Improving the Production of Secondary Metabolites via the Application of Biogenic Zinc Oxide Nanoparticles in the Calli of Delonix elata: A Potential Medicinal Plant

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The implementation of nanotechnology in the field of plant tissue culture has demonstrated
an interesting impact on in vitro plant growth and development. Furthermore, the plant tissue culture
accompanying nanoparticles has been showed to be a reliable alternative for the biosynthesis of secondary
metabolites. Herein, the effectiveness of zinc oxide nanoparticles (ZnONPs) on the growth of
Delonix elata calli, as well as their phytochemical profiles, were investigated. Delonix elata seeds were
collected and germinated, and then the plant species was determined based on the PCR product sequence
of ITS1 and ITS4 primers. Afterward, the calli derived fromDelonix elata seedlings were subjected
to 0, 10, 20, 30, 40, and 50 mg/L of ZnONPs. The ZnONPs were biologically synthesized using the
Ricinus communis aqueous leaf extract, which acts as a capping and reducing agent, and zinc nitrate
solution. The nanostructures of the biogenic ZnONPs were confirmed using different techniques like
UV–visible spectroscopy (UV), zeta potential measurement, Fourier transform infrared spectra (FTIR),
X-ray diffraction (XRD), and scanning electron microscopy (SEM). Adding 30 mg/L of ZnONPs to
the MS media (containing 2.5 M 2,4-D and 1 M BAP) resulted in the highest callus fresh weight
(5.65 g) compared to the control and other ZnONP treatments. Similarly, more phenolic accumulation
(358.85 g/g DW) and flavonoid (112.88 g/g DW) contents were achieved at 30 mg/L. Furthermore,
the high-performance liquid chromatography (HPLC) analysis showed significant increments in gallic
acid, quercetin, hesperidin, and rutin in all treated ZnONP calli compared to the control. On the other
hand, the gas chromatography and mass spectroscopy (GC-MS) analysis of the calli extracts revealed
that nine phytochemical compounds were common among all extracts. Moreover, themost predominant
compound found in calli treatedwith 20, 30, 40, and 50mg/L of ZnONPswas bis(2-ethylhexyl) phthalate,
with percentage areas of 27.33, 38.68, 22.66, and 17.98%, respectively. The predominant compounds in
the control and in calli treated with 10 mg/L of ZnONPs were octadecanoic acid, 2-propenyl ester and
heptanoic acid. In conclusion, in this study, green ZnONPs exerted beneficial effects on Delonix elata calli
and improved their production of bioactive compounds, especially at a dose of 30 mg/L.