Dominika Kidaj

Nod factors stimulate seed germination and promote growth and nodulation of pea and vetch under competitive conditions

Nod factors are lipochitooligosaccharide (LCO) produced by soil bacteria commonly known as rhizobia acting as signals for the legume plants to initiate symbiosis. Nod factors trigger early symbiotic responses in plant roots and initiate the development of specialized plant organs called nodules, where biological nitrogen fixation takes place. Here, the effect of specific LCO originating from flavonoid induced Rhizobium leguminosarum bv. viciae GR09 culture was studied on germination, plant growth and nodulation of pea and vetch. A crude preparation of GR09 LCO significantly enhanced symbiotic performance of pea and vetch grown under laboratory conditions and in the soil. Moreover, the effect of GR09 LCOs seed treatments on the genetic diversity of rhizobia recovered from vetch and pea nodules was presented.

Enhancing Rhizobium-Legume Symbiosis Using Signaling Factors

Rhizobial symbiosis with leguminous plants affects the supply of organic nitrogen. Soil bacteria comprising members of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium, and Azorhizobium, commonly referred to as rhizobia, are taxonomically diverse members of the α and β subclasses of the Proteobacteria. They possess the ability to induce root nodules on legume plants and provide these plants with fixed nitrogen, enabling them to grow in nitrogen-limited soils. Rhizobia colonize root nodules, fix nitrogen inside, transport usable form of N to plants, and concurrently facilitate the growth and grain yields of legumes. Rhizobium–legume symbiosis is a multi-step process requiring the exchange of numerous molecular signals between bacteria and the plant host. Precise fulfilling of all stages of this molecular dialogue is prerequisite to the effective symbiosis, allowing bacteria to invade the host and, conversely, enabling the host to derive benefits from the presence of bacteria. Individual legumes are often nodulated by multiple bacterial strains with varying symbiosis-establishing capabilities. Thus, selection of highly effective strains that successfully compete with less effective ones is required when developing legume inoculants. Various factors that influence symbiotic rhizobial interactions under competitive soil environment, including the exchange of plant and bacterial signaling molecules, such as flavonoids and nodulation factor (Nod factor), in the early stages of symbiosis is highlighted. Beneficial responses of rhizobial inoculants on to legumes, as well as manipulations of symbiotic signaling factors, is likely to increase their potential as biofertilizers for sustainable agriculture to promote growth and nodulation of legume plants.

The effect of biotic and physical factors on the competitive ability of Rhizobium leguminosarum

BackgroundRhizobium leguminosarum bv. viciae (Rlv) is a soil bacterium which can form nitrogen-fixing symbiotic relationships with leguminous plants. Numerous rhizobial strains found in soils compete with each other. Competition can occur both during the saprophytic growth phase in the rhizosphere and inside plant tissues, during the symbiotic phase. Competition is important as it may affect the composition of rhizobial populations present in the soil and in the root nodules of plants.MethodologyWe examined the link between physiological traits and bacterial competitive ability in eighteen Rhizobium leguminosarum bv. viciae (Rlv) isolates during root nodule colonization using laboratory and field experiments. The competitive ability of R/v strains was measured as the percentage of root nodules colonized by gusA-tagged rhizobia in two types of host plants, peas and vetch.ResultsThe competitiveness of Rlv strains was significantly affected by soil type and the identity of the host plant. Of the eighteen bacterial traits examined in this study, the metabolic potential (number of utilized carbon and energy sources) and the responsiveness of nod genes to flavonoid activation were most important in affecting the competitive ability of Rlv strains. The amount of acylated homoserine lactones (AHL) produced by the strains was less important in influencing competitiveness. Finally, the preactivation of strains with flavonoids or the addition of AHL to gus-tagged Rlv strains did not significantly enhance competitiveness: of the gus-tagged inoculants in comparison to indigenous soil populations of vetch microsymbionts.ConclusionsThe competitiveness of Rlv strains is dependent upon numerous physiological traits. However, environmental factors such as soil type and the type of host plant may be even more important in affecting rhizobial competitiveness.

The response of rhizosphere microbial properties to flavonoids and Nod factors

Flavonoids and Nod factors (lipo-chitooligosaccharides) influence legume growth, plant–microbe communication and regulate microbial community. To assess the effects of pea (Pisum sativum) seed inoculation with flavonoids and Nod factors on microbial parameters, field study was performed on a Haplic Luvisol. Flavonoids were extracted from sprouted pea seeds, and Nod factors were isolated from Rhizobium leguminosarum bv. viciae strain GR09. Pea seeds were soaked with flavonoids (F), Nod factors (Nod), mixture of both compounds (F + Nod) or water (control, C) and then planted. Enzyme activities (dehydrogenases, protease and acid phosphatase), the numbers of cultivable fungi, bacteria, Pseudomonas and Bacillus were determined in pea rhizosphere. Generally, a significant increase was observed in dehydrogenases and protease activities, bacteria, Pseudomonas and Bacillus numbers under F, Nod and F + Nod treatments compared to control and decrease in fungi number. The effect of F and Nod factors was significant for most of the microbial parameters analysed. Significant interactions between F and Nod factors for a number of fungi and Pseudomonas population were found. The results indicate that the application of F and Nod factors on seeds could improve root–microbes interactions through enhanced microbe number and activity in pea rhizosphere.

