Abstract
Plants can obtain superinduction of defense against unpredictable challenges based on prior acclimation, but the mechanisms involved in the acclimation memory are little known. The objective of this study was to characterize mechanisms of heat acclimation memory in Rhododendron hainanense, a thermotolerant wild species of azalea. Pretreatment of a 2-d recovery (25/18°C, day/night) after heat acclimation (37°C, 1 h) (AR-pt) did not weaken but enhanced acquired thermotolerance in R. hainanense with less damaged phenotype, net photosynthetic rate, and membrane stability than non-acclimation pretreated (NA-pt) plants. Combined transcriptome and proteome analysis revealed that a lot of heat-responsive genes still maintained high protein abundance rather than transcript level after the 2-d recovery. Photosynthesis-related genes were highly enriched and most decreased under heat stress (HS: 42°C, 1 h) with a less degree in AR-pt plants compared to NA-pt. Sustainably accumulated chloroplast-localized heat shock proteins (HSPs), Rubisco activase 1 (RCA1), beta-subunit of chaperonin-60 (CPN60β), and plastid transcriptionally active chromosome 5 (pTAC5) in the recovery period probably provided equipped protection of AR-pt plants against the subsequent HS, with less damaged photochemical efficiency and chloroplast structure. In addition, significant higher levels of RCA1 transcripts in AR-pt compared to NA-pt plants in early stage of HS showed a more important role of RCA1 than other chaperonins in heat acclimation memory. The novel heat-induced RCA1, rather than constitutively expressed RCA2 and RCA3, showed excellent thermostability after long-term HS (LHS: 42/35°C, 7 d) and maintained balanced Rubisco activation state in photosynthetic acclimation. This study provides new insights into plant heat acclimation memory and indicates candidate genes for genetic modification and molecular breeding in thermotolerance improvement.
Highlights
Sessile plants constantly experience daily and seasonal fluctuations of environmental temperatures in nature
To characterize defense responses to heat acclimation in the thermotolerant azalea R. hainanense, three groups of plants were exposed to non-acclimation pretreatment (NA-pt), acclimation pretreatment at 37°C for 1 h (AC-pt), and acclimation with a 2-d recovery pretreatment (AR-pt) (Figure 1A), respectively, before heat stress (HS) (42/35°C, day/night)
We found that a 2-d recovery to control temperature following heat acclimation did not weaken but even enhance the acquired thermotolerance in the heat-tolerant azalea R. hainanense
Summary
Sessile plants constantly experience daily and seasonal fluctuations of environmental temperatures in nature. The ability to retain “a memory” or “stress imprint” of prior exposure to certain priming conditions for a certain length of time can make a plant more tolerant to future stress (Bruce et al, 2007; Wu et al, 2013; Martinez-Medina et al, 2016). Defense priming is an adaptive trait for promoting the plant to a persistently primed state of defense readiness for unpredictable environments, resulting in enhanced pest and disease resistance and abiotic stress tolerance (Conrath et al, 2006; Walter et al, 2013; Martinez-Medina et al, 2016). Frequent recurring stress memory can extend into future generations (RamirezCarrasco et al, 2017), which may be an evolutionary force for plants adapting to rugged environments. It is necessary to investigate the mechanisms involved in stress memory and apply it into tolerance improvement or genetic modification for cultivar breeding by regulation of plant natural defense system
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