Transcription factor which promotes the expression of heat stress-inducible genes by contributing to the formation of a heat stress-specific transcriptional complex with NF-Y subunits (e.g. NF-YA2 and NF-YB3) and DREB2A at the promoter of target genes, thus promoting heat tolerance (PubMed:25490919). Together with the glyceraldehyde-3-phosphate dehydrogenase (GAPC), enhances heat tolerance (PubMed:32651385). Interacts with DREB2A in the nucleus during heat-stress (PubMed:25490919). Component of a heat stress-inducible transcriptional complex with NF-YA and NF-YB subunits made, at least, of NFYA2, NFYB3 and DPB3-1 in cooperation with DREB2A (PubMed:25490919). Binds directly with NFYB1, NFYB2, NFYB3, NFYB6, NFYB7, and DPB4 (PubMed:25490919). Binds to GAPC in response to heat-stress; this interaction promotes DNA-binding ability to its target promoter (PubMed:32651385). Expressed in seedlings, petioles, cotyledons, hypocotyls and root tips. In seedlings, first expressed in petioles and leaf blades of the cotyledons as well as tops and bottoms of the hypocotyls (PubMed:25490919). Later present in whole hypocotyls and weakly in the cotyledons and root tips (PubMed:25490919). Enhanced by heat stress but repressed by dehydration stress in young seedlings but not in older plants. Reduced expression of heat stress-inducible genes leading to an enhanced sensitivity to heat stress (PubMed:25490919). Impaired ability of GAPC to enhance heat tolerance of seedlings and to promote the expression of heat-inducible genes (PubMed:32651385). Belongs to the NFYC/HAP5 subunit family.