Regulation of Endogenous Proline Biosynthesis Influences Boron Toxicity Response in <i>Arabidopsis thaliana</i>

The relation between endogenous proline and the tolerance to boron toxicity (BT) was investigated in Arabidopsis thaliana. BT resulted in chlorosis, although esk1-1 was the least affected. Under BT, esk1-1, the proline-overproducing mutant, had the highest osmolarity and proline values, whereas p5cs1-4, the proline-deficient mutant, had the lowest values. Catalase, peroxidase, and superoxide dismutase activities were the most stimulated in Col-0 under BT. Ascorbic acid content was the highest in esk1-1 under BT. The highest expression levels of BOR1 and BOR2 was determined at esk1-1. BOR2 expression was the lowest in p5cs1-4. BOR4 expression level were higher than p5cs1-4 under BT. The NIP6;1 gene expression in Col-0 was higher than esk1-1 and p5cs1-4. Boron content was highest in esk1-1 and lowest in p5cs1-4 leaves. Osmotic regulation, GSH biosynthesis, and induction of B transport proteins were the main mechanisms of esk1-1's tolerance to BT. To avoid BT, p5cs1-4 plants prefer to raise the water potential and to keep boron out by balancing boron uptake and excretion.