Molecular chaperone. Involved in RPM1-mediated resistance. Component of the RPM1/RAR1/SGT1 complex. May stabilize RPM1 and protect it from SGT1-mediated degradation. Associates with RAR1 which may function as co-chaperone. Possesses ATPase activity (PubMed:14592967, PubMed:19487680). In the absence of heat shock, negatively regulates heat-inducible genes by actively suppressing heat shock transcription factor A1D (HSFA1D) function (PubMed:17965410). Involved in the induction of heat shock transcription factor A2 (HSFA2) expression in response to oxidative stress (PubMed:20147301). Required for stomatal closure and modulates transcriptional and physiological responses to abscisic acid (ABA) (PubMed:21586649). Regulates RPP4-mediated temperature-dependent cell death and defense responses (PubMed:24611624). May assist SGT1B in the formation of SCF E3 ubiquitin ligase complexes that target the immune receptors SNC1, RPS2 and RPS4 for degradation, to regulate receptor levels and avoid autoimmunity (PubMed:24889324). Homodimer (PubMed:19487680, PubMed:24036116). Interacts with RPM1, RAR1 and SGT1B (PubMed:14592967, PubMed:19487680). Interacts with OEP61, OEP64 and OM64 (PubMed:24036116). Interacts with HSFA1D (PubMed:17965410). A number of isoforms are produced. According to EST sequences. Present in all tissues. Most abundantly expressed in roots followed by floral bud clusters, flowers and young fruits. In contrast to other major heat shock proteins, this one is also expressed at normal growth temperatures. Levels increase only slightly after heat shock. The TPR repeat-binding motif mediates interaction with TPR repeat-containing proteins. No visible phenotype under normal growth condition. In case of infection, plants are altered in RPM1-mediated disease resistance. Belongs to the heat shock protein 90 family.