Photoprotection contributes to freezing tolerance as revealed by RNA-seq profiling ofRhododendronleaves during cold acclimation and deacclimation over time.

Abstract

Cold acclimation (CA) and deacclimation (DA), which are often accompanied by changes in freezing tolerance (FT), carbohydrates and hormones, are crucial for winter survival, especially under global warming. Plants with weak CA and premature DA caused by warm winters and/or unseasonal warm spells can be easily injured by adverse reactions to cold. Thus, understanding the molecular mechanisms of FT is imperative. In this study, we used high-throughput RNA-seq to profile the CA and DA of leaves of overwintering Rhododendron “Miyo-no-Sakae” over time; these leaves do not undergo dormancy but do undergo photoprotection during CA, and they do not grow during DA. Using Mfuzz and weighted gene coexpression network analysis, we identified specific transcriptional characteristics in each phase of CA and DA and proposed networks involving coexpressed genes and physiological traits. In particular, we discovered that the circadian rhythm is critical for obtaining the strongest FT, and high expression of circadian rhythm-related genes might be linked to sugar accumulation during winter. Furthermore, evergreen leaves exhibited robust photoprotection during winter, as revealed by high values of nonphotochemical quenching, high expression of transcripts annotated as “early light-induced proteins”, loss of granum stacks and destacking of thylakoids, all of which were alleviated during DA. The strong requirement of photoprotection could be the reason for decreased abscisic acid (ABA) and jasmonic acid (JA) contents during CA, and decreases in ABA and JA contents may contribute to decreases in lignin content. Our data suggest that the molecular mechanisms of FT in overwintering leaves are unique, which may be due to the high requirements for photoprotection during winter.

Photoprotection conferring plant tolerance to freezing stress through rescuing photosystem in evergreen Rhododendron

“…Although ELIPs have been found for so many years and proved to be photoprotective and low temperature‐sensitive (Liu et al, 2020, 2021; Noren et al, 2003; Wang et al, 2009; Wei et al, 2005), no research has yet looked into whether they directly contribute to plants' freezing tolerance. In this study, we demonstrated that RhELIP3 ‐OEs did show less freeze‐injury when compared with the WT after freezing treatment (Figure 3d,e), which could through protecting the PSII system due to the higher F v / F m , ΦII, qP, ETR and qL in RhELIP3 ‐OEs after 3‐day resumption (Figure 3f,g).…”

“…We found that RhHY5 ‐OEs were more freezing tolerant than RhELIP3 ‐OEs, while the resumption of PSII‐related parameters of RhHY5 ‐OEs was not as good as RhELIP3 ‐OEs (Figures 3d,e and 5d,e), which suggests that the RhELIP3 ‐OEs improved the plants' freezing tolerance primarily through the pathway of photoprotection while RhHY5‐OEs did not. The upregulation of AtELIP1/2 in RhHY5‐OEs suggested that RhHY5 might partially participate in inducing the RhELIP3 expression, and the large amount of upregulation of AtELIP1/2 in the WT suggests that other low‐temperature sensing factors, such as CRY1, may also play a role in inducing the expressions of ELIPs (Liu et al, 2022). Thus, although RhHY5 could significantly induce the expressions of RhELIP3 or AtELIP1/2 , its capacity to induce the massive expressions of ELIPs under low temperatures might be limited.…”

“…Transcripts identified as ELIPs exhibited 100-fold raises in winter than | 2093 that in summer in evergreens Rhododendron 'Elsie Lee' and 'Miyo-no-Sakae' (Liu et al, 2020(Liu et al, , 2022, as well as in Subalpine fir and Lodgepole pine (Zarter et al, 2006).…”

“…For instance, the photosynthetic activity was severely downregulated, while the NPQ was significantly upregulated in evergreen leaves of Rhododendron catawbiense under low temperatures when compared with that under warm temperatures (Liu et al, 2019). Transcripts identified as ELIPs exhibited 100‐fold raises in winter than that in summer in evergreens Rhododendron ‘Elsie Lee’ and ‘Miyo‐no‐Sakae’ (Liu et al, 2020, 2022), as well as in Subalpine fir and Lodgepole pine (Zarter et al, 2006).…”

Physiological and molecular mechanisms of the response of roots of Pinus massoniana Lamb. to low-temperature stress

“…L., Pinus sylvestris , and Picea abies ( Joosen et al, 2006 ; Maestrini et al, 2009 ; Benedict et al, 2010 ; Vergara et al, 2022 ), and have addressed antioxidant enzymes ( Renaut et al, 2006 ) and TFs ( Vergara et al, 2022 ). Recently, researchers have paid more attention to the relationship between cold resistance and lignin and starch synthesis pathways ( Zhao et al, 2019 ; Liang et al, 2022 ; Liu et al, 2022 ).…”