The tobacco ntf4 mitogen-activated protein (MAP) kinase gene (and its encoded protein p45Ntf4) is expressed at later stages of pollen maturation. We have found that the highly related MAP kinase SIPK is also expressed in pollen and, like p45Ntf4, is activated upon pollen hydration. The MAP kinase kinase NtMEK2 activates SIPK, and here we show that it can also activate p45Ntf4. In an attempt to inhibit the function of both MAP kinases simultaneously we constructed a loss-of-function mutant version of NtMEK2, which, in transient transformation assays, led to an inhibition of germination in the transformed pollen grains. These data indicate that NtMEK2, and by inference its substrates p45Ntf4 and/or SIPK, are involved in pollen germination.
