Introduction

The mitogen-activated protein (MAP) kinase cascade is a signal transmission pathway conserved among eukaryotes. The cascade mediates external and internal signals for cellular responses. A typical cascade is composed of three classes of kinases, MAP kinase (MAPK), MAPK kinase (MAPKK) and MAPKK kinase (MAPKKK). MAPKKK phosphorylates MAPKK to activate the latter. MAPKK in turn phosphorylates MAPK to activate the latter. In the course of this signal transmission, the first, subtle signal is amplified sufficiently to induce various cellular responses including transcription, cytoskeleton dynamics and cell division. The MAP kinase cascade exists in higher plants and contributes to the mediation of a variety of signals, including those involving hormones, the environment, pathogenic attacks and internal signals generated during the cell cycle. While the MAP kinase pathways in plants have been studied intensively in research on cellular responses to environmental and pathogenic stimuli, understanding of the roles of MAP kinase pathways in cell division and plant growth and development is still very limited. In this section, the roles of MAP kinase pathways in these latter events are summarized.

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