Characterization of fungi resistance in two autotetraploid apple cultivars
Introduction
Plants constantly face threats from various biotic stresses, and disease seriously affects the yield and quality of fruit production (Freeman et al., 1998, Li et al., 2013). Apple (Malus × domestica) is a fruit tree that occupies large planting areas worldwide, and important losses in apple production are caused by the hemibiotrophic fungal pathogens Alternaria alternata (Abe et al., 2010, Harteveld et al., 2014) and Colletotrichum gloeosporioides (Lee and Hong, 2015). Alternaria mali causes leaf blight and is one of the most destructive apple diseases (Woudenberg et al., 2015). The symptoms begin as brown spots on the leaf, with the centre of the disease spot gradually expanding and turning to reddish-brown with a puce border (Jones and Aldwinckle, 1990). Anthracnose disease is a major economic constraint to the apple industry and causes fruit rot and canker (González et al., 2006). This disease also occurs on leaves (Tanahashi et al., 2016), and infection limits apple production.
Polyploidization is a natural evolutionary phenomenon in plants (Masterson, 1994). Polyploid cells contain more than two basic sets of chromosomes (Osabe et al., 2012). Polyploidy is usually divided into autopolyploid and allopolyploid at the level of the genome (Stebbins, 1947). Autopolyploidy results from chromosome doubling only, whereas allopolyploids achieve chromosome doubling by species hybridization (Doyle et al., 2008, Mayrose et al., 2010). Studies have shown that inducing polyploidy is an effective way to increase resistance to fungal pathogens in agricultural production (Hoshino et al., 2011; Shahid et al., 2012, Xu et al., 2014). Tetraploid wheat has a greater ability than diploid wheat to resist powdery mildew (Zhu et al., 2003) and leaf rust (Yuan et al., 2007). Allopolyploid tobacco has a higher plant virus resistance (Gottula et al., 2014). Autotetraploid watermelons have a higher resistance to Fusarium wilt than diploid watermelons (Liu et al., 2009). Most studies have focused on herbaceous allopolyploid plants. Research on artificially induced autotetraploids is less abundant. Due to the long growth cycle of woody plants, information on breeding disease-resistant plants is limited. Compared with allopolyploids, studies on disease-resistant autopolyploid woody plants of is more plentiful.
In this study, we compared the differences in morphology and disease incidence in diploid and in autotetraploid apple cultivars ‘Hanfu’ and ‘Gala’ when exposed to infection with Alternaria alternata and Colletotrichum gloeosporioides. We also examined the differential expression of the disease resistance-related genes CERK1, PR1, WRKY29, CDPK and MPK4. The results show that autotetraploid apple trees display higher fungal pathogen resistance than diploid apple trees. The results of the present study provide a meaningful contribution to the current understanding of disease resistance breeding in autopolyploid woody plants (Table 1).
Section snippets
Plant materials
We induced autotetraploidy in the apple cultivars ‘Hanfu’ and ‘Gala’ by treating diploid cultivars (Malus × domestica, 2n = 2x = 34) with colchicine (Xue et al., 2015). After the induction of the autotetraploid cultivars, both diploid and autotetraploid apples were grown in the Fruit Molecular Biology Laboratory at Shenyang Agricultural University at the same time. After root growth, plants grown under two kinds of tissue culture were transplanted into the field at the same time in 2008 and managed
Disease assessment
The A. alternata initially caused brown spots. As the lesions expanded, the spots became dark in the centre and developed maroon borders. The results showed that autotetraploid plants exhibit better resistance than diploid plants in ‘Hanfu’ and ‘Gala’ (Fig. 1). Autotetraploid plants showed a significantly higher level of resistance than diploid plants to A. alternata. Symptoms of leaf injury plants appeared earlier on the diploid apple leaves than on those of the autotetraploids. The response
Polyploidy is widespread
Polyploidy is an evolutionary process (Cavanagh et al., 2013, Liu et al., 2011). Common crops such as rice, cotton, corn, potato, soybean, wheat and rape are polyploid (Feldman and Levy, 2012, Jiao et al., 2011). Approximately half of fruit tree species contain polyploid types, which tend to grow vigorously and show increased fruit yield and quality (Einset, 1952, Tan et al., 2015, Yin et al., 2010, Zhang et al., 2015). Polyploid plants can survive better under abiotic stresses such as salt
Conclusions
Previous research revealed polyploidization to be an effective means by which plants adapt to adverse environmental conditions (Thao et al., 2003). However, the role of polyploidy in adapting to changes in molecules is poorly known. In order to assess disease resistance of artificially induced autotetraploid apples, we inoculated autotetraploid and diploid plants with the pathogens A. alternata and C. gloeosporioides. At the same time, we also explored the related genes that may be involved in
Conflicts of interest
The authors declare that they have no conflict of interest.
Author contribution statement
M. Chen and Y. Ma performed the experiment and interpreted the results; M. Chen and Y. Ma drafted the manuscript; F. Wang, Z. Zhang and J. Fu participated in analysis; and Y. Ma and J. Fu proposed and supervised the overall project. All authors read and approved the final manuscript.
Key message
Disease resistance traits of diploid and autotetraploid apple plants (‘Hanfu’ and ‘Gala’) are reported. The differences indicate that apple plants (‘Hanfu’ and ‘Gala’) may differ in their response to biotic stress depending on ploidy level.
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