Mitogen-Activated Protein Kinase Cascades in Plant Hormone Signaling

despite the impressive current knowledge of MAPK cascades, their participation in BR and AUX signaling remains relatively unexplored. The very limited amount of data available on crosstalk between MAPK and GA signaling pathways (Huttly and Phillips, 1995; Marcote and Carbonell, 2000; Li Y. et al., 2014; Lu et al., 2015) highlights significant opportunities for further study in this area.

The molecular mechanisms that regulate MAPK assembly, activity (both activation and inactivation) and substrate binding require further elucidation.

the concentration of the protein is tightly controlled within the target compartment.

The very complexity of MAPK cascades means that it is often difficult to define them in detail and to assign them a specific role in a particular biological process. Thus, to date, no MAPK cascade, together with its downstream substrates, has been defined in its entirety in any plant system.

We also need a better characterization of the functional diversity and redundancy of MAPK complexes.

Importantly, we also need to understand the consequences of phosphorylation by MAPKs for the function, localization or stability of their protein targets. Therefore, many questions remain, some of which are listed below:

• Which cellular elements function as molecular switches to support precise crosstalk and interaction outcomes between MAPK cascades? How do plants discriminate between hormone signaling pathways? How do MAPK cascades maintain specificity?

• What governs MAPK distribution within the cell? What post-transcriptional and translational mechanisms are employed to regulate this distribution?

• Which signaling systems are responsible for MAPK inactivation? How do these work? Which ligands target MAPK pathways to regulate their activity?

• What is the relative importance of different MAPK pathways in hormonal responses?

We believe that the answers to these questions will provide exciting discoveries and establish further the crucial role of MAPKs in plants. MAPK cascades, like other signaling networks, display a wide range of regulatory properties and the extension of MAPK research to all economically important crops is particularly relevant for ensuring sustainable food production globally.