Mastoparan Rapidly Activates Plant MAP Kinase Signaling Independent of Heterotrimeric G Proteins

It has long been known that mastoparan (MP), a cationic, amphiphilic tetradecapeptide isolated from wasp venom, is capable of directly stimulating the guanine nucleotide exchange reaction of the α-subunit of animal heterotrimeric G proteins via a mechanism analogous to that of G protein coupled receptors. This leads to a myriad of downstream events including the activation of mitogen-activated protein kinases (MAPKs). Here, we show that MP induction of plant MAPK signaling does not require the participation of either the Gα- or Gβ-subunits of the plant heterotrimeric G proteins, but is reliant on reactive oxygen species (ROS), a cognate MAPKK, and influx of extracellular Ca2+ ions. While this does not preclude a role for a heterotrimeric G protein in MAPK signaling, it raises concern about the conclusions drawn from published experiments using MP. Gα-, Gβ-, and Gγ-homologs have been identified in Arabidopsis and other plant species (Ma et al., 1990; Gotor et al., 1996; Lee and Assmann, 1999; Saalbach, et al., 1999; Mason and Botella, 2000). In Arabidopsis, a single prototypical Gα-(Arabidopsis G PROTEIN, ALPHA SUBUNIT1 [GPA1]) and one prototypical Gβ-(Arabidopsis G PROTEIN, BETA SUBUNIT1 [AGB1]) subunit and potentially, two Gγ-subunits (Arabidopsis G PROTEIN, GAMMA SUBUNIT1 [AGG1 and AGG2]), are found. Interaction has been detected between the Arabidopsis Gγ- and Gβ-subunits (Mason and Botella, 2000; for review, see Jones, 2002), and evidence has been obtained for in vitro and in vivo interaction of GPA1 and Gβ (J.-G. Chen, J.S. Liang, and A.M. Jones, unpublished data).