Identification of a light-regulated protein kinase activity from seedlings of Arabidopsis thaliana

MAP-kinase activity in etiolated Cucumis sativus cotyledons: the effect of red and far-red light irradiation

Phytochrome (phy) signalling in plants may be transduced through protein phosphorylation. Mitogen-activated protein kinase (MAP-kinase, MAPK) activity and the effect of R (red) and FR (far-red) light irradiation on MAPK activity were studied in etiolated Cucumis sativus L. cotyledons. By in vitro protein phosphorylation and in-gel assays with myelin basic protein (MBP), a protein band (between 48 and 45 kDa) with MAPK-like activity was detected. The addition to the phosphorylation buffer of specific protein phosphatase (PTP) inhibitors (Na(3)VO(4) and NaF) and genistein, apigenin or PD98059 as MAPK inhibitors allowed us to confirm the MAPK activity of the protein band. Irradiation of etiolated cotyledons with FR light for 5, 10 or 60 min rapidly and transiently stimulated the MAPK activity of the protein band. This suggests that there was a very low fluence response (VLFR) of phys. In addition, 15 min of R light irradiation or a sequential treatment of 15 min of R plus 5 min of FR also increased MAPK activity. The stimulatory effect of R light was also attributed to the same photoreceptor, which suggests that MAPKs are involved in phytochrome signal transduction. Protein immunoprecipitation and immunoblotting analysis with the polyclonal antibody anti-pERK1/2 (Tyr 204) and the monoclonal antibody anti-phosphotyrosine PY20 allowed us to recognize the above mentioned protein band as two proteins with molecular masses (M(r)) of approximately 47 and 45 kDa, and MAPK activity. The biochemical and immunological properties showed by the proteins detected indicated that they were members of the MAPK family phosphorylated in tyrosine residues.

Protein kinase activity in Cucumis sativus cotyledons: effect of calcium and light

Light signals received by phytochromes in plants may be transduced through protein phosphorylation. Ca(2+) as second messenger was involved in phytochrome-mediated cellular events. Our experiments with Cucumis sativus cotyledons, treated with red (R) and far-red (FR) light, showed a stimulatory effect on in vitro protein phosphorylation of histone, added as exogenous substrate to the cotyledon extracts, and also modified the phosphorylation of endogenous polypeptides. The effect of light treatments was mimicked by the addition of Ca(2+) to the phosphorylation buffer, indicating phytochrome- and Ca(2+)-dependence on activity of some protein kinases (PKs). In-gel kinase assays were performed to characterize the PKs involved at the cotyledon stage of cucumber plants. Three proteins of about 75, 57 and 47kDa with PK activity were detected between M(r) markers of 94 and 45kDa. All three were able to phosphorylate histone and undergo autophosphorylation. However, only the 75 and 57kDa proteins autophosphorylated and phosphorylated the substrate in a Ca(2+)-dependent manner, and were inhibited when calmodulin (CaM) antagonists were added to the incubation buffer. Western-blot analysis with polyclonal antibodies directed against calcium-dependent protein kinase of rice (OsCDPK11) or Arabidopsis (AtCPK2) recognised 57 and 75kDa polypeptides, respectively. These results indicate the presence in cucumber cotyledons of at least two proteins (ca. 75 and 57kDa) with activity of PKs that could be calcium-dependent protein kinases (CDPKs). Both CDPKs could be modulated by phytochromes throughout FR-HIR and VLFR responses.

Phytochrome phosphorylation in plant light signaling

Reversible protein phosphorylation is a switching mechanism used in eukaryotes to regulate various cellular signalings. In plant light signaling, sophisticated photosensory receptor systems operate to modulate growth and development. The photoreceptors include phytochromes, cryptochromes and phototropins. Despite considerable progresses in defining the photosensory roles of these photoreceptors, the primary biochemical mechanisms by which the photoreceptor molecules transduce the perceived light signals into cellular responses remain to be elucidated. The signal-transducing photoreceptors in plants are all phosphoproteins and/or protein kinases, suggesting that light-dependent protein phosphorylation and dephosphorylation play important roles in the function of the photoreceptors. This review focuses on the role of phytochromes' reversible phosphorylation involved in the light signal transduction in plants.

Phytochrome-mediated signal transduction pathways in plants

Phytochromes are photoreceptors that regulate plant growth and development in response to the solar radiation environment. Recent studies reveal how phytochrome-mediated light signals can be transduced to the cells for their responses. The possible signal transduction pathways of phytochromes include: (a) direct regulation of gene transcription and (b) typical kinase-involved signaling pathways and its regulation by phosphorylation, dephosphorylation, and proteolytic degradation. This review highlights some of the recent findings.