Time-Resolved Analysis of Candidate Gene Expression and Ambient Temperature During Bud Dormancy in Apple.

Janne Lempe,Henryk Flachowsky,Andreas Peil

doi:10.3389/fpls.2021.803341

Abstract

Winter dormancy – a period of low metabolic activity and no visible growth – appears as an adaptation to harsh winter conditions and can be divided into different phases. It is tightly controlled by environmental cues, with ambient temperature playing a major role. During endodormancy, a cultivar-specific amount of cold needs to be perceived, and during ecodormancy, heat hours accumulate before bud burst and anthesis in spring. Expression analysis, performed in several key fruit tree species, proved to be very useful in elucidating the molecular control of onset and release of dormancy. However, the time resolution of these experiments has been limited. Therefore, in this study, dense time-series expression analysis was conducted for 40 candidate genes involved in dormancy control, under the cool-temperate climate conditions in Dresden. Samples were taken from the cultivars ‘Pinova’ and ‘Gala,’ which differ in flowering time. The set of candidate genes included well-established dormancy genes such as DAM genes, MdFLC-like, MdICE1, MdPRE 1, and MdPIF4. Furthermore, we tested genes from dormancy-associated pathways including the brassinosteroid, gibberellic acid, abscisic acid (ABA), cytokinin response, and respiratory stress pathways. The expression patterns of well-established dormancy genes were confirmed and could be associated with specific dormancy phases. In addition, less well-known transcription factors and genes of the ABA signaling pathway showed associations with dormancy progression. The three ABA signaling genes HAB1_chr15, HAI3, and ABF2 showed a local minimum of gene expression in proximity of the endodormancy to ecodormancy transition. The number of sampling points allowed us to correlate expression values with temperature data, which revealed significant correlations of ambient temperature with the expression of the Malus domestica genes MdICE1, MdPIF4, MdFLC-like, HAB1chr15, and the type-B cytokinin response regulator BRR9. Interestingly, the slope of the linear correlation of temperature with the expression of MdPIF4 differed between cultivars. Whether the strength of inducibility of MdPIF4 expression by low temperature differs between the ‘Pinova’ and ‘Gala’ alleles needs to be tested further.

Highlights

In deciduous fruit trees such as apple (Malus × domestica Borkh.), phases of high metabolic activity, growth, and reproduction alternate with phases of no growth and low levels of activity, called dormancy
The MADS-box transcription factor genes MdDAM1, MdDAM2, MdDAM4, MdSVPa, and MdSVPb are well known to control the progression of dormancy in fruit trees of the Rosacea (Falavigna et al, 2019)
The transcription factor gene Inducer of CBF Expression1 (ICE1) was found in a pear to be associated with endodormancy (Takemura et al, 2015) and was identified as the candidate gene in previous QTL studies investigating the timing of bud break, similar to PACLOBUTRAZOL RESISTANCE 1 (MdPRE1) (Miotto et al, 2019)

Summary

Introduction

In deciduous fruit trees such as apple (Malus × domestica Borkh.), phases of high metabolic activity, growth, and reproduction alternate with phases of no growth and low levels of activity, called dormancy. The cycles of activity and dormancy need to be synchronized well with environmental conditions in order to maximize survival during winter, optimize the timing of reproduction and yield, and maximize the length of growing seasons (Cooke et al, 2012). Which environmental cues regulate onset, progression, and release of dormancy in deciduous fruit trees is not entirely resolved, but ambient temperature and photoperiod appear to play major roles (Heide, 2008; Cooke et al, 2012; Maurya and Bhalerao, 2017). ×domestica is not sensitive to photoperiod and uses ambient temperature as a major environmental cue to control the timing of dormancy (Heide and Prestrud, 2005; Tanino et al, 2010; Cooke et al, 2012; Singh et al, 2017). Whether light quality serves as an additional cue needs to be determined

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Conclusion