Growth, biochemical, and ultrastructural analyses of Acacia modesta callus Wall. in response to different concentrations of growth regulators and heavy metals

Four concentrations (2, 4, 6, and 8 mg/L) of benzyladenine (BA) and kinetin (Kn) along with three concentrations (5, 10, and 15 mg/L) of 2,4-dichlorophenoxyacetic acid (2.4-D) were incorporated into Murashige and Skoog (MS) medium to investigate their effectiveness in inducing callus formation from axillary buds of A. modesta . It was found that the 2,4-D concentration of 10 and 15 mg/L had the most favorable effect on the induction rates and growth characteristics of A. modesta. Afterwards, the impacts of various concentrations (0, 0.125, 0.25, 0.5, and 1 ppm) of cadmium (Cd), nickel (Ni), and lead (Pb), on the morphological attributes and primary metabolites were measured in A. modesta calli grown at MS medium supplemented with 10 and 15 mg/L 2,4-D. In addition, ultrastructural attributes were investigated using transmission electron microscopy (TEM) in A. modesta calli grown at the highest metal concentrations. At low concentrations of the three heavy metals, the calli exhibited minimal morphological changes. However, at the highest concentrations, callus growth was significantly reduced, as evidenced by lower fresh weights and altered morphological characteristics. High metal concentrations caused compact, dehydrated, and necrotic tissues. Cd exposure induced the most severe effects, including cell wall thickening, cytoplasmic shrinkage, and plasmolysis, which were observed. Ni treatment led to reduced vascular bundle size, mesophyll thickness, and chloroplast integrity, while Pb exposure resulted in extensive vacuolation and mitochondrial disruption. The concentrations (0.125 and 0.25 ppm) of the three metals exhibited the highest contents of carbohydrates and proteins. The highest concentrations (1 ppm) of the metals had the lowest contents of carbohydrates and proteins. A. modesta callus cultures displayed varying degrees of tolerance to Cd, Ni, and Pb indicated by its ability to form callus at low metal concentrations, as well as the morphological adaptations to high metal levels, suggests potential for phytoremediation applications.