Effect of heterocystous nitrogen-fixing cyanobacteria against rice sheath blight and the underlying mechanism

Rice sheath blight causes serious damage to rice crops in China. The most common practice for rice sheath blight control is still chemical treatment, an environmentally unfriendly way. Studies have demonstrated the use of nitrogen-fixing cyanobacteria to prevent crop diseases as well as to improve soil fertility and crop productivity. However, little information is available on the biocontrol of rice sheath blight using cyanobacteria. Therefore, the objective of the present study was to screen and evaluate the biocontrol effect of nitrogen-fixing cyanobacteria against the disease through microchamber and microarea experiments. Based on the results of the microchamber experiment, three strains (SCAU04, SCAU15, and SCAU26) out of nine nitrogen-fixing cyanobacterial strains with a significant inhibition (80%) on rice sheath blight were selected. Further, microarea experiments during two seasons demonstrated the highest disease control efficacy (54.5%) on day 20 with SCAU26 treatment. Moreover, the algal treatments significantly promoted root length, underground biomass, and number of grains per panicle. SCAU04 and SCAU26 treatments significantly improved the rice theoretical yield and significantly (P < 0.05) enhanced soil potassium content compared with the control group without algal treatment. The extract of SCAU04, SCAU15, and SCAU26 algal cells inhibited the mycelial growth of R. solani (inhibition rate; 90.12%, 78.33%, and 89.87%, respectively). Based on gas chromatography-mass spectrometry results, eight metabolites were selected and evaluated for their activity against R. solani. Among them, seven inhibited R. solani growth; diisooctyl adipate showed the highest inhibitory effect. All three cyanobacteria produced the phytohormones salicylic acid, auxins, and cytokines. The salicylic acid and trans-zeatin-riboside contents of SCAU04 were 3654.74 ng g(-1) and 5266.67 ng g(-1), respectively. The isopentenyladenine content of SCAU26 reached 2839.98 ng g(-1). Morphological and molecular analysis identified the best two strains SCAU04 and SCAU26 as Nostoc piscinale and Anabaena variabilis, respectively. To conclude, our findings indicate that cyanobacteria produce bioactive substances to inhibit R. solani, secrete phytohormones to promote plant growth and induce plant disease resistance, and improve soil nutrient content to support plant growth.