Summary The plasma membrane (PM)‐localized receptor kinase FLAGELLIN SENSING 2 (FLS2) recognizes bacterial flagellin or its immunogenic epitope flg22, and initiates microbe‐associated molecular pattern‐triggered immunity, which inhibits infection by bacterial pathogens. The localization, abundance and activity of FLS2 are under dynamic control. Here, we demonstrate that Arabidopsis thaliana EXO70B1, a subunit of the exocyst complex, plays a critical role in FLS2 signaling that is independent of the truncated Toll/interleukin‐1 receptor‐nucleotide binding sequence protein TIR‐NBS2 (TN2). In the exo70B1‐3 mutant, the abundance of FLS2 protein at the PM is diminished, consistent with the impaired flg22 response of this mutant. EXO70B1‐GFP plants showed increased FLS2 accumulation at the PM and therefore enhanced FLS2 signaling. The EXO70B1‐mediated trafficking of FLS2 to the PM is partially independent of the PENETRATION 1 (PEN1)‐containing secretory pathway. In addition, EXO70B1 interacts with EXO70B2, a close homolog of EXO70B1, and both proteins associate with FLS2 and contribute to the accumulation of FLS2 at the PM. Taken together, our data suggest that the exocyst complex subunits EXO70B1 and EXO70B2 regulate the trafficking of FLS2 to the PM, which represents a new layer of regulation of FLS2 function in plant immunity.

Many bacterial pathogens secret effectors into host cells to disable host defenses and thus promote infection. The exocyst complex functions in the transport and secretion of defense molecules, and loss of function of the EXO70B1 subunit leads to autoimmunity by activation of a truncated Toll/interleukin-1 receptor–nucleotide-binding sequence protein (TIR-NBS2; herein referred to as TN2). Here, we show that EXO70B1 is required for pathogen-associated molecular pattern-triggered immune responses in Arabidopsis thaliana . The effector AvrPtoB, an E3 ligase from Pseudomonas syringae pv. tomato ( Pto ) strain DC3000, associates with EXO70B1. AvrPtoB ubiquitinates EXO70B1 and mediates EXO70B1 degradation via the host’s 26S proteasome in a manner requiring E3 ligase activity. AvrPtoB enhances Pto DC3000 virulence by overcoming EXO70B1-mediated resistance. Moreover, overexpression of AvrPtoB in Arabidopsis leads to autoimmunity, which is partially dependent on TN2. Expression of TN2 in tobacco ( Nicotiana tabacum and Nicotiana benthamiana ) triggers strong and rapid cell death, which is suppressed by co-expression with EXO70B1 but reoccurs when co-expressed with AvrPtoB. Taken together, our data highlight that AvrPtoB targets the Arabidopsis thaliana EXO70 protein family member EXO70B1 to manipulate the defense molecule secretion machinery or immunity.