Text by Ülo Niinemets
A collaboration between the scientists from Estonian University of Life Sciences and Chungbuk National University, South Korea demonstrates how plant growth-promoting bacteria enhance plant salinity tolerance
Soil salinity is one of the key abiotic stress factor affecting agricultural productivity worldwide. Every day, nearly 2000 hectares of fertile agricultural land degrades due to salinity. There are only limited agricultural options to cope with increasing salinification of soils, especially in the case of salt sensitive staple crops such as rice and wheat, productivity of which is seriously curbed due to salinity in many Earth locations. Among the possible options, plant growth-promoting bacteria (PGPB) have a large potential to improve crop plant productivity under salinity, but the progress in application of PGPB has been slow due to lack of non-invasive methodology for testing the efficiency of different bacteria in increasing plant salt resistance. The present collaborative study by scientists of Chungbuk National University, South Korea and the Estonian University of Life Sciences looked at foliage volatile emission and photosynthetic traits as potential non-invasive markers to estimate improvements in salinity resistance upon inoculation of rice plants with plant growth-promoting rhizosphere bacterium (PGPR) Brevibacterium linens RS16 .
This work was mainly focused on controlling the volatile organic compound (VOC) emission of plants. Though VOC emission is a part of plant defense, but it has a large impact to the environment and climate change, so the control of VOC emission from plants is still a complex and open question. “This study gives us a preliminary idea about the efficiency of 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing plant growth promoting bacteria Brevibacterium linens RS16 in altering volatile emission and photosynthetic traits in conjunction with ameliorating salt stress. It also shows a strong negative relationship with foliage ethylene and volatile emission responses and a strong positive correlation with net carbon assimilation rate” said Poulami Chatterjee, the doctoral student from Chungbuk National University.
Enhanced salinity induces oxidative stress in plants, primarily by reducing the osmotic potential of soil water. Salt stress-driven accumulation of cellular Na+ above a threshold in chloroplasts causes tissue damage and increases the rate of chlorosis. The enhanced salinity above a threshold in soil declines osmotic potential of mesophyll cells and in turn, it curbs photosynthetic electron transport activities, ultimately leading to a significant reduction in photosynthesis. In the present study, we used IR29 (salt-sensitive) and FL478 (moderately salt resistant) rice cultivars, to assess foliage carbon assimilation and stress volatile emission rates in response to inoculation by the halotolerant PGPB B. linens RS16 followed by the application of salt in soil.
Inoculation of rice plants with ACC deaminase-containing B. linens RS16 alleviated the severity of salt-stress, characterized by enhanced foliage photosynthetic traits and decreased stress volatile emissions after stress applications, greater changes detected for salt-sensitive cultivar IR29 than for moderately salt resistant genotype FL478. In addition, maintenance of high level of Fv/Fm in bacterially-inoculated salt-stressed plants suggests that the integrity of photosynthetic apparatus was maintained upon bacterial inoculation. In fact, reduction in Na+ uptake from saline-soil water is likely associated with the ability of bacteria to bind Na+ with surface polysaccharides since the sequestration of Na+ into the vacuole is also a key strategy for adjusting osmotic potential.
“Salt stress is observed to have a strong effect on photosynthetic traits and volatile emissions, thus screening photosynthetic characteristics and volatile emissions as non-invasive tools under salinity can provide illuminative insight into the severity of stress as well as induction of various primary and secondary metabolic pathways through progression of stress” said Professor Ülo Niinemets of Estonian University of Lifesciences, who led the present study along with Professor Tong-Min Sa of Chungbuk National University.
In addition, this study showed that salt stress negatively affected foliage photosynthetic characteristics in both rice cultivars with more progressive reduction observed in salt sensitive cultivar, IR29 than moderately salt resistant, FL478. Moreover, salinity enhanced the emission rates of foliage stress volatiles, particularly ethylene, lipoxygenase pathway volatiles, light-weight oxygenated volatiles, long-chained saturated aldehydes, benzenoids, geranylgeranyl diphosphate pathway products, and mono- and sesquiterpenes. However, B. linens RS16 inoculation significantly improved photosynthetic characteristics and reduced the volatile emission in salt stressed rice cultivars, compared to control plants. The decrease in volatile emission upon bacterial inoculation may likely be associated with reduced foliar ACC accumulation and ACC oxidase activity catalyzed by ACC deaminase. “Overall, this work will be useful to explore novel characteristics of PGPR under saline environments and further outlines the feasibility of foliage spray of B. linens RS16 in realistic biological settings in fields as a cost-effective strategy to cope with salt-stress, associated with greater economic losses in salt-sensitive crops” mentioned by Prof. Tongmin Sa, from Chungbuk National University.
A – healthy rice plants, and B- salt-affected rice plants (Photo courtesy: IRRI, Philippines)
C – salt-affected rice field, and B – healthy rice field (Photo courtesy: Chungbuk National University, South Korea)
The collaboration among the participating teams was supported by a collaboration with the framework of European Research Council (ERC) – Korean National Science Foundation collaborative agreements that allowed the Estonian partner team led Prof. Ülo Niinemets working in the field of plant stress biology and volatile emissions to team up with the Korean microbiology team led by Prof. Tongming Sa.
Citation: Chatterjee, P., Kanagendran, A., Samaddar, S., Pazouki, L., Sa, T. M., & Niinemets, Ü. (2018). Inoculation of Brevibacterium linens RS16 in Oryza sativa genotypes enhanced salinity resistance: Impacts on photosynthetic traits and foliar volatile emissions. Science of the Total Environment, 645, 721-732. (link to full text)
The emission of volatiles in response to salt stress in rice cultivars has not been studied much to date. Studies addressing the regulation of stress induced volatile emission by halotolerant plant growth promoting bacteria containing ACC (1-aminocyclopropane-1-carboxylate) deaminase are also limited. The objective of the present study was to investigate the salt alleviation potential of bacteria by regulating photosynthetic characteristics and volatile emissions in rice cultivars, and to compare the effects of the bacteria inoculation and salt responses between two rice genotypes. The interactive effects of soil salinity (0, 50, and 100 mM NaCl) and inoculation with Brevibacterium linens RS16 on ACC accumulation, ACC oxidase activity, carbon assimilation and stress volatile emissions after stress application were studied in the moderately salt resistant (FL478) and the salt-sensitive (IR29) rice (Oryza sativa L.) cultivars. It was observed that salt stress reduced foliage photosynthetic rate, but induced foliage ACC accumulation, foliage ACC oxidase activity, and the emissions of all the major classes of volatile organic compounds (VOCs) including the lipoxygenase pathway volatiles, light-weight oxygenated volatiles, long-chained saturated aldehydes, benzenoids, geranylgeranyl diphosphate pathway products, and mono- and sesquiterpenes. All these characteristics scaled up quantitatively with increasing salt stress. The effects of salt stress were more pronounced in the salt-sensitive genotype IR29 compared to the moderately salt resistant FL478 genotype. However, the bacterial inoculation significantly enhanced photosynthesis, and decreased ACC accumulation and the ACC oxidase activity, and VOC emissions both in control and salt-treated plants. Taken together, these results suggested that the ACC deaminase-containing Brevibacterium linens RS16 reduces the temporal regulation of VOC emissions and increases the plant physiological activity by reducing the availability of ethylene precursor ACC and the ACC oxidase activity under salt stress.