2018/2019 from Anuradhapura, Kurunegala, Gampaha and Kalutara districts in Sri Lanka. Endophytic fungi were isolated from leaves, stems and roots of the four rice varieties using previously optimized protocols. Identifications of the isolated fungal spp. was carried out using morphological and molecular characteristics. Species were identified by PCR amplification of the Internal Transcriber Spacer (ITS) regions and comparing their sequences with those of well characterized/type strains in the National Centre for Biotechnology Information (NCBI) database. 1,920 plant segments used for isolations yielded 26 fungal genera and 39 fungal spp. The most frequently isolated and dominant species among all rice varieties were Microdochium fisheri, Dendryphiella sp and Penicillium oxalicum. Species diversity was analyzed using Shannon Wiener’s (H’) and Simpson’s dominance (1-D) indices and a high diversity of different fungal spp. were observed in Bg 352 collected from Kalutara during the Yala season. As EF have been reported to produce extra cellular enzymes as a means of showing mycoparasitic activity, selected EF isolates were screened in enriched media for production of Chitinase and Protease enzymes using standard plate assays and Glucanase enzyme production was analyzed by Dinitro Salicylic Acid assay. Out of the tested isolates, Rhizopus microsporus produced a significantly high (p≤0.05) level of chitinase, while Aspergillus fischeri showed a significantly high (p≤0.05) protease production. Penicillium oxalicum showed a significantly high (p≤0.05) glucanase production. Thus, findings of this study postulate rich EF assemblages in Sri Lankan rice varieties which could potentially be a foundation for studies on novel models of rice-fungal mutualism. Rice endophytes also produce extra-cellular hydrolytic enzymes that may contribute towards controlling rice pathogens

Morphological and molecular characterization of two graminicolous <i>Exserohilum</i> species associated with cultivated rice and early barnyard grass from Sri Lanka

The genus Exserohilum (Order Pleosporales, Class Dothideomycetes) comprises plant pathogenic hyphomycetous fungi, associated with poaceous hosts. Although numerous pathogenic species of Exserohilum are known globally, only E. turcicum and E. rostratum have been reported from Sri Lanka. In the present study, samples showing the symptoms of leaf blight of Oryza sativa (cultivated rice) and sheath blight of Echinochloa oryzoides (early barnyard grass) were collected and causal agents were primarily identified as Exserohilum spp. based on morphological characters. Molecular phylogenetic analyses based on three loci namely, nuclear ribosomal internal transcribed spacer (ITS), partial glyceraldehyde 3-phosphate dehydrogenase (GPDH) and translational elongation factor (TEF-1α) were used to infer evolutionary relationships and accurate identification. These isolates from O. sativa and Echinochloa oryzoides were identified as Exserohilum rostratum and E. oryzicola respectively. Both records are novel plant-fungal associations from Sri Lanka based on available data. This study suggests the need for morphological and molecular reassessments of emerging and poorly known species of fungi associated with cereals, their wild relatives and other economically important hosts in Sri Lanka