The adaptation of root cell wall pectin to copper toxicity in two citrus species differing in copper tolerance: remodeling and responding.

Citrus species are prone to be suffered from copper (Cu) toxicity because of improper application of Cu-based agrochemicals. The Cu immobilization mediated by the pectin methylesterase (PME) in the root cell wall (CW) is effective for Cu-detoxification. However, the underlying mechanisms of the structural modification and stress responses of citrus root CW pectin to Cu toxicity were less discussed. In the present study, seedlings of 'Shatian pummelo' [Citrus grandis (L.) Osbeck] and 'Xuegan' [Citrus sinensis (L.) Osbeck], which differed in Cu tolerance, were irrigated with nutrient solution containing 0.5 (as control), 100, 300 or 500 μM Cu for 18 weeks in sandy culture or within 24 hours in hydroponics. By the end of treatments in 18 weeks' sandy culture, the Cu toxicity on the CW pectin content, the Cu distribution, the degree of pectin methylesterification, and the PME enzyme activity were discussed. At the genome-wide level, the PME gene family was identified from two citrus species and the qRT-PCR array of citrus PMEs under control and 300 μM Cu stress for 18 weeks were performed to screen the Cu-responsive PME genes. Moreover, the candidate genes that responded to Cu toxicity were further examined within 24 hours. The results showed that Cu toxicity increased the root CW pectin content. The root CW pectin under Cu toxicity was remodeled by upregulating the expression of the Cu-responsive PME genes followed by increasing PME activity, which mainly promoted the low methylesterased pectin level and the Cu content on the root CW pectin. Compared to C. sinensis, C. grandis root CW had a lower degree of pectin methylesterification and higher Cu content on the Cu-stressed root CW pectin, contributing to its higher Cu tolerance. Our present study provided theoretical evidence on the root CW pectin remodeling in response to Cu toxicity of citrus species.