Membrane trafficking plays a crucial role in organizing plant signaling and membrane compartments in response to environmental stimuli. In this thesis, I focused on PEN1-mediated membrane trafficking in response to pathogen attack. The interaction between the barley powdery mildew fungus, Bh, and the model plant, Arabidopsis, was explored. This was not only because it is a paradigm of the nonhost plant-pathogen interaction, but also because powdery mildew is one of the most common diseases threatening the economically important crops. Studying the interaction between Bh and Arabidopsis will help to understand the mechanism of how plants resist potential pathogen invasion. Firstly, I have shown that PEN1 mediates general immunity to prevent Arabidopsis from being infected by filamentous pathogens. This evolutionarily conserved function is redundant with SYP122 in the SYP12 clade of plant syntaxins. These two syntaxins have specialized in polarized secretion at defense structures in response to pathogen invasion. In addition, PEN1 is further specialized in response to a specific group of pathogens, which are the powdery mildews. This functional specialization is reflected in high penetration resistance of Arabidopsis and high papillary accumulation of PEN1 during infection with Bh. In addition, PEN1 is more mobile between the plasma membrane (PM) and trans-Golgi network (TGN). I then investigated the regulatory mechanism for PEN1 recycling and narrowed down the functional specialization to a predicted SUMOylation site, VK108LE. The VK108-dependent PEN1-specific SUMOylation was demonstrated by immunoprecipitation, and the mechanism of SUMOylated PEN1 is proposed to initiate the endocytosis of PEN1 for its rapid recycling back to the PM. Finally, the involvement the adaptor protein 2 (AP2) subunit, AP2M, in the endocytosis of PEN1 was preliminarily tested. The ap2m-1 mutant shows an impaired recycling of PEN1 and resistance to Bh. However, how AP2M is involved in PEN1-specific penetration resistance to Bh requires further exploration. Summarizing, this work deepens our understanding of how syntaxins regulate membrane trafficking in response to pathogen infection. Here, PEN1 functions in at least two regulatory pathways: one is shared with SYP122, which exhibits a general immunity and is responsible for the secretion of defense materials to the defense structures. Second, PEN1 is specialized in the timely recycling of defense materials to papillae during response to Bh attack. This is proposed to depend on a likely SUMOylation, implying a novel mechanism for SUMOylation in regulating the movement and functional specificity of a syntaxin.