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 a-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 Gaor Gb-subunits of the plant heterotrimeric G proteins, but is reliant on reactive oxygen species (ROS), a cognate MAPKK, and influx of extracellular Ca 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. Ga-, Gb-, and Gg-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 Ga-(Arabidopsis G PROTEIN, ALPHA SUBUNIT1 [GPA1]) and one prototypical Gb(Arabidopsis G PROTEIN, BETA SUBUNIT1 [AGB1]) subunit and potentially, two Gg-subunits (Arabidopsis G PROTEIN, GAMMA SUBUNIT1 [AGG1 and AGG2]), are found. Interaction has been detected between the Arabidopsis Ggand Gb-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evidence has been obtained for in vitro and in vivo interaction of GPA1 and Gb (J.-G. Chen, J.S. Liang, and A.M. Jones, unpublished data). Mutational and pharmacological studies have implicated plant heterotrimeric G protein subunits in numerous physiological processes and phenotypic changes, including auxin and gibberellin signaling, K channel regulation, Ca regulation, cell division, and stomatal function (Aharon et al., 1998; Jones et al., 1998; Saalbach et al., 1999; Ullah et al., 2001; Wang et al., 2001). MP has been widely used to implicate G protein regulated processes in both plants and animals (Higashijima et al., 1988; Legendre et al., 1992; Legendre et al., 1993; Wise et al., 1993; Höller et al., 1999). For example, short-term responses to MP treatment reported for plants include increases in cellular Ca ions, induction of an oxidative burst, stimulation of 1,4,5-inositol triphosphate turnover, and activation of phospholipase C, phospholipase D2, and myelin basic protein (MBP) kinases (Scherer, 1992; Kauss and Jeblick, 1996; Chahdi et al., 1998; Takahashi et al., 1998; Chahdi et al., 2003). Although it is generally assumed that these responses are mediated by the initial activation of heterotrimeric G proteins, there is limited direct evidence for this, particularly in plants.