Freezing treatment under light conditions leads to a dramatic enhancement of freezing tolerance in cold‐acclimated Arabidopsis

Abstract

AbstractOverwintering plants survive subzero temperatures by cold acclimation (CA), wherein they acquire freezing tolerance through short‐term exposure to low temperatures above 0°C. The freezing tolerance of CA plants increases when they are subsequently exposed to mild subzero temperatures, a phenomenon known as second‐phase cold hardening (2PH). Here, we explored the molecular mechanism and physiological conditions of 2PH. The results show that, compared with supercooling, a freezing treatment during 2PH after CA enhanced the freezing tolerance of Arabidopsis. This required CA as a pretreatment, and was designated as second‐phase freezing acclimation (2PFA). Light increased the effect of 2PFA to enhance freezing tolerance after CA. C‐repeat binding factor and cold‐regulated genes were downregulated by light during the 2PFA treatment, a different transcription profile from that during CA. The freezing tolerance of 2PFA plants was decreased by the presence of the photosynthetic electron transfer inhibitor 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea during the 2PFA treatment. Compared with wild‐type plants, phototropin1,2 and phyb mutants showed lower freezing tolerance after 2PFA treatment. These results show that exposure to freezing after CA increases freezing tolerance as a secondary process, and that freezing under light conditions further increases freezing tolerance via pathways involving photoreceptors and photosynthetic electron transfer.