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belong to filamentous bacteria (gram positive) with a high guanine–cytosine ratio in their DNA, and they are good sources of natural compounds used in the pharmaceutical industry (Golinska et al. 2015). Endophytic actinomycetes showed enhanced growth and decreased disease infestation through the production of bioactive secondary metabolites. Some important bioactive compounds that are released by host plants and their therapeutic properties along with microorganisms are presented in Table 3.1. Moreover, they alter plant physiology by stimulating the acquired resistance in medicinal plants. Specific antibiotics produced by genera of Penicillium, Trichoderma, Aspergillus, and Actinomycetes may enhance producer diligence when competitors or predators are present in soil habitats. Moreover, Azospirillum, Gluconacetobacter, Pseudomonas, and Rhizobium, Bacillus, and Paenibacillus and gram-positive genera are also included in this circle. Plant–microbe mutualism produces diverse biochemical mechanism that improves bacterial health and ultimately plant growth (Bulgarelli et al. 2013). It also enables plants to maintain microbiota in the soil. The concentration of the bacteria found around the roots of plants is greater than the rest of the soil. It might also be due to the presence of nutrients including sugars, amino acids, and organic acids as well as other small molecules from plant root exudates that fix one-third of carbon by the plant (Badri et al. 2009). However, it mostly depends on soil type and host genotype interaction. Both the type and number of bacteria found in different soils are influenced by soil conditions. It was also influenced by soil temperature, moisture, chemicals as well as the concentration of the salts, and the number and types of crop plants present in the soil. Moreover, the effect of specific bacterium on the plant may change with the change of conditions. For example, a bacterium facilitates plant growth either by fixing phosphorus compounds or by fixing nitrogen that benefits only plants (Kumar et al. 2015, 2016). Plants with medicinal properties, their use, and secondary metabolites production under abiotic stress condition are presented in Table 3.1 .
Table 3.1 Plants with Medicinal Properties Under Abiotic Stress, Their Use, and Their Secondary Metabolites.
| Medicinal plants | Uses | Secondary metabolites | Abiotic stresses | References |
|---|---|---|---|---|
| Pluchea lanceolata | Bronchitis, dyspepsia, and rheumatoid arthritis | Quercetin | Heavy metals | Kumar et al. (2004) |
| Dioscorea bulbifera | Antispasmodic, analgesic, aphrodisiac, and diuretic | Diosgenin | Metal | Narula et al. (2005) |
| Catharanthus roseus | Cancer and diabetes mellitus | Vinblastine | Salinity | Jaleel et al. (2009) |
| Jatropha curcas | Skin diseases and rheumatism, piles | Curcin | Salinity | Gao et al. (2008) |
| Orthosiphon stamineus | Antiallergenic, antihypertensive, anti-inflammatory, and nephritis | Polyphenols | Salinity | Ting et al. (2009) |
| Thymus maroccanus Ball | Antitussive, antiseptic, antispasmodic, and antihelmintic | Thymol | Salinity | Belaqziz et al. (2009) |
| Matricaria chamomilla | Mucositis and irritable bowel syndrome | Umbelliferone | CuCl2 | Eliasova et al. (2004) |
| Bacopa monnieri | Antioxidant, epilepsy, and asthma | Bacoside | Salinity and Drought | Debnath et al. (2011) |
| Olea europaea L. | Antipruritic, antiseptic, astringent, and cholagogue | Oleosides | Salinity | Rejskova et al. (2007) |
| Populus euphratica | Anodyne, anti-inflammatory, febrifuge, and vermifuge | Gallic acid | Salinity | Zhang et al. (2004) |
| Matricaria chamomilla | Sore stomach, irritable bowel syndrome, and oral mucositis | Apigenin | Salinity | Razmjoo et al. (2008) |
| Ziziphora clinopodioides | Antibacterial and sedative stomachache | Leucoanthocyanins | Salinity and Defoliation | Koocheki et al. (2008) |
| Dioscorea dregeana | Sedative and anti-inflammatory | Paclobutrazol | Smoke, temperature | Kulkarni et al. (2007) |
| Datura innoxia | Anodyne, antispasmodic, hallucinogenic, hypnotic, and narcotic | Scopolamine | Light, dark, and HCHO deprivation | Laszlo et al. (2004) |
However, the composition of the secondary metabolite, which is common in the rhizosphere belongs to these bacteria, such as actinobacteria, bacteroidetes, firmicutes, and proteobacteria. Microbial communities provide protection directly or indirectly from the pathogen in the leaf regions while root microbiota provides an additional host function by getting nutrients from the soil. Other functions of the microbiota associated with these medicinal plants such as phytohormone production, nutrient solubilization, and nitrogen metabolism are also important (Kumar et al. 2015).
3.3 Physiological and Molecular Response of Plant and Microbiota against Stress
Salinity reduces mineral nutrients transport into the plants that retard growth and yield of medicinal plant as well as their active components. Some studies showed that the composition of essential oil of marjoram (Origanum majorana), coriander (Coriandrum sativum), and peppermint (Mentha piperita) have