Calcium influxes and mitogen-activated protein kinase kinase activation mediate ethylene inducing ipomoelin gene expression in sweet potato.

The ipomoelin gene (IPO) was identified to be a wound-inducible gene from Ipomoea batatas, and its expression was stimulated by methyl jasmonate (MeJA) and hydrogen peroxide. IPO protein was also characterized as a defence-related protein, and it is also a carbohydrate-binding protein. In this study, the expression of IPO was used as a molecular probe to study the effects of Ca2+ on the signal transduction of ethylene. A confocal microscope monitored the Ca2+ within cells, and Northern blotting examined IPO expression. The presence of Ca2+ channel blocker, including diltiazem, neomycin or ruthenium red, abolished the increase of cytosolic Ca2+, and reduced the IPO expression in the cells induced by ethylene. Furthermore, both Ca2+ influxes and IPO expression stimulated by ethylene were prohibited in the presence of 10 mm ethylene glycol-bis(2-aminoethyl ether)-N, N, N', N'-tetraacetic acid (EGTA). These results indicated that Ca2+ influxes into the cytosol induced by ethylene are from both apoplast and organelles, and are required for activating IPO expression. However, in the presence of 1 mm EGTA, ethylene can still stimulate IPO expression, but mechanical wounding failed to do it. Therefore, Ca2+ channels in the plasma membrane induced by ethylene have higher affinity to Ca2+ than that stimulated by wounding. Moreover, the addition of A23187, an ionophore, raised cytosolic Ca2+, but was unable to stimulate IPO expression. These findings showed that IPO induction did not solely depend on Ca2+, and Ca2+ elevation in cytosol is necessary but not sufficient for IPO expression. The application of PD98059, a mitogen-activated protein kinase kinase (MAPKK) inhibitor, did not prevent Ca2+ from increasing in the cytosol induced by ethylene, but inhibited the IPO expression stimulated by staurosporine (STA), a protein kinase inhibitor. Conclusively, elevation of cytosolic Ca2+ by ethylene may stimulate protein phosphatase and MAPKK, which finally activates IPO expression.