Chung Huy Nguyen

Plant growth-promoting rhizobacteria (PGPR) have been collected and used to promote plant growth and enhance disease tolerance of various crops. In the current work, Pseudomonas aeruginosa CAKS2, an endophytic strain isolated from the rhizosphere of sweet orange, exhibited both growth promotion and antimicrobial activities. Under the in vitro condition, the CAKS2 showed multiple plant growth-promoting properties such as phosphate, potassium, and calcium solubilization, nitrogen fixation as well as production of siderophores, IAA, ammonia, exopolysaccharides, hydrogen cyanide, and biofilm formation. This P. aeruginosa strain inhibited the growth of different tested fungal and bacterial pathogens. Under the in vivo condition, the CAKS2 enhanced sweet orange plant growth, indicated by increases in the root and shoot lengths, the leaf number, and the total biomass. The biochemical components and the transcription levels of genes related to plant hormone biosynthesis were altered in the CAKS2-inoculated sweet orange. Under the in vivo infection of C. gloeosporioides, the CAKS2 reduced the diameter of lesions on orange leaves and harvested fruits and decreased disease severity and incidence at the whole plant level. The whole genome sequence of CAKS2 showed the presence of candidate genes involved in different molecular pathways contributing to plant-promoting and biocontrol properties. Importantly, certain changes in the expression of gene response for plant growth promotion and biocontrol were observed when the CAKS2 was exposed to sweet orange root exudates. This study highlights P. aeruginosa CAKS2 as a potential PGPR strain for enhancing plant growth and C. gloeosporioides tolerance in sweet orange and other citrus plants.

Fusarium wilt (Foc) TR4 was first reported in Northern Vietnam in 2018. Since then, it has rapidly spread across most northern provinces along the Red River basin banana production landscapes, impacting Cavendish (Musa AAA genome) production. The other main banana cultivars which are widely grown in this production zone are Pisang Awak (Musa ABB genome) and Pisang Mas (Musa AA genome). Field surveys were conducted in 2022/2023 across this banana production region to assess pathogen spread from Cavendish monocropping systems into adjacent smaller-scale mixed cultivar systems. Across 130 sites, a total of 210 banana pseudostem tissue samples were collected from symptomatic Cavendish, Pisang Awak and Pisang Mas plants. Foc TR4 incursions into mixed small-to-mid-sized Cavendish–Pisang Awak plantations were confirmed, and the pathogen was also recorded in Pisang Awak plantations and backyard gardens that did not contain any Cavendish mats. A screenhouse-based Foc TR4 screening trial including seven commonly cultivated Musa varieties in Northern Vietnam indicated that Pisang Awak and Pisang Mas are susceptible to the pathogen. While Pisang Awak, an important local variety, is known to be susceptible to both Foc Race 1 and TR4, recent field observations suggest a limited susceptibility of Pisang Awak to Foc TR4 in mixed cultivar plantation settings. Local farmers similarly reported observing reduced susceptibility, with several having already replanted TR4-affected Cavendish fields with Pisang Awak as part of their disease management strategy. No infections were observed on field-grown Pisang Mas plants in TR4-affected mixed banana cultivar production landscapes. These results and insights provide solutions for the revival of TR4-affected Cavendish production fields or landscapes, through the cultivation of less susceptible local cultivars. In addition, the introduction, validation and scaling of Formosana (i.e., GCTCV-218, a Cavendish somaclone with moderate resistance to Foc TR4) should be envisaged.

The rice bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) bacteria is rice’s most destructive disease. Due to the potential biomagnification caused by bactericides, deploying host resistance is the most effective method to protect the rice harvest. Studying the pathogenicity structure of the Xoo population helps propose the best breeding strategies to use the resistance gene pool. Among 12 resistance genes validated, xa5 and Xa21 maintained their efficacy against the Xoo population in Vietnam. Other genes, including Xa7 and xa13, remained effective in certain regions. The most destructive Xoo races distribute in the South, where intensive rice production was practiced. The same Xoo races found in different ecological regions illustrated the pathogen migration between adjacent areas. This manuscript proposed a designation system that converts the response of the differential set to each Xoo isolate into a binary, then into an octal system to name Xoo races appropriately.

