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Abiotic stresses, mainly salinity and drought, are the most important environmental threats that constrain worldwide food security by hampering plant growth and productivity. Plants cope with the adverse effects of these stresses by implementing a series of morpho-physio-biochemical adaptation mechanisms. Accumulating effective osmo-protectants, such as proline and glycine betaine (GB), is one of the important plant stress tolerance strategies. These osmolytes can trigger plant stress tolerance mechanisms, which include stress signal transduction, activating resistance genes, increasing levels of enzymatic and non-enzymatic antioxidants, protecting cell osmotic pressure, enhancing cell membrane integrity, as well as protecting their photosynthetic apparatus, especially the photosystem II (PSII) complex. Genetic engineering, as one of the most important plant biotechnology methods, helps to expedite the development of stress-tolerant plants by introducing the key tolerance genes involved in the biosynthetic pathways of osmolytes into plants. Betaine aldehyde dehydrogenase (BADH) is one of the important genes involved in the biosynthetic pathway of GB, and its introduction has led to an increased tolerance to a variety of abiotic stresses in different plant species. Replacing down-regulated ferredoxin at the acceptor side of photosystem I (PSI) with its isofunctional counterpart electron carrier (flavodoxin) is another applicable strategy to strengthen the photosynthetic apparatus of plants under stressful conditions. Heterologous expression of microbially-sourced flavodoxin (Fld) in higher plants compensates for the deficiency of ferredoxin expression and enhances their stress tolerance. BADH and Fld are multifunctional transgenes that increase the stress tolerance of different plant species and maintain their production under stressful situations by protecting and enhancing their photosynthetic apparatus. In addition to increasing stress tolerance, both BADH and Fld genes can improve the productivity, symbiotic performance, and longevity of plants. Because of the multigenic and complex nature of abiotic stresses, the concomitant delivery of BADH and Fld transgenes can lead to more satisfying results in desired plants, as these two genes enhance plant stress tolerance through different mechanisms, and their cumulative effect can be much more beneficial than their individual ones. The importance of BADH and Fld genes in enhancing plant productivity under stress conditions has been discussed in detail in the present review.

Bioactive compounds of medicinal plants have a wide range of applications in pharmaceutical, food and other industries. In vitro culture systems have great potential for sustainable production of bioactive compounds of medicinal plants. In the present study, the individual and combined effects of a stress tolerance-inducing (salicylic acid) and a stress-inducing elicitor (polyethylene glycol) were evaluated on regeneration efficiency, antioxidants activity and phytochemical profile of in vitro shoot cultures of ajowan. Different concentrations of salicylic acid (SA) (0, 10, 20, 40, 80 µM) and polyethylene glycol (PEG 6000) (0, 1, 2, 5%) were added to the shoot regeneration Murashige and Skoog medium containing Kin (1.5 mg/L) and NAA (0.25 mg/L) plant growth regulators. Salicylic acid reduced the adverse effect of PEG treatment on number of regenerated shoots and in vitro rooting. The activities of catalase, superoxide dismutase, and peroxidase enzymatic antioxidants were significantly increased in SA + PEG treated plants. The gas chromatography-mass spectrometry (GC-MS)-profiling revealed quantitative and qualitative phytochemical differences between control and SA + PEG treated plants. The greatest means of p-cymene and thymol bioactive compounds were obtained from in vitro shoots treated with 5% PEG + 40 µM SA. The inter-simple sequence repeats (ISSR) markers proved the genetic stability of in vitro regenerated plants. The presented protocol is useful for large-scale sustainable production of secondary metabolites (SMs) of medicinal plants. The same strategy (stress tolerance-inducing elicitor + stress-inducing elicitor) is applicable to increase valuable SMs in other production systems such as hydroponic, greenhouse and field conditions.

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Bioactive compounds of medicinal plants have a wide range of applications in pharmaceutical, food and other industries. In vitro culture systems have great potential for sustainable production of bioactive compounds of medicinal plants. In the present study, the individual and combined effects of a stress tolerance-inducing (salicylic acid) and a stress-inducing elicitor (polyethylene glycol) were evaluated on regeneration e ciency, antioxidants activity and phytochemical pro le of in vitro shoot cultures of ajowan. Different concentrations of salicylic acid (SA) (0, 10, 20, 40, 80 µM) and polyethylene glycol (PEG 6000) (0, 1, 2, 5%) were added to the shoot regeneration Murashige and Skoog medium containing Kin (1.5 mg/L) and NAA (0.25 mg/L) plant growth regulators. Salicylic acid reduced the adverse effect of PEG treatment on number of regenerated shoots and in vitro rooting. The activities of catalase, superoxide dismutase, and peroxidase enzymatic antioxidants were signi cantly increased in SA + PEG treated plants. The gas chromatography-mass spectrometry (GC-MS)-pro ling revealed quantitative and qualitative phytochemical differences between control and SA + PEG treated plants. The greatest means of p-cymene and thymol bioactive compounds were obtained from in vitro shoots treated with 5% PEG + 40 µM SA. The inter-simple sequence repeats (ISSR) markers proved the genetic stability of in vitro regenerated plants. The presented protocol is useful for large-scale sustainable production of secondary metabolites (SMs) of medicinal plants. The same strategy (stress tolerance-inducing elicitor + stress-inducing elicitor) is applicable to increase valuable SMs in other production systems such as hydroponic, greenhouse and eld conditions.