The Diversity of Pea Microsymbionts in Various Types of Soils and Their Effects on Plant Host Productivity

The growth and yield of peas cultivated on eight different soils, as well as the diversity of pea microsymbionts derived from these soils were investigated in the present study. The experimental plot was composed of soils that were transferred from different parts of Poland more than a century ago. The soils were located in direct vicinity of each other in the experimental plot. All soils examined contained pea microsymbionts, which were suggested to belong to Rhizobium leguminosarum sv. viciae based on the nucleotide sequence of the partial 16S rRNA gene. PCR-RFLP analyses of the 16S-23S rRNA gene ITS region and nodD alleles revealed the presence of numerous and diversified groups of pea microsymbionts and some similarities between the tested populations, which may have been the result of the spread or displacement of strains. However, most populations retained their own genetic distinction, which may have been related to the type of soil. Most of the tested populations comprised low-effective strains for the promotion of pea growth. No relationships were found between the characteristics of soil and symbiotic effectiveness of rhizobial populations; however, better seed yield was obtained for soil with medium biological productivity inhabited by high-effective rhizobial populations than for soil with high agricultural quality containing medium-quality pea microsymbionts, and these results showed the importance of symbiosis for plant hosts.

Symbiotic Activity of Pea (Pisum sativum) after Application of Nod Factors under Field Conditions

Growth and symbiotic activity of legumes are mediated by Nod factors (LCO, lipo-chitooligosaccharides). To assess the effects of application of Nod factors on symbiotic activity and yield of pea, a two-year field experiment was conducted on a Haplic Luvisol developed from loess. Nod factors were isolated from Rhizobium leguminosarum bv. viciae strain GR09. Pea seeds were treated with the Nod factors (10−11 M) or water (control) before planting. Symbiotic activity was evaluated by measurements of nitrogenase activity (acetylene reduction assay), nodule number and mass, and top growth by shoot mass, leaf area, and seed and protein yield. Nod factors generally improved pea yield and nitrogenase activity in the relatively dry growing season 2012, but not in the wet growing season in 2013 due to different weather conditions.

The pleiotropic effects of extract containing rhizobial Nod factors on pea growth and yield

Rhizobial lipochitooligosacharides (Nod factors) influence the development of legume roots, including growth stimulation, nodule induction and root hair curling. However, their effect on the green parts of plants is less known, therefore we evaluated seed and foliar application of an extract containing Nod factors on pea growth and yield. Pea plants were examined from emergence to full maturity, including growth dynamics and morphological (nodule number and weight, the quantity and surface area of leaves) or physiological (photosynthesis and transpiration intensity, chlorophyll and nitrogen content) parameters. The foliar application Nod factor extract, or seed dressing followed by foliar application, resulted in the best outcomes. The number and weight of root nodules, the chlorophyll content in leaves, and the intensity of net photosynthesis were all elevated. As a consequence of Nod factor treatment, the dynamics of dry mass accumulation of pea organs were improved and the pod number was increased. A significant increase in pea yield was observed after Nod factor application. Increase of nodule and pod numbers and improved growth of roots appear to be amongst the beneficial effects of Nod factor extract on the activation of secondary plant meristems.

Effect of Sulfur and Nod Factors (LCOs) on Some Physiological Features and Yield of Pea (Pisum sativum L.)

The response of pea var. Medal to treatment with Nod factors (LCOs) and mineral sulfur was estimated in a pot experiment with a completely randomized design. Foliar spraying of plants was performed at the 5–6 leaf stage (BBCH 15) at concentrations of 10−12 M dm−3 and 12 g S dm−3 for LCOs and sulfur, respectively. The use of these factors, both individually and in combination, caused an increase in leaf area and “greenness” (SPAD), gas exchange parameters, straw and seed yields and in the root system. The number of nodules and respective nodule dry weight also increased with these treatments. A significant increase in seed yield resulted from the beneficial effects of LCOs and sulfur with an increase in the number of pods and seeds per plant compared to control plants, is clearly significant from the agricultural point of view. Although each factor improved the traits studied, the best results were achieved in the case of plants treated with both LCOs and sulfur.

Flavonoids and Nod Factors: Importance in Legume-Microbe Interactions and Legume Improvement

Biological nitrogen fixation is one of the most important physiological processes in which atmospheric nitrogen is reduced to ammonia by symbiotic bacteria called rhizobia belonging to α- and β-Proteobacteria. Legume plants (Fabaceae) enter into mutualistic symbiosis with nitrogen-fixing rhizobia which enable them to grow in nitrogen-limited agricultural soils. Infection of legumes by rhizobia involves a series of sequential steps in which plant flavonoids and rhizobial Nod factors activate plant transmission signaling and initiate nodule development. Inside the nodule, rhizobia multiply and differentiate into nitrogen-fixing bacteroids. Here, besides an overview of symbiosis, the role of signal molecules, flavonoids, and Nod factors in legume growth and yield enhancement is highlighted. Recent progress in the understanding of the functions of the symbiotic signaling factors in initiation and development of symbiosis is likely to facilitate successful application thereof in sustainable agriculture to promote growth and nodulation of legume plants.