Combating photooxidative stress in green hairy roots of Daucus carota cultivated under light irradiation

Light spectra manipulation stimulates growth, specialized metabolites and nutritional quality in Anethum graveolens

Light-Emitting Diodes (LED) play a major role in manipulating light spectra that helps in regulating the growth and specialized metabolite synthesis relevant to the plant defence system. In this study, we assessed photosynthetic performance, phytonutrients, and anatomical variations of an aromatic herb Anethum graveolens (also known as dill), grown under various combinations of LED lights viz. red (100R:0B), red:blue (50R:50B); blue (0R:100B) and warm white (WW, served as control). Exposure to 0R:100B LED lights led to the tallest stem height, whereas, the number of leaves were highest under 50R:50B LED lights. The photosynthetic performance was observed to be highest under 50R:50B LED lights. HPLC analysis revealed chlorogenic acid and rosmarinic acid as the major phenolic compounds accumulated under different spectral irradiations. The highest chlorogenic acid content was observed in 50R:50B LED treated dill plants, while 100R:0B light showed the highest accumulation of rosmarinic acid. Dill plants grown under 50R:50B light displayed a relatively higher content of volatile compounds including, myristicin (phenylpropene), psi-limonene, and α-phellandrene (monoterpenoids). Expression analyses of candidate genes of phenylpropanoid and monoterpenoid biosynthetic pathways showed good correlations with the enhanced phenolic compounds and monoterpenes detected under appropriate light treatments. Further, the stem anatomy revealed higher vascularization under the influence of 0R:100B LED lights, whereas, intense histochemical localization of specialized metabolites could be correlated with enhanced accumulation of phenolic compounds and terpenoids observed in this study. Taken together, these studies suggest that proper combinations of blue and red spectra of light could play important role to augment the growth and phytochemical characteristics of dill, thus improving its value addition in the food industry.

Hairy Root Cultures as a Source of Phenolic Antioxidants: Simple Phenolics, Phenolic Acids, Phenylethanoids, and Hydroxycinnamates

Plant-derived antioxidants are intrinsic components of human diet and factors implicated in tolerance mechanisms against environmental stresses in both plants and humans. They are being used as food preservatives and additives or ingredients of cosmetics. For nearly forty years, Rhizobium rhizogenes-transformed roots (hairy roots) have been studied in respect to their usability as producers of plant specialized metabolites of different, primarily medical applications. Moreover, the hairy root cultures have proven their value as a tool in crop plant improvement and in plant secondary metabolism investigations. Though cultivated plants remain a major source of plant polyphenolics of economic importance, the decline in biodiversity caused by climate changes and overexploitation of natural resources may increase the interest in hairy roots as a productive and renewable source of biologically active compounds. The present review examines hairy roots as efficient producers of simple phenolics, phenylethanoids, and hydroxycinnamates of plant origin and summarizes efforts to maximize the product yield. Attempts to use Rhizobium rhizogenes-mediated genetic transformation for inducing enhanced production of the plant phenolics/polyphenolics in crop plants are also mentioned.

Steviol glycosides profile in Stevia rebaudiana Bertoni hairy roots cultured under oxidative stress-inducing conditions

The ability to synthesize particular steviol glycosides (SvGls) was studied in Stevia rebaudiana Bertoni hairy roots (HR) grown in the light or in the dark under the influence of different osmotic active compounds. Manipulation of culture conditions led to changes in the morphology and growth rate of HR, as well as to an increase in oxidative stress manifested as an enhancement in endogenous hydrogen peroxide concentration in the cultured samples. The highest level of H₂O₂ was noted in HR cultured under light or in the medium with the highest osmotic potential. This correlated with the highest increase in the expression level of ent-kaurenoic acid hydroxylase, responsible for the redirection of metabolic route to SvGls biosynthesis pathway. An analysis of transcriptional activity of some UDPglucosyltransferase (UGT85c2, UGT74g1, UGT76g1) revealed that all of them were upregulated due to the manipulation of culture conditions. However, the level of their upregulation depended on the type of stress factor used in our experiment. Analysis of SvGls content revealed that HR grown under all applied conditions were able to synthesize and accumulate several SvGls but their concentration differed between the samples across the different conditions. The level of rebaudioside A concentration exceeded the content of stevioside in HR in all tested conditions. Concomitantly, the presence of some minor SvGls, such as steviolbioside and rebaudioside F, was confirmed only in HR cultured in the lowest osmotic potential of the medium while rebaudioside D was also detected in the samples cultured in the media supplemented with NaCl or PEG.Key Points● Several steviol glycosides are synthesized in hairy roots of S. rebaudiana.● Light or osmotic factors cause enhancement in oxidative stress level in hairy roots.● It correlates with a significant increase in the level of KAH expression.● UGTs expression and steviol glycosides content depends on culture conditions.

