Genome-Wide Analysis and Expression Pattern of the AP2/ERF Gene Family in Kiwifruit under Waterlogging Stress Treatment

The APETALA2/ethylene response factor (AP2/ERF) superfamily is defined by the AP2/ERF domain which consists of about 60 to 70 amino acids, and can be classified into at least five subfamilies: AP2 (containing two repeated AP2/ERF domains), DREB (dehydration responsive element binding, containing a single AP2/ERF domain), ERF (containing a single AP2/ERF domain), RAV (containing a B3domain and a single AP2/ERF domain), and other s(Zhang et al. 2012b ). The ERF and DREB subfamilies can be further divided into six subfamilies: the B1 –6 subfamilies and A1 –6 subfamilies, respectively (Zhang et al . 2012b ). It has been demonstrated that AP2/ERF play important roles in the plant cell cycle, growth and development, as well as the response to biotic and abiotic stresses (Zhang et al. 2012b ).

 Waterlogging is one of the most common stresses affecting plant growth and development. Many important crop plants are sensitive to waterlogging conditions caused by heavy rain. Waterlogging and submergence conditions impose a variety of challenges on the plants (Hinz et al. 2010 ). Previous research has shown that the AP2/ERF genes play an important role in the regulation of gene expression during waterlogging stress. RAP2.2 is induced in shoots by ethylene and functions in an ethylene -controlled signal transduction pathway and the overexpression of RAP2.2 resulted in improved plant survival under hypoxia (low -oxygen) stress, whereas lines containing T -DNA knockouts of the gene had poorer survival rates than the wild type(Hinz et al. 2010 ). The RAP2.2 gene plays a significant role in the metabolic adaptation to flooding stress in Arabidopsis (Hinz et al. 2010 ). The SUB1A -1 allele could reduce elongation growth and carbohydrate consumption, and to confer submergence tolerance (Fukao 2006 ; Xu et al. 2006 ; Jung et al. 2010 ). Floodin g sensitive japonica cultivar over - expressing SUB1A -1 could increase A DH1 expression and flooding tolerance (Fukao 2006 ; Xu et al. 2006; Jung et al. 2010) Flooding sensitive japonica cultivar overexpressing SUB1A -1 could increase ADH1 expression and flooding tolerance(Fukao 2006; Xu et al. 2006). Four ERF subfamily genes in Arabidopsis, namely RAP2.2 (At3g14230), RAP2.12 (At1g53910), HRE1 (At1g72360), and HRE2 (At2g47520) have been documented that play important role in the response to hypoxia (Hinz et al. 2010; Licausi et al. 2010). HRE1 over-expressing plants showed an increased activity in the fermentative enzymes pyruvate decarboxylase and alcohol dehydrogenase together with increased ethanol production under hypoxia, showed an improved tolerance of anoxia (Licausi et al. 2010). RAP2.2 was induced in shoots by ethylene and functions in an ethylene-controlled signal transduction pathway(Hinz et al. 2010). Overexpression of RAP2.2 resulted in improved plant survival under hypoxia stress, whereas lines containing T-DNA knockouts of the gene had poorer survival rates than the wild type(Hinz et al. 2010). Deepwater rice requires SNORKEL1 (SK1) and SK2 ERF transcription factors to elongate stem internodes and extend the hollow stems to the water surface for survival (Hattori et al. 2009). Du et al. reported (Du et al. 2014) that 38 of 184 AP2/ERF transcript factor genes were responsive to waterlogging stress and 25 genes were ERF subfamily. 

Kiwifruit is a major fruit worldwide. However, the majority of currently growing kiwifruit cultivars, like ‘Hongyang’, are susceptible to waterlogging stress in East China. A thorough knowledge of kiwifruit resistance mechanisms will help to limit crop loss due to waterlogging stress, and to decrease the economic losses. It’s known to us that AP2/ERF transcript factor play important roles in the response to waterlogging stresses. It is possible to identify the AP2/ERF genes in many species as these plant genome projects have been completed. Previous studies showed that there are 147, 184, 132, 200, and 116 AP2/ERF genes in the Arabidopsis (Nakano 2006) , maize(Du et al. 2014), grapevine(Zhuang et al. 2009), poplar(Zhuang et al. 2008), and Chinese plum (Du et al. 2013), respectively. However, few reports of the AP2/ERF superfamily are available in kiwifruit. Kiwifruit genome projects have been completed(Huang and al. 2013). In this article, AP2-like genes from kiwifruit were surveyed and comparatively analyzed. Here, 183 AP2/ERF transcription factors were identified from the kiwifruit genome database and the transcriptome sequencing database (Zhang et al., 2015). The expressions of kiwifruit AP2/ERF genes under waterlogged stress were performed. These analyses will be valuable to isolate and understand the molecular mechanism of AP2/ERF genes responded to waterlogging stress in kiwifruit.