It can be seen that the kinetics of disease development in all three groups of plants are substantially the same and that all three groups of plants showed similar SA-induced delays in disease development (Fig. virus (Xie et al., 2001). Although this experiment showed that NtRdRp1 alone cannot mediate SA-induced resistance to TMV, it does not rule out the possibility that NtRdRp1 activity and RNA silencing may, along with other factors, contribute to the overall phenomenon of SA-induced resistance to viruses. In tobacco treated with SA the replication of and TMV, as well as the cell-to-cell movement of TMV, are inhibited in directly inoculated leaf tissue (Chivasa et al., 1997; Naylor et al., 1998; Murphy and Carr, 2002). However, replication and cell-to-cell movement of (CMV) are not inhibited by SA but the chemical does inhibit the systemic movement of CMV through the phloem tissue (Naylor et al., 1998). The ability of CMV to evade the primary layers of SA-induced virus resistance is conferred by its 2b protein. This multifunctional protein influences virus movement and symptom development (Soards et al., 2002), but most importantly it can counter induction of RNA silencing (Bclin et al., 1998; Brigneti et al., 1998) and SA-induced resistance (Ji and Ding, 2001). The ability of the CMV 2b protein to act as a counter defense factor is dependent on its localization to the cell nucleus (Lucy et al., 2000; Mayers et al., 2000), where it affects expression of host genes including at least one SA-inducible gene: the mitochondrial alternative oxidase (AOX; Ji and Ding, 2001). All plants possess AOX, which by itself constitutes a distinct branch of the cytochrome pathway (CYT) linking the oxidation of the ubiquinol/ubiquinone CDK9 inhibitor 2 (UQ) pool directly to the reduction of oxygen to water. This branch is usually referred to as the alternative respiratory pathway (AP; Affourtit et al., 2001, 2002). AP activity is not coupled to ATP generation. Instead, it is thought to play a potentially crucial role in protecting all plant cells against the lethal effects of reactive oxygen varieties (ROS; Maxwell et al., 1999; Yip and Vanlerberghe, 2001), and in the maintenance of flower homeostasis under varying growth conditions (Affourtit et al., 2001, 2002; Sakano, 2001; Moore et al., 2002). AOX is definitely a homodimeric protein and activity is definitely regulated from the redox-sensitive formation or breakage of an intersubunit disulfide bridge (Rhoads et al., 1998). AOX activity and transcription of mRNA can be stimulated by inhibitors of the CYT (antimycin A [AA] or cyanide), as well as by SA and the synthetic resistance-inducing chemical, 2,6-dichloroisonicotinic acid (Raskin et al., 1987; Rhoads and McIntosh, 1992; Chivasa and Carr, 1998; Chivasa et al., 1999). While investigating the possible involvement of AOX in signaling during pathogen resistance induction in tobacco and Arabidopsis, we found that the defensive signal transduction pathway branches downstream of SA. One branch induces PR proteins and resistance to bacteria and fungi, whereas another causes induction of resistance to viruses (Murphy et al., 1999; Wong et al., 2002). Initial evidence for this was based on pharmacological data. Specifically, resistance to viruses can be triggered using AA and cyanide, or inhibited with salicylhydroxamic acid (SHAM, an AOX inhibitor), individually of the induction of gene manifestation (Chivasa et al., 1997; Chivasa and Carr, 1998). Subsequent experiments using Arabidopsis mutants confirmed the existence LMAN2L antibody of this branch point downstream of SA (Kachroo et CDK9 inhibitor 2 al., 2000; Wong et al., 2002). The results of our pharmacological experiments were consistent with a role for AOX in the rules of CDK9 inhibitor 2 induced resistance to viruses. In addition, it was mentioned by ourselves as well CDK9 inhibitor 2 as others (Lennon et al., 1997; Chivasa and Carr, 1998; Lacomme and Roby, 1999; Simons et al., 1999) that gene manifestation and AOX protein accumulation are elevated in plant cells expressing the HR, further suggesting an association between AOX and pathogen resistance. Although.
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