Crosstalk in cellular signaling: background noise or the real thing?

Abstract

During the past two decades, molecular biologists and geneticists have deconstructed intracellular signaling pathways in individual cells, revealing a great deal of crosstalk among key signaling pathways in the animal kingdom. Fewer examples have been reported in plants, which appear to integrate multiple signals on the promoters of target genes or to use gene family members to convey signal-specific output. For both plants and animals, the question now is whether the "crosstalk" is biologically relevant or simply noise in the experimental system. To minimize such noise, we suggest studying signaling pathways in the context of intact organisms with minimal perturbation from the experimenter.

Figure 1

A myriad of signals constantly bombard plants. Some of these signals must be dealt with immediately (e.g., shading by other plants), while other cues are the source of more long-term information, (e.g., photoperiod, temperature in seasonal time and location on the planet). Plant hormone signaling pathways then determine whether the plant grows or not—often a matter of survival.

Figure 2. Direct crosstalk between BR, auxin and MAMP signaling pathways

Binding of BRs to the BRI1 receptor triggers association with its co-receptor BAK1 at the cell surface. The activated receptor complex in turns leads to inhibition of BIN2-dependent phosphorylation of the BES1/BZR1 transcription factors. Dephosphorylated BES1/BZR1 bind to and activate BR-responsive gene expression, among which a significant subset also respond to auxin. BR signaling triggers the BIN2-mediated phosphorylation of the ARF2 repressor to inhibit its binding to auxin-response elements in the promoters of BR/auxin-regulated genes. Auxin perception removes the Aux/IAA-dependent repression on activator ARFs (A-ARF) that now can bind to auxin-response elements and further activate transcription of BR/auxin-responsive genes. The BAK1 co-receptor is also used by FLS2 to signal flagellin perception and trigger MAMP responses. A mechanism of crosstalk between BRs and MAMPs was recently proposed where ligand perception by BRI1 titrate BAK1 away from other receptors that use BAK1 as a co-receptor (Belkhadir et al., 2011). As a consequence, BRs negatively impact on several pattern recognition receptors, such as the flagellin receptor FLS2, and MAMP responses.