Abstract
Understanding the defence mechanisms used by apple leaves against Alternaria alternate pathogen infection is important for breeding purposes. To investigate the ultrastructural differences between leaf tissues of susceptible and resistant seedlings, in vitro inoculation assays and transmission electron microscopy (TEM) analysis were conducted with two different inoculation assays. The results indicated that the resistant leaves may have certain antifungal activity against A. alternate that is lacking in susceptible leaves. To elucidate the two different host responses to A. alternate infection in apples, the proteomes of susceptible and resistant apple leaves that had or had not been infected with pathogen were characterised using two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionisation time-of-flight tandem mass spectrometry (MALDI-TOF-TOF MS). MS identified 43 differentially expressed proteins in two different inoculation assays. The known proteins were categorised into 5 classes, among these proteins, some pathogenesis-related (PR) proteins, such as beta-1,3-glucanase, ascorbate peroxidase (APX), glutathione peroxidase (GPX) and mal d1, were identified in susceptible and resistant hosts and were associated with disease resistance of the apple host. In addition, the different levels of mal d1 in susceptible and resistant hosts may contribute to the outstanding anti-disease properties of resistant leaves against A. alternate. Taken together, the resistance mechanisms of the apple host against A. alternate may be a result of the PR proteins and other defence-related proteins. Given the complexity of the biology involved in the interaction between apple leaves and the A. alternate pathogen, further investigation will yield more valuable insights into the molecular mechanisms of suppression of the A. alternate pathogen. Overall, we outline several novel insights into the response of apple leaves to pathogen attacks. These findings increase our knowledge of pathogen resistance mechanisms, and the data will also promote further investigation into the regulation of the expression of these target proteins.
Highlights
Plants are usually under numerous threats of pathogen infection, and some of them act as hosts to invasive pathogens [1, 2]
A recent proteomic study investigated the differential protein expression of transgenic mint infected by A. alternate pathogen [49]
The high percentage of pathogen-responsive proteins signifies the necessity for studying the pathogen-responsive proteomes of apple plants during A. alternata pathogen infection, which are thought to be related to resistance
Summary
Plants are usually under numerous threats of pathogen infection, and some of them act as hosts to invasive pathogens [1, 2]. A comprehensive understanding of the proteins induced by pathogens will help reveal the complex molecular mechanisms that mediate plant disease resistance and will aid the development of new strategies to increase disease resistance in some economically important crops. Apple (Malus domestica) is considered a model fruit plant due to its world-wide economic importance; a large number of apple cultivars dominate world fruit production [4, 5]. To date, apple cultivation has been limited by many kinds of fungal diseases, and the domestic apple has become an important fruit crop in which to study commercial traits such as disease resistance [6, 7]. Among the many fungal diseases affecting apple trees, Alternaria blotch, which is caused by the pathogen Alternaria alternata, has been a destructive apple disease in China and other East Asian countries [8]. Management of the Alternaria blotch occurs mainly through traditional chemical control agents instead of resistant cultivars
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