The Mitogen Activated Protein (MAP) kinase pathway is a three-tiered protein phosphorylation module common to eukaryotic cells. In yeast, different MAP kinase pathways are involved in the regulation of the switch in mating types and responses to nutrient starvation and osmotic stress. In humans, MAP kinase pathways also play important roles in the regulation of cell proliferation, differentiation, and death.
MAP kinase (MPK) signaling cascades are known to transmit both biotic and abiotic stress signals in plants. However, their roles in plant growth regulation are mostly unknown. We took advantage of a strong mammalian MPK phosphatase (MKP1 to investigate potential functions of MAPK cascades in plant growth and development. Unexpectedly, expression of MKP1 in cultured tobacco cells led to shoot differentiation from microcalli, indicating enhanced cytokine sensitivity. Transgenic tobacco plants expressing MKP1 showed remarkable phenotypes, including early flowering, delayed leaf senescence, and increased longevity of floral meristems, which continued to produce new flowers even after seed set. Stem segments from MKP-1 plants were consistently able to produce shoots on medium without exogenous cytokinin, and these shoots also flowered precociously in culture. Flowers from most of the MKP1 transformed lines had protruding stigma and enhanced anthocyanin accumulation in petals, resembling flowers of tobacco plants that have reduced ethylene sensitivity due to the expression of a dominant mutant of ethylene receptor ETR1.
All the phenotypes associated with MKP1 depended on the phosphatase activity of MKP1, since expression of a phosphatase-inactive mutant, MKPCS, did not result in the same phenotypes. Analysis of gene expression in MKP1 plants showed elevated expression of a type A response regulator, a pathogenesis-related protein (PR1-b), and a Flowering locust T gene (FT) gene while the expression level of ACC oxidase (ACO, the enzyme for the final step of ethylene biosynthesis) was decreased compared to that of the wild-type plants. These results suggest that MKP1 might inactivate stress-related MPK pathways, and as a result increase the sensitivity to cytokinin, MAPK pathways may be a convergent point through which stress and growth signals interact in shaping the final outcome of plant development.
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