Intrinsic Mechanism of CaCl₂ Alleviation of H₂O₂ Inhibition of Pea Primary Root Gravitropism

Normal root growth is essential for the plant uptake of soil nutrients and water. However, exogenous H₂O₂ inhibits the gravitropic growth of pea primary roots. It has been shown that CaCl₂ application can alleviate H₂O₂ inhibition, but the exact alleviation mechanism is not clear. Therefore, the present study was carried out by combining the transcriptome and metabolome with a view to investigate in depth the mechanism of action of exogenous CaCl₂ to alleviate the inhibition of pea primordial root gravitropism by H₂O₂. The results showed that the addition of CaCl₂ (10 mmol·L^-1) under H₂O₂ stress (150 mmol·L^-1) significantly increased the H₂O₂ and starch content, decreased peroxidase (POD) activity, and reduced the accumulation of sugar metabolites and lignin in pea primary roots. Down-regulated genes regulating peroxidase, respiratory burst oxidase, and lignin synthesis up-regulated PGM1, a key gene for starch synthesis, and activated the calcium and phytohormone signaling pathways. In summary, 10 mmol·L^-1 CaCl₂ could alleviate H₂O₂ stress by modulating the oxidative stress response, signal transduction, and starch and lignin accumulation within pea primary roots, thereby promoting root gravitropism. This provides new insights into the mechanism by which CaCl₂ promotes the gravitropism of pea primary roots under H₂O₂ treatment.