Biochemical synthesis of gold nanoparticles from leaf protein of Nicotiana tabacum L. cv. xanthi and their physiological, developmental, and ROS scavenging responses on tobacco plant under stress conditions.

The stress conditions imposed by the impact of metal and non-metal oxide nanoparticles over plant systems enhances the synthesis of reactive oxygen species (ROS), resulting in oxidative damage at cellular level. The objective of this study was to synthesise the gold nanoparticles (GNps) from the leaves protein of Nicotiana tabacum L. cv. xanthi, its characterisation, and response on plant physiology and ROS scavenging activity on plants after exposure to different stresses. The authors have treated N. tabacum L. cv. xanthi plants with 100, 200, 300, 400, and 500 ppm biochemically synthesised GNps and examined physiological as well as biochemical changes. Results showed that biochemically synthesised GNps exposure significantly increased the seed germination (P < 0.001), root (P < 0.001), shoot growth (P < 0.001), and antioxidant ability (P < 0.05) of plants depending on bioengineered GNPs concentrations. Low concentrations (200-300 ppm) of GNps boosted growth by ∼50% and significantly increase in photosynthetic parameters such as total chlorophyll content (P < 0.05), membrane ion leakage (P < 0.05) as well as malondialdehyde (P < 0.05) content with respect to untreated plants under stress conditions. The high concentration (400-500 ppm) of GNps affected these parameters in a negative manner. The total antioxidant activity was also elevated in the exposed plants in a dose-dependent manner.

RNAi down-regulation of cinnamate-4-hydroxylase increases artemisinin biosynthesis in Artemisia annua.

Cinnamate-4-hydroxylase (C4H) converts trans-cinnamic acid (CA) to p-coumaric acid (COA) in the phenylpropanoid/lignin biosynthesis pathway. Earlier we reported increased expression of AaCYP71AV1 (an important gene of artemisinin biosynthesis pathway) caused by CA treatment in Artemisia annua. Hence, AaC4H gene was identified, cloned, characterized and silenced in A. annua with the assumption that the elevated internal CA due to knock down may increase the artemisinin yield. Accumulation of trans-cinnamic acid in the plant due to AaC4H knockdown was accompanied with the reduction of p-coumaric acid, total phenolics, anthocyanin, cinnamate-4-hydroxylase (C4H) and phenylalanine ammonia lyase (PAL) activities but increase in salicylic acid (SA) and artemisinin. Interestingly, feeding trans-cinnamic acid to the RNAi line increased the level of artemisinin along with benzoic (BA) and SA with no effect on the downstream metabolites p-coumaric acid, coniferylaldehyde and sinapaldehyde, whereas p-coumaric acid feeding increased the content of downstream coniferylaldehyde and sinapaldehyde with no effect on BA, SA, trans-cinnamic acid or artemisinin. SA is reported earlier to be inducing the artemisinin yield. This report demonstrates the link between the phenylpropanoid/lignin pathway with artemisinin pathway through SA, triggered by accumulation of trans-cinnamic acid because of the blockage at C4H.

Leaf Senescence and GABA Shunt.

Leaf senescence is highly regulated and complex developmental process that involves degradation of macromolecules as well as its recycling. Senescence process involves loss of chlorophyll, degradation of proteins, nucleic acid, lipid and mobilization of nutrients through its transport to the growing parts, developing fruits and seeds. Nitrogen is the most important nutrient to be recycled in senescence process. GABA-transaminase (γ-aminobutyric acid) is found to play very important role in nitrogen recycling process through GABA-shunt. Therefore, it is of interest to review the significance of GABA shunt in leaf senescence.