Arabidopsis TSPO and porphyrins metabolism: a transient signaling connection?

What goes up should come down and vice versa. Cellular homeostasis requires that every signaling process involving up- or down-regulation of a given pathway should only be transient, and returning to steady state after a signaling process is as vital to living cells as being able to perceive and transduce changes of their environment. One of the best studied responses of plant cells subjected to water-related stress is the transient increase of the phytohormone abscisic acid (ABA). The increase in active ABA regulates the expression of ABA-responsive genes, some of which are strictly ABA-dependent in that their expression is almost undetectable in absence of elevated levels of cellular ABA. Since the function of these proteins may only be required transiently, a regulatory mechanism for transcriptionally and/or post-translationally regulate their expression should exist. In general during stress, molecular mechanisms aimed at shutting down the ABA-dependent signaling, as required at some point for the homeostasis of the plant cell, are poorly understood. The arabidopsis TSPO (translocator protein)-related protein is transiently induced by abiotic stresses and ABA treatment. Our recent work aiming at understanding the function and regulation of At-TSPO yielded exciting insights into the interplay among a stress-regulated protein, ABA responses, tetrapyrrole biosynthesis/scavenging, and autophagy. We discuss these findings in relation to tetrapyrroles metabolism/trafficking and the regulation of ABA-dependent signaling by the plant cell.