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Plants have a sophisticated immune system to defend against a wide range of invaders, including insects, nematodes, bacteria, oomycetes, fungi, and viruses. Microbes may manipulate or suppress immunity by delivering effector proteins, either to the inside or outside of plant cells. Much attention has been focused on identifying the targets of effector proteins in the host and on characterizing how effector activities suppress immunity. The best studied effector proteins are bacterial type III effectors (T3Es), many of which target positive regulators of immunity in order to inhibit their activity (Deslandes and Rivas, 2012). Yet some bacterial T3E effectors have been found to target host proteins that either negatively regulate immunity, or directly benefit bacterial nutrition. Examples include the Psuedomonas syringae T3E AvrB, which mediates phosphorylation and consequent activation of MPK4, a suppressor of immunity …