Intrinsic Mechanism of CaCl Alleviation of HO Inhibition of Pea Primary Root Gravitropism.

Normal root growth is essential for the plant uptake of soil nutrients and water. However, exogenous HO inhibits the gravitropic growth of pea primary roots. It has been shown that CaCl application can alleviate HO 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 CaCl to alleviate the inhibition of pea primordial root gravitropism by HO. The results showed that the addition of CaCl (10 mmol·L) under HO stress (150 mmol·L) significantly increased the HO 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 , a key gene for starch synthesis, and activated the calcium and phytohormone signaling pathways. In summary, 10 mmol·L CaCl could alleviate HO 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 CaCl promotes the gravitropism of pea primary roots under HO treatment.