The TPX family of co-repressors: a hub amidst the hubbub.

The phytohormone auxin affects a wide range of plant responses through global shifts in gene expression. The TOPLESS/TOPLESS RELATED (TPL/TPR) co-repressors (here collectively called the TPX family for simplicity) play a central role in this transcriptional regulation, acting through a variety of mechanisms, including modifying chromatin accessibility and assembling the machinery needed for transcription initiation. Structure-function analysis has mapped multiple repression domains within the founding TPL protein, and uncovered several forms of post-translational modifications that alter the function of TPL or other TPX proteins. Recent examination of the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) proteins suggests that differential affinity for TPXs can set the threshold for auxin sensitivity and subsequent growth dynamics. Beyond well-established roles in development, the TPX family has also emerged as a hub in plant immunity with effectors from diverse pathogens directly targeting TPX proteins. In one particularly striking case, a species of insect reduces the fitness of its competitors by manipulating TPX activity in the shared host plant to increase a selective suite of plant defenses. The subtle and effective reprogramming of critical developmental and immunity networks via modification of the pool of available TPX proteins could guide engineering strategies to optimize growth-defense trade-offs in crops. In this review, we will summarize recent studies highlighting how modifying the available pool of TPX family members results in subtle and effective reprogramming of critical developmental and immunity networks, and how this mode of regulation could provide a blueprint for optimizing growth-defense trade-offs in crops.