Insights into the biophysical and molecular mechanisms underlying enhanced growth in Brassica juncea L. under lead-induced hormesis

Abstract

Constant mining, industrial production of Lead (Pb)-based herbicides, pesticides, and batteries contributed to elevated levels of Pb in the soil. The adverse effects of high Pb doses on plants are extensively documented, however, research demonstrating any beneficial effects of low to moderate Pb levels on plants is limited, even though these effects probability are common in nature. This present study aims to examine the hormetic effects of Pb doses causing eustress enhancement in morphological, physiological, biochemical, and molecular parameters of 10 days old Brassica juncea L. seedlings. Our finding revealed that hormetic 18 µM and 36 µM Pb²⁺ doses, significantly increased germination percentage, root-shoot length, and fresh-dry weight of seedlings. Similar hormetic doses result in a significant increase in photosynthetic pigments, antioxidants, and chlorophyll fluorescence parameters while decreasing the pheophytin, reactive oxygen species (ROS), and malondialdehyde (MDA) content in seedlings. These hormetic Pb doses treated seedlings also enhanced maximum gene expression of growth and photosynthesis-related genes such as Chlorophyll synthase (Chls), Plastid terminal oxidase (PTOX), Photosystem II protein D1 (psbA), Phytoene Synthase (PSY), Violaxanthin de-epoxidase (VD), Chalcone synthase (CHL), and Phenylalanine ammonia-lyase (PAL). Also, these hormetic Pb²⁺ doses regulate the expression of genes involved in reducing oxidative stress such as Respiratory burst oxidase homolog (RBOH), Chlorophyllase 1 (Chlase), Ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit (rbcs), Mitogen-activated protein kinases 2, 4, and 6 (MAPK2, MAPK4, MAPK6). The hormetic response of seedlings to Pb²⁺ suggests a potential adaptive underlying mechanism that could increase plant growth and yield in Pb-contaminated environments. This research improves our understanding of plant-metal interactions, provides insights into changes in plant biophysiology under mild Pb-contaminated soils and strategies for crop enhancement programs.