Redirection of metabolite biosynthesis from hydroxybenzoates to volatile terpenoids in green hairy roots of Daucus carota

A metabolic shift in green hairy root cultures of carrot from phenylpropanoid/benzenoid biosynthesis toward volatile isoprenoids was observed when compared with the metabolite profile of normal hairy root cultures. Hairy roots cultures of Daucus carota turned green under continuous illumination, while the content of the major phenolic compound p-hydroxybenzoic acid (p-HBA) was reduced to half as compared to normal hairy roots cultured in darkness. p-Hydroxybenzaldehyde dehydrogenase (HBD) activity was suppressed in the green hairy roots. However, comparative volatile analysis of 14-day-old green hairy roots revealed higher monoterpene and sesquiterpene contents than found in normal hairy roots. Methyl salicylate content was higher in normal hairy roots than in green ones. Application of clomazone, an inhibitor of 1-deoxy-D-xylulose 5-phosphate synthase (DXS), reduced the amount of total monoterpenes and sesquiterpenes in green hairy roots compared to normal hairy roots. However, methyl salicylate content was enhanced in both green and normal hairy roots treated with clomazone as compared to their respective controls. Because methyl-erythritol 4-phosphate (MEP) and phenylpropanoid pathways, respectively, contribute to the formation of monoterpenes and phenolic acids biosynthesis, the activities of enzymes regulating those pathways were measured in terms of their in vitro activities, in both green and normal hairy root cultures. These key enzymes were 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR), an early regulatory enzyme of the MEP pathway, pyruvate kinase (PK), an enzyme of primary metabolism related to the MEP pathway, shikimate dehydrogenase (SKDH) which is involved in biosynthesis of aromatic amino acids, and phenylalanine ammonia-lyase (PAL) that catalyzes the first step of phenylpropanoid biosynthesis. Activities of DXR and PK were higher in green hairy roots as compared to normal ones, whereas the opposite trend was observed for SKDH and PAL activities. Gene expression analysis of DXR and PAL showed trends similar to those for the respective enzyme activities. Based on these observations, we suggest a possible redirection of metabolites from the primary metabolism toward isoprenoid biosynthesis, limiting the phenolic biosynthetic pathway in green hairy roots grown under continuous light.

Metabolic shift from withasteroid formation to phenylpropanoid accumulation in cryptogein-cotransformed hairy roots of Withania somnifera (L.) Dunal

Cotransformed hairy roots containing a gene that encodes a fungal elicitor protein, β-cryptogein, were established in Withania somnifera, a medicinal plant widely used in Indian systems of medicine. To find out whether β-cryptogein protein endogenously elicits the pathway of withasteroid biosynthesis, withaferin A and withanolide A contents along with transcript accumulation of farnesyl pyrophosphate (FPP) synthase, 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGR), and sterol glycosyltransferase (SGT) were analyzed in both cryptogein-cotransformed and normal hairy roots of W. somnifera. It was observed that the withaferin A and withanolide A contents were drastically higher in normal hairy roots than cryptogein-cotransformed ones. Similar trends were also observed on the levels of transcript accumulation. Subsequently, the enzyme activity of phenylalanine ammonia lyase (PAL), one of the key enzymes of phenylpropanoid pathway, was measured in both cryptogein-cotransformed and normal hairy roots of W. somnifera along with the levels of PAL transcript accumulation. Upliftment of PAL activity was observed in cryptogein-cotransformed hairy roots as compared to the normal ones, and the PAL expression also reflected a similar trend, i.e., enhanced expression in the cryptogein-cotransformed lines. Upliftment of wall-bound ferulic acid accumulation was also observed in the cryptogein-cotransformed lines, as compared to normal hairy root lines. Thus, the outcome of the above studies suggests a metabolic shift from withanolide accumulation to phenylpropanoid biosynthesis in cryptogein-cotransformed hairy roots of W. somnifera.