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Crop plants are continuously exposed to various abiotic stresses like drought, salinity, ultraviolet radiation, low and high temperatures, flooding, metal toxicities, nutrient deficiencies which act as limiting factors that hampers plant growth and low agricultural productivity. Climate change and intensive agricultural practices has further aggravated the impact of abiotic stresses leading to a substantial crop loss worldwide. Crop plants have to get acclimatized to various environmental abiotic stress factors. Though genetic engineering is applied to improve plants tolerance to abiotic stresses, these are long-term strategies, and many countries have not accepted them worldwide. Therefore, use of microbes can be an economical and ecofriendly tool to avoid the shortcomings of other strategies. The microbial community in close proximity to the plant roots is so diverse in nature and can play an important role in mitigating the abiotic stresses. Plant-associated microorganisms, such as endophytes, arbuscular mycorrhizal fungi (AMF), and plant growth-promoting rhizobacteria (PGPR), are well-documented for their role in promoting crop productivity and providing stress tolerance. This mini review highlights and discusses the current knowledge on the role of various microbes and it's tolerance mechanisms which helps the crop plants to mitigate and tolerate varied abiotic stresses.

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The effects of boron (B) and high irradiance (HI) on the growth and activities of antioxidant enzymes have been investigated in cowpea plants (Vigna unguiculata L. Walp. 'P152'). A significant decrease in root and shoot lengths were observed in B-deficient (0 ppm) and B-excess (50 ppm) plants compared to B-sufficient (0.5 ppm) plants. Under B and B + HI stress, significant increase in membrane permeability (EC), lipid peroxidation (MDA) and hydrogen peroxide (H O ) were observed in B-deficient and B-excess leaves. Under B and B + HI stress, the superoxide dismutase (SOD) activity was found to be significantly high whereas the peroxidase (POX), polyphenol oxidase (PPO) activities and the non-enzymatic antioxidants, ascorbic acid and proline accumulation were found to be significantly decreased in B-deficient and B-excess leaves which showed the B inefficiency and susceptible nature of the cowpea plants to B and B + HI stress.

Canola plants were fumigated in open-top chambers with ozone (O 3) (120 ppb) under well-watered (WW) and water-stressed (WS) conditions for 4 weeks. Non-fumigated plants were also studied to facilitate comparison between treatments for the same week and over time. Therefore, the treatments were: WW, WW-O 3 , WS and WS-O 3. The fast chlorophyll a fluorescence transients OJIP for the four treatments emitted upon illumination of dark-adapted leaves were measured after week 1, 2, 3, 4 and analysed by the JIP-test to evaluate the resulting changes in photosynthetic performance. Ozone fumigation led to a decline of total performance index (PI total) in well-watered plants. The effect of O 3 was minor under drought conditions, as revealed by a decrease of PI total by 3%. The PI total decreased as the treatment was prolonged, due to leaf ageing for all cases and the decline was more pronounced under WW-O 3. Taking the average of all weeks, WW had the highest PI total and the lowest WW-O 3 (decrease by 27%), while in WS and WS-O 3 , it was lower than WW (14 and 17%, respectively). We found that the absorption (ABS)/reaction centre (RC) increases, while the maximum quantum yield of primary photochemistry (φ Po) undergoes slight changes, and trapping (TR 0)/RC closely followed the increase in ABS/RC. This indicates that O 3 and drought caused an increase in the functional antenna size. The maximum quantum yield of primary photochemistry showed slight differences for all treatments and over time, suggesting that this parameter is less sensitive to drought and O 3 stress. Therefore, the more sensitive components of the photosynthetic electron transport chain appeared to be the probability that an electron from the intersystem electron carriers is transferred to reduce end electron acceptors at the PSI acceptor side (δ Ro) and the RC density on a chlorophyll basis (RC/ABS).

The effects of elevated CO2 (700 ppm) and O3 (80 ppb) alone and in combination on the photosynthetic efficiency of canola and wheat plants were investigated in open-top chambers (OTCs). The plants were fumigated for four weeks under well-watered and water-stressed (water deficit) conditions. The fast chlorophyll a fluorescence transients were measured after 2 and 4 weeks of fumigation, as well as in control plants, and analyzed by the JIP-test, which is a non-destructive, non-invasive, informative, very fast and inexpensive technique used to evaluate the changes in photosynthetic efficiency. Biomass measurements were taken only after 4 weeks of fumigation. The performance index (PItotal), an overall parameter calculated from the JIP-test formulae, was reduced by elevated CO2 and O3 under well-watered conditions. In the absence of any other treatment, water stress caused a decrease of the PItotal, and it was partly eliminated by fumigation with elevated CO2 and CO2 + O3. This finding was also supported by the biomass results, which revealed a higher biomass under elevated CO2 and CO2 + O3. The decrease in biomass induced by elevated O3 was likely caused by the decline of photosynthetic efficiency. Our findings suggest that elevated CO2 reduces the drought effect both in the absence and presence of O3 in canola and wheat plants. The study also indicates that elevated O3 would pose a threat in future to agricultural crops.

In the original publication's Fig. 1b, the labels J and I, should be placed at approximately 2 ms and 30 ms respectively. Also, Fig. 3C y-axis title should be written as ψ Eo /(1-ψ Eo). The corrected Figs. 1 and 3 are provided here.