Optimization of Molecular Methods for Detecting Duckweed-Associated Bacteria

The bacterial colonization dynamics of plants can differ between phylogenetically similar bacterial strains and in the context of complex bacterial communities. Quantitative methods that can resolve closely related bacteria within complex communities can lead to a better understanding of plant-microbe interactions. However, current methods often lack the specificity to differentiate phylogenetically similar bacterial strains. In this study, we describe molecular strategies to study duckweed-associated bacteria. We first systematically optimized a bead-beating protocol to co-isolate nucleic acids simultaneously from duckweed and bacteria. We then developed a generic fingerprinting assay to detect bacteria present in duckweed samples. To detect specific duckweed-bacterium associations, we developed a genomics-based computational pipeline to generate bacterial strain-specific primers. These strain-specific primers differentiated bacterial strains from the same genus and enabled the detection of specific duckweed-bacterium associations present in a community context. Moreover, we used these strain-specific primers to quantify the bacterial colonization of duckweed by normalization to a plant reference gene and revealed differences in colonization levels between strains from the same genus. Lastly, confocal microscopy of inoculated duckweed further supported our PCR results and showed bacterial colonization of the duckweed root-frond interface and root interior. The molecular methods introduced in this work should enable the tracking and quantification of specific plant-microbe associations within plant-microbial communities.

The Metabolic Response of Brachypodium Roots to the Interaction with Beneficial Bacteria Is Affected by the Plant Nutritional Status

The potential of plant growth promoting (PGP) bacteria in improving the performance of plants in suboptimal environments is increasingly acknowledged, but little information is available on the mechanisms underlying this interaction, particularly when plants are subjected to a combination of stresses. In this study, we investigated the effects of the inoculation with the PGP bacteria Azospirillum brasilense (Azospirillum) on the metabolism of the model cereal Brachypodium distachyon (Brachypodium) grown at low temperatures and supplied with insufficient phosphorus. Investigating polar metabolite and lipid fluctuations during early plant development, we found that the bacteria initially elicited a defense response in Brachypodium roots, while at later stages Azospirillum reduced the stress caused by phosphorus deficiency and improved root development of inoculated plants, particularly by stimulating the growth of branch roots. We propose that the interaction of the plant with Azospirillum was influenced by its nutritional status: bacteria were sensed as pathogens while plants were still phosphorus sufficient, but the interaction became increasingly beneficial for the plants as their phosphorus levels decreased. Our results provide new insights on the dynamics of the cereal-PGP bacteria interaction, and contribute to our understanding of the role of beneficial microorganisms in the growth of cereal crops in suboptimal environments.

Rhizobacteria Inoculation Effects on Phytohormone Status of Potato Microclones Cultivated In Vitro under Osmotic Stress

Water deficits inhibit plant growth and decrease crop productivity. Remedies are needed to counter this increasingly urgent problem in practical farming. One possible approach is to utilize rhizobacteria known to increase plant resistance to abiotic and other stresses. We therefore studied the effects of inoculating the culture medium of potato microplants grown in vitro with Azospirillum brasilense Sp245 or Ochrobactrum cytisi IPA7.2. Growth and hormone content of the plants were evaluated under stress-free conditions and under a water deficit imposed with polyethylene glycol (PEG 6000). Inoculation with either bacterium promoted the growth in terms of leaf mass accumulation. The effects were associated with increased concentrations of auxin and cytokinin hormones in the leaves and stems and with suppression of an increase in the leaf abscisic acid that PEG treatment otherwise promoted in the potato microplants. O. cytisi IPA7.2 had a greater growth-stimulating effect than A. brasilense Sp245 on stressed plants, while A. brasilense Sp245 was more effective in unstressed plants. The effects were likely to be the result of changes to the plant's hormonal balance brought about by the bacteria.

Functioning of plant-bacterial associations under osmotic stress in vitro

The search for effective plant-growth-promoting strains of rhizospheric bacteria that would ensure the resistance of plant-microbial associations to environmental stressors is essential for the design of environmentally friendly agrobiotechnologies. We investigated the interaction of potato (cv. Nevsky) microplants with the plant-growth-promoting bacteria Azospirillum brasilense Sp245 and Ochrobactrum cytisi IPA7.2 under osmotic stress in vitro. The bacteria improved the physiological and biochemical variables of the microplants, significantly increasing shoot length and root number (1.3-fold, on average). Inoculation also led a more effective recovery of the plants after stress. During repair, inoculation contributed to a decreased leaf content of malonic dialdehyde. With A. brasilense Sp245, the decrease was 1.75-fold; with O. cytisi IPA7.2, it was 1.4-fold. During repair, the shoot length, node number, and root number of the inoculated plants were greater than the control values by an average of 1.3-fold with A. brasilense Sp245 and by an average of 1.6-fold with O. cytisi IPA7.2. O. cytisi IPA7.2, previously isolated from the potato rhizosphere, protected the physiological and biochemical processes in the plants under stress and repair better than did A. brasilense Sp245. Specifically, root weight increased fivefold during repair, as compared to the noninoculated plants, while chlorophyll a content remained at the level found in the nonstressed controls. The results indicate that these bacteria can be used as components of biofertilizers. A. brasilense Sp245 has favorable prospects for use in temperate latitudes, whereas O. cytisi IPA7.2 can be successfully used in saline and drought-stressed environments.