Abstract
Apple (Malus domestica Borkh.), an economically important tree fruit worldwide, frequently suffers from temperature stress during growth and development, which strongly affects the yield and quality. Heat shock protein 20 (HSP20) genes play crucial roles in protecting plants against abiotic stresses. However, they have not been systematically investigated in apple. In this study, we identified 41 HSP20 genes in the apple ‘Golden Delicious’ genome. These genes were unequally distributed on 15 different chromosomes and were classified into 10 subfamilies based on phylogenetic analysis and predicted subcellular localization. Chromosome mapping and synteny analysis indicated that three pairs of apple HSP20 genes were tandemly duplicated. Sequence analysis revealed that all apple HSP20 proteins reflected high structure conservation and most apple HSP20 genes (92.6%) possessed no introns, or only one intron. Numerous apple HSP20 gene promoter sequences contained stress and hormone response cis-elements. Transcriptome analysis revealed that 35 of 41 apple HSP20 genes were nearly unchanged or downregulated under normal temperature and cold stress, whereas these genes exhibited high-expression levels under heat stress. Subsequent qRT-PCR results showed that 12 of 29 selected apple HSP20 genes were extremely up-regulated (more than 1,000-fold) after 4 h of heat stress. However, the heat-upregulated genes were barely expressed or downregulated in response to cold stress, which indicated their potential function in mediating the response of apple to heat stress. Taken together, these findings lay the foundation to functionally characterize HSP20 genes to unravel their exact role in heat defense response in apple.
Highlights
Temperature is an important factor affecting plant growth and geographical distribution (Wang et al, 2016)
After growing for 70 days, uniform seedlings were randomly divided into three groups and transferred to different chambers for temperature stress: (1) control (CK), growth chamber were maintained at 25 ± 1◦C, 16-h photoperiod (160 μmol m−2 s−1) and relative humidity of 60–70%; (2) heat stress (HS), growth chamber maintained at 40 ± 1◦C; (3) cold stress (CS), growth chamber maintained at 4 ± 1◦C, 16-h photoperiod (160 μmol m−2 s−1) and relative humidity of 60–70%
A total of 45 Heat shock protein 20 (HSP20) protein sequences were found in the apple cultivar “Golden Delicious” genome
Summary
Temperature is an important factor affecting plant growth and geographical distribution (Wang et al, 2016). Most plants undergo optimal growth and development within a narrow temperature range and can only tolerate minor fluctuations. Fluctuations beyond optimal range result in temperature stress, which is one of the most severe environmental stresses affecting plant growth, development and survival worldwide (Peleg and Blumwald, 2011). Plants have developed a series of physiological and molecular strategies to overcome temperature stress over evolutionary time (Asea et al, 2016; Huo et al, 2020). Heat shock proteins (HSPs) are one of the strategies, and HSPs are essential in regulating growth, development and stress response in plants (Waters et al, 1996; Wang et al, 2004; Asea et al, 2016; He et al, 2019)
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