Laetitia Herrmann

Aims: Intensive agricultural practices have led to severe soil degradation in the Central Highlands of Vietnam. Integrating legumes as cover crops is a promising strategy to restore soil fertility via biological nitrogen fixation (BNF). Arachis pintoi has been increasingly promoted in Vietnam for adoption as a cover crop but the diversity and efficiency of its native rhizobial partners remain poorly documented. This study assessed the diversity of indigenous rhizobial strains nodulating A. pintoi in the Central Highlands of Vietnam and evaluated their symbiotic efficiency and ability to support plant growth under greenhouse conditions. Methods and results: Nodule occupancy was analyzed by Restriction Fragment Length Polymorphism and 16rDNA sequencing was performed for representative strains of each group. Seven distinct profiles were found, all identified as Bradyrhizobium spp. While the diversity of rhizobia was relatively similar across both provinces, nodule occupancy varied markedly between locations. Strains from IGS 04 group dominated in Dak Lak (66.3%), whereas IGS 01 was the most prevalent group in Gia Lai (78.3%). A greenhouse trial was conducted to assess the performance of the different rhizobial groups. Inoculation significantly improved plant growth and nitrogen levels compared to uninoculated treatment. Plants inoculated with IGS 03 and IGS 06 groups exhibited the highest performance, exceeding that of the positive control. Conversely, IGS 01 and IGS 07 strains demonstrated the poorest symbiotic efficiency. Conclusions: These results highlight the critical need to select high-efficiency native strains for bioinoculants to maximize BNF potential and support the long-term sustainability of perennial agroecosystems in Vietnam.

Salinity is one of the most devastating abiotic stresses hampering the growth and production of rice. Nine indole-3-acetic acid (IAA)-producing salt-tolerant plant-growth-promoting rhizobacteria (ST-PGPR) were inoculated into Thai jasmine rice (Oryza sativa L.) variety Khao Dawk Mali 105 (KDML105) seedlings grown under different concentrations of NaCl (0, 50, 100, and 150 mM). The ST-PGPR strains significantly promoted the growth parameters, chlorophyll content, nutrient uptake (N, P, K, Ca, and Mg), antioxidant activity, and proline accumulation in the seedlings under both normal and saline conditions compared to the respective controls. The K⁺/Na⁺ ratio of the inoculated seedlings was much higher than that of the controls, indicating greater salt tolerance. The most salt-tolerant and IAA-producing strain, Sinomonas sp. ORF15-23, yielded the highest values for all the parameters, particularly at 50 mM NaCl. The percentage increases in these parameters relative to the controls ranged from >90% to 306%. Therefore, Sinomonas sp. ORF15-23 was considered a promising ST-PGPR to be developed as a bioinoculant for enhancing the growth, salt tolerance, and aroma of KDML105 rice in salt-affected areas. Environmentally friendly technologies such as ST-PGPR bioinoculants could also support the sustainability of KDML105 geographical indication (GI) products. However, the efficiency of Sinomonas sp. ORF15-23 should be evaluated under field conditions for its effect on rice nutrient uptake and growth, including the 2AP level.

Indole-3-acetic acid (IAA) synthesis is a major property of rhizosphere bacteria. The IAA-producing ability of rhizobacteria may be influenced by agricultural management. We therefore evaluated the IAA-producing potential of rhizobacteria isolated during organic rice farming (ORF) and conventional rice farming (CRF) in Thung Kula Rong Hai areas of Thailand. The results indicated that ORF gave a significantly higher percentage of IAA producers (95.8%) than CRF (69.9%). The average IAA values of the ORF isolates were around two times higher than those of the CRF isolates both in the absence (12.8 and 5.8 μg IAA mL^-1, respectively) and presence of L-tryptophan (L-Trp) (35.2 and 17.2 μg IAA mL^-1, respectively). The 16S rRNA gene sequence analysis indicated that the 23 selected isolates belonged to 8 different genera—Sinomonas sp., Micrococcus sp., Microbacterium sp., Fictibacillus sp., Bacillus sp., Burkholderia sp., Leclercia sp. and Enterobacter sp. Interestingly, only three ORF isolates i.e., ORF15-20 (Micrococcus sp.), ORF15-21 (Sinomonas sp.) and ORF15-23 (Sinomonas sp.), exhibited high IAA production ability without L-Trp (128.5, 160.8 and 174.7 μg IAA mL^-1, respectively). Meanwhile, a slight decrease in IAA production with L-Trp was noticed,suggesting the L-Trp was not used for the IAA synthesis of these isolates. Biopriming with rhizobacterial isolates significantly enhanced the rate of germination of KDML 105 rice seeds compared to the control.

Thai jasmine rice (Oryza sativa L. KDML105), particularly from inland salt-affected areas in Thailand, is both domestically and globally valued for its unique aroma and high grain quality. The key aroma compound, 2-acetyl-1-pyrroline (2AP), has undergone a gradual degradation due to anthropogenic soil salinization driven by excessive chemical input and climate change. Here, we propose a cheaper and an ecofriendly solution to improve the 2AP levels, based on the application of plant growth-promoting rhizobacteria (PGPR). In the present study, nine PGPR isolates from rice rhizosphere were investigated for the 2AP production in liquid culture and the promotion potential for 2AP content in KDML105 rice seedlings under four NaCl concentrations (0, 50, 100, and 150 mM NaCl). The inoculation of 2AP-producing rhizobacteria resulted in an increase in 2AP content in rice seedling leaves with the maximum enhancement from Sinomonas sp. ORF15-23 at 50 mM NaCl (19.6 µg·kg⁻¹), corresponding to a 90.2% increase as compared to the control. Scanning electron microscopy confirmed the colonization of Sinomonas sp. ORF15-23 in the roots of salinity-stressed KDML105 seedlings. Our results provide evidence that Sinomonas sp. ORF15-23 could be a promising PGPR isolate in promoting aroma level of Thai jasmine rice KDML105 under salt stress.

Khao Dawk Mali 105 (KDML105) is a premium fragrant rice variety and is widely grown in Thung Kula Rong Hai (TKR), northeast Thailand. In the present study, the influence of organic and conventional rice farming (ORF and CRF, respectively) in TKR farmers’ paddy fields on soil properties and their relationship with 2-acetyl-1-pyrroline (2AP) in KDML105 rice grains were investigated. The results indicated that the ORF system had a strong positive effect on major soil quality indicators and the 2AP content in the rice grains. The soil organic matter (SOM) was approximately twice as much in the ORF than in the CRF system, thus leading to much higher total nitrogen (TN), humic acid (HA), and microbial populations in the ORF system. The higher SOM in the ORF system not only enhanced the soil quality indicators but also contributed to approximately 3.5 times higher 2AP than in the CRF system. Principle component analysis indicated a close correlation among SOM, TN, HA, and microbial population under the ORF system; these variables exhibited strong correlations with the 2AP contents in KDML105 rice grains.