Heterologous expression of HpBHY and CrBKT increases heat tolerance in Physcomitrella patens

Heat stress can restrict plant growth, development, and crop yield. As essential plant antioxidants, carotenoids play significant roles in plant stress resistance. β-carotene hydroxylase (BHY) and β-carotene ketolase (BKT), which catalyze the conversions of β-carotene to zeaxanthin and β-carotene to canthaxanthin, respectively, are key enzymes in the carotenoid biosynthetic pathway, but little is known about their potential functions in stress resistance. Here, we investigated the roles of β-carotene hydroxylase and β-carotene ketolase during heat stress in Physcomitrella patens through expressing a β-carotene ketolase gene from Chlamydomonas reinhardtii (CrBKT) and a β-carotene hydroxylase gene from Haematococcus pluvialis (HpBHY) in the moss P. patens. In transgenic moss expressing these genes, carotenoids content increased (especially lutein content), and heat stress tolerance increased, with reduced leafy tissue necrosis. To investigate the mechanism of this heat stress resistance, we measured various physiological indicators and found a lower malondialdehyde level, higher peroxidase and superoxide dismutase activities, and higher endogenous abscisic acid and salicylate content in the transgenic plants in response to high-temperature stress. These results demonstrate that CrBKT and HpBHY increase plant heat stress resistance through the antioxidant and damage repair metabolism, which is related to abscisic acid and salicylate signaling.