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Oxidative browning is a common and severe problem in plant tissue cultures including callus cultures of Glycyrrhiza glabra , where browning is caused by accumulation and oxidation of phenolic compounds. Brown callus results in decreased biosynthetic potential and poor growth, eventually leading to cell death. In this study, the effect of different concentrations and combinations of antioxidants-ascorbic acid, and citric acid on browning of G.glabra callus was studied. Morphological and ultra-structural changes related to browning and antioxidant treatment were also studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to evaluate the cellular characteristics and metabolic activity of control and treated callus. Intensity of browning in callus remarkably decreased with increasing concentrations of ascorbic acid; the browning was completely overcome at a concentration of 20 mg/l ascorbic acid, in combination with 10 mg/l citric acid. An electron microscopy analysis showed that brown callus was found to have ruptured cell surface and a disorganized cell structure with the absence of cell organelles that affected callus development and lead to cell death. Non-brown callus had all the characteristics typical for a non-embryogenic callus cells with high-level activity showing all vital cell organelles including abundant and elongated mitochondria. Therefore, we conclude that ascorbic acid and citric acid have wide applications in tissue cultures of Glycyrrhiza glabra where browning restricts the callus development, and lead to the establishment of non-embryogenic callus with high metabolic activity for further use in secondary metabolite production.

Plant-associated microbes include taxonomically diverse communities of bacteria, archaebacteria, fungi, and viruses, which establish integral ecological relationships with the host plant and constitute the phyto-microbiome. The phyto-microbiome not only contributes in normal growth and development of plants but also plays a vital role in the maintenance of plant homeostasis during abiotic stress conditions. Owing to its immense metabolic potential, the phyto-microbiome provides the host plant with the capability to mitigate the abiotic stress through various mechanisms like production of antioxidants, plant growth hormones, bioactive compounds, detoxification of harmful chemicals and toxins, sequestration of reactive oxygen species and other free radicals. A deeper understanding of the structure and functions of the phyto-microbiome and the complex mechanisms of phyto-microbiome mediated abiotic stress mitigation would enable its utilization for abiotic stress alleviation of crop plants and development of stress-resistant crops. This review aims at exploring the potential of phyto-microbiome to alleviate drought, heat, salinity and heavy metal stress in crop plants and finding sustainable solutions to enhance the agricultural productivity. The mechanistic insights into the role of phytomicrobiome in imparting abiotic stress tolerance to plants have been summarized, that would be helpful in the development of novel bioinoculants. The high-throughput modern approaches involving candidate gene identification and target gene modification such as genomics, metagenomics, transcriptomics, metabolomics, and phyto-microbiome based genetic engineering have been discussed in wake of the ever-increasing demand of climate resilient crop plants.

Lycopersicon esculentum respond to UV-B by enhanced synthesis of flavonoid quercetin, a strong antioxidant that