Fusarium wilt, caused by the fungus Fusarium oxysporum f. sp. cubense (Foc), poses a major threat to global banana production. The tropical race 4 (TR4) variant of Foc is a highly virulent form with a large host range, and severely affects Cavendish bananas. Foc TR4 was recently observed within the Greater Mekong Subregion, after Chinese private companies expanded Cavendish production to the region. In this study, extensive surveys conducted across Laos and Vietnam show that Foc TR4 is still mainly constricted to the northern regions of these countries and is limited to Cavendish cultivation settings. In Laos, Foc TR4 is associated with large-scale Cavendish plantations owned by or involved with Chinese companies through which infected planting material could have been imported. In Vietnam, mostly small-holder Cavendish farmers and backyard gardens were affected by Foc TR4. In Vietnam, no direct link is found with Chinese growers, and it is expected the pathogen mainly spreads through local and regional movement of infected planting materials. Foc TR4 was not recorded on banana cultivars other than Cavendish. The extensively cultivated ‘Pisang Awak’ cultivar was solely infected by VCGs belonging to Foc race 1 and 2, with a high occurrence of VCG 0123 across Laos, and of VCG 0124/5 in Vietnam. Substantial diversity of Foc VCGs was recorded (VCGs 0123, 0124/5, 01218 and 01221) from northern to southern regions in both countries, suggesting that Fusarium wilt is well established in the region. Interviews with farmers indicated that the local knowledge of Fusarium wilt epidemiology and options for disease management was limited. Clear communication efforts on disease epidemiology and management with emphasis on biosecurity practices need to be improved in order to prevent further spread of Foc TR4 to mixed variety smallholder settings.

Background: Orange jasmine has a complex nomenclatural history and is now known asMurraya paniculata(L.) Jack. Our interest in this common ornamental stemmed from the need to resolve its identity and the identities of closely related taxa as hosts of the pathogen ‘CandidatusLiberibacter asiaticus’ and its vectorDiaphorina citri. Understanding these microbe-vector-plant relationships has been hampered by taxonomic confusion surroundingMurrayaat both the generic and specific levels.Results: To resolve the taxonomic uncertainty, six regions of the maternally-inherited chloroplastal genome and part of the nuclear-encoded ITS region were amplified from 85 accessions ofMurrayaandMerrilliausing the polymerase chain reaction (PCR). Clustering used maximum parsimony (MP), maximum likelihood (ML) and Bayesian inference (BI). Chronograms were produced for molecular dating, and to test the monophyly ofMurrayarigorously, using selected accessions ofMurrayaand 26 accessions of the Rutaceae and Simarubaceae. Sequence data from the ITS and chloroplastal regions suggest thatMurraya paniculata(sensu (Swingle WT and Reece CR, The Citrus Industry, p. 190–430, 1967)) can be separated into four distinct but morphologically somewhat cryptic taxa:Murraya paniculata(sensu (Mabberley DJ, Taxon 65:366–371, 2016)),M. elongata,M. sumatranaandM. lucida. In addition,Murraya omphalocarpawas identified as a putative hybrid ofM. paniculataandM. lucidawith two geographically isolated nothovarieties representing reciprocal crosses.Murrayais monophyletic, and molecular dating suggests that it diverged fromMerrilliaduring the Miocene (23–5 Ma) with thisMurrayagroup speciating and dispersing during the Middle Miocene onwards.Conclusions: The accessions from Asia and Australasia used in this study grouped into biogeographical regions that match herbarium specimen records for the taxa that suggest natural allopatric distributions with limited overlap and hybridity.Murraya paniculatahas been distributed around the world as an ornamental plant. The division of theMurraya paniculatacomplex into four species with a rare hybrid also confirms morphological studies.