1. Introduction
Fruit abscission is a complex process that is regulated by internal and external factors. During the abscission process, the abscission-zone (AZ) tissues are induced and lead to cell separation. The abscission signals are regulated by hormones, such as ethylene and abscisic acid (ABA), and signal transduction pathways (Botton et al., 2011). The formation of AZ cell layers is induced by the JOINTLESS andMACROCALYX genes (Nakano et al., 2012) accompanied by the modification of cell wall components, such as lignin and pectin (Kim et al., 2019). Previous studies have mainly focused on elucidating the intrinsic factors of the abscission signal including fruitlet abscission or pre-harvest drop (Dal Cin et al., 2008; Li et al., 2019; Shaya et al., 2019). However, the abscission studies on early developing fruit affected by environmental stresses have rarely been reported.
The cold response of trees includes C-repeat binding factor (CBF)-mediated signalling (Sakuma et al., 2002; Wisniewskiet al., 2011; Yang et al., 2011; reviewed by Miura et al., 2013). During the cold stress response, the hormone ABA is synthesized, which can activate the CBF-mediated signals (Knight et al., 2004). Subsequently, the expression of cold-responsive (COR ) genes is amplified and as a result, trees undergo various physiological changes involving cell dehydration correlated with dehydrins (Wisniewski et al., 2006).
Apple (Malus × domestica Borkh.) is one of the major perennial fruit crops, which occupies a large portion of the world fruit industry. Extreme change in weather conditions are expected to increase due to climate change (Lee et al., 2012). Orchard trees are exposed to abnormally low-temperature conditions in the primary apple production regions in Korea and the frequency of this trend is found to further increase in the prediction model using regional climate change scenarios (Kim et al., 2018). During early stages of development, fruit can be exposed to extreme changes/fluctuations in temperature during spring; however, the mechanism underlying premature fruit drop affected by the environmental stress is largely unknown. Changes in temperature conditions of the orchard may cause fruit drop if the abscission signal transduction is induced. Yuan et al. (2004) demonstrated that exposure to varying temperatures contributes to different responses of leaf and fruit abscission under ethylene application. During the fruit development process, unexpected fruit drop decreases the yield of the orchard. Therefore, from the perspective of the fruit industry, it is important to understand the abnormal fruit abscission mechanism triggered by environmental factors, such as cold stress.
In late May 2018, abnormal fruit drop was reported in many orchards in the apple production areas of South Korea. This damage was also observed in the Apple Research Institute (Gunwi; 36.28°N, 128.47°E) following 2 days of cold stress along with a sharp fluctuation in daily mean temperature compared with the annual mean for the 10 years (Figure 1). In this study, we conducted RNA-seq and metabolomics analyses from young apple fruit collected in-field and hypothesized that cold stress may induce the abscission of early developing fruits undergoing abscission after cold exposure in the field. To elucidate the molecular basis of premature fruit drop, we investigated the morphological traits of AZ cortical cells by electron microscopy and the tissue-specific quantitative reverse transcription PCR (qRT-PCR) of excised tree subunits consisting of a branch, pedicel, and fruit inex -vivo cold shock experiments.