Cytokinin Production by Azospirillum brasilense Contributes to Increase in Growth, Yield, Antioxidant, and Physiological Systems of Wheat (Triticum aestivum L.).

Plant growth-promoting rhizobacteria are known to associate with several cereal crops. The rhizobacterium exerts its function by synthesizing diverse arrays of phytohormones, such as cytokinin (Ck). However, it is difficult to determine the plant growth promotion when a bacterium produces many different kinds of phytohormones. Therefore, to assess the involvement of Ck in growth promotion and activation of antioxidant and physiological systems, we set up this experiment. Wheat seeds (Triticum aestivum L.) were inoculated with Azospirillum brasilense RA-17 (which produces zeatin type Ck) and RA-18 (which failed to produce Ck). Results showed that seed inoculation with RA-17 significantly improved growth and yield-related parameters compared with RA-18. The activity of enzymes, proline contents, and endogenous hormonal levels in wheat kernels were improved considerably with RA-17 than with RA-18. Strain RA-17 enhanced grain assimilation more than strain RA-18 resulting in a higher crop yield. These results suggest that microbial Ck production may be necessary for stimulating plant growth promotion and activating antioxidant and physiological systems in wheat.

Inoculation of Azospirillum brasilense and exogenous application of trans-zeatin riboside alleviates arsenic induced physiological damages in wheat (Triticum aestivum).

Due to increased industrialization, arsenic (As) in the soil has become a serious issue for wheat production since past few decades. We investigated the role of Azospirillum brasilense and trans-zeatin riboside (tZR) in the mitigation of arsenic toxicity in wheat for 2 years (2018-2019 and 2019-2020) in pot experiments. Wheat plants grown in soil artificially spiked with arsenic (50, 70, and 100 µM) was left alone or amended with A. brasilense, tZR, or their combination as mitigation strategies. A treatment without arsenic or amendments was maintained as control. Arsenic-induced physiological damages were noticed in the wheat plants. Detrimental effects on the plant physiological functions, such as disruption of cell membrane stability, reduced water uptake, and stomatal functions, were noticed with increase in As toxicity. Application of biological amendments reversed the effects of As toxicity by increasing wheat plant growth rate, leaf area, and photosynthesis and also yield. Therefore, application of tZR and wheat seed inoculation with A. brasilense could be a sustainable and environmentally friendly strategy to mitigate arsenic-induced crop physiological damages.

Physiological and molecular responses of pearl millet seedling to atrazine stress.

Pearl millet has been recommended beneficial for several therapeutic purposes. However, little is known of the physiological responses to abiotic stressors, especially of atrazine. In order to elucidate the physiological and molecular responses of pearl millet to atrazine stress, we studied the response of various biomarkers under increasing herbicide concentrations (0, 5, 10, and 50 mg/kg). We also quantified the levels of malondialdehyde (MDA) and reactive oxygen species (ROS) (H2O2 and O2•-) produced in the leaves to evaluate the extent of oxidative damage. Increasing atrazine concentrations significantly increased ROS and MDA production in the plant leaves. Ascorbate peroxidase (APX) and peroxidase (POD) activities increased, while catalase (CAT) and superoxide dismutase activities reduced with increasing atrazine concentrations. Generally, atrazine applied at 50 mg/kg suppressed chlorophyll contents, whereas, chlorophyll (a/b) ratio was increased. Atrazine applied at 50 mg/kg significantly suppressed antioxidant gene expressions to the lowest. The APX gene showed overall low response to the atrazine treatments. The chloroplastic psbA gene showed highest expression with 10 mg/kg atrazine, whereas atrazine at 50 mg/kg significantly suppressed the gene expression to its lowest. Pearl millet was able to suppress oxidative stress under low atrazine levels, but high atrazine concentration could induce more oxidative damage.

Effect of dimethyl phthalate (DMP) on germination, antioxidant system, and chloroplast ultrastructure in Cucumis sativus L.

Pollution of agricultural soils caused by widely employed plastic products, such as phthalic acid esters (PAEs), are becoming widespread in China, and they have become a threat to human health and the environment. However, little information is available on the influence of PAEs on vegetable crops. In this study, effects of different dimethyl phthalate (DMP) treatments (0, 30, 50, 100, and 200 mg L(-1)) on seed germination and growth of cucumber seedlings were investigated. Although germination rate showed no significant difference compared to control, seed germination time was significantly delayed at DMP greater than 50 mg L(-1). Concentrations of DMP greater than 30 mg L(-1) reduced cucumber lateral root length and number. The measurement of five physiological indexes in cucumber leaves with increasing DMP concentration revealed a decrease in leaf chlorophyll content, while proline and H2O2 contents increased. Peroxidase (POD) and catalase (CAT) activities increased in cucumber plants under 30 and 50 mg L(-1) DMP treatments compared to control; while after a 7-day treatment, these activities were seriously reduced under 100 and 200 mg L(-1) DMP treatments. According to transmission electron microscopy (TEM) micrographic images, the control and 30 mg L(-1) DMP treatments caused no change to leaf chloroplast shape with well-structured thylakoid membrane and parallel pattern of lamellae. At concentrations higher than 30 mg L(-1), DMP altered the ultrastructure of chloroplast, damaged membrane structure, disordered the lamellae, and increased the number and volume of starch grains. Moreover, the envelope of starch grains began to degrade under 200 mg L(-1) DMP treatment.

Effect of di-n-butyl phthalate on root physiology and rhizosphere microbial community of cucumber seedlings.

The authors investigated the effects of di-n-butyl phthalate (DBP) on root physiology and rhizosphere microbial communities of cucumber seedlings (sativus L. cv Jinyan No. 4). Root protein content and root activity were observed to decrease. From the ultrastructural micrographs, visible impact on the mitochondria, endoplasmic reticulum and vacuole were detected. Moreover, the number of starch grains increased, and some were adhered to other cell components which might be the most direct evidence of DBP causing cellular damage. Results of PCR-DGGE (denaturing gradient gel electrophoresis) indicated that DBP significantly changed the abundance, structure and composition of rhizosphere bacteria when the concentration was higher than 50 mg L(-1). The relative abundances of Firmicutes increased while that of Bacteroidetes decreased. Bacillus was detected as the dominant bacteria in DBP contaminated cucumber rhizospheric soil. The amount of Actinobacteridae and Pseudomonas decreased until it disappeared in the rhizosphere soil when exposed to DBP concentrations higher than 50 mg L(-1).