Physiological and molecular responses of tomato and citrus to chromium (III) stress at early growth stage.

Chromium (Cr) contamination poses food safety and environmental challenges, yet the early-stage physiological and molecular responses to Cr(III) stress remain unclear. Citrus and tomato are economically important crops representing woody and herbaceous species, making them valuable models for studying heavy metal toxicity in plants. This study investigates the impact of Cr (III) exposure on citrus and tomato seedlings, with a focus on physiological phenotypes and transcriptional response. Citrus seed germination declines with increasing Cr(III) concentrations, while low Cr(III) levels promote tomato germination, with inhibition occurring above 1 g/L. Under hydroponic conditions, Cr (III) severely hampers root and leaf growth in both citrus and tomato plants, accompanied by decreased net photosynthetic rate. Using a GFP-based confocal microscopy system, we observed reduced fluorescence intensity within three days of Cr(III) exposure (100 mg/L and 500 mg/L), indicating early cellular damage. Biochemical assays revealed oxidative stress, marked by increased H₂O₂, malondialdehyde (MDA), and antioxidant enzyme activity. Additionally, low Cr (III) concentrations could result in the death of various microorganisms, including Escherichia coli, Agrobacterium rhizogenes, and Agrobacterium tumefaciens. Transcriptomic analysis identified differentially expressed genes related to "MAPK signaling pathway" and "Plant hormone signal transduction pathway". Transcription of many transcription factors, such as bHLH, WRKY, and MYB, also underwent significant changes.