Proteomic analysis of the Actinidia deliciosa leaf apoplast during biotrophic colonisation by Pseudomonas syringae pv. actinidiae.

For plant pathogenic bacteria, adaptation to the apoplast is considered as key in the establishment of the parasitic lifestyle. Pseudomonas syringae pv. actinidiae (Psa), the causal agent of the bacterial canker of kiwifruit, uses leaves as the entry site to colonise plants. Through a combined approach based on 2-DE, nanoLC-ESI-LIT-MS/MS and quantitative PCR, we investigated Psa colonisation of the Actinidia deliciosa "Hayward" leaf apoplast during the bacterial biotrophic phase. A total of 58 differentially represented protein species were identified in artificially inoculated leaves. Although the pathogen increased its population density during the initial period of apoplast colonisation, plant defence mechanisms were able to impede further disease development. We identified a concerted action of different proteins mainly belonging to the plant defence and metabolism category, which intervened at different times and participated in reducing the pathogen population. On the other hand, bacterial BamA was highly represented during the first week of leaf apoplast colonisation, whereas OmpA and Cpn60 were induced later. In addition to presenting further proteomic information on the molecular factors actively participating in this pathosystem, our data characterise the early events of host colonisation and will promote the eventual development of novel bioassays for pathogen detection in kiwiplants.BIOLOGICAL SIGNIFICANCE:This original study evaluates on a proteomic perspective the interaction occurring into the leaf apoplast between Actinidia deliciosa and its specific pathogen Pseudomonas syringae pv. actinidiae. Despite the initial bacterial multiplication, a concerted action of the plant defence mechanisms blocked the infection during 21 days of apopalst colonization, as revealed by the number of differentially-represented proteins identified in artificially-inoculated and control leaves. Three bacterial proteins were also recognized among the over-represented molecules in infected plants. This study may contribute to improve breeding programs aimed at selecting resistant/tolerant kiwifruit cultivars toward Pseudomonas syringae pv. actinidiae, which present a high representation of the plant proteins here shown to be involved in resistance mechanisms. In addition to present additional information on the molecular players actively participating in this pathosystem, our data will also facilitate the technological development of future bioassays for the detection of this pathogen in kiwiplants.