NUCLEAR FACTOR Y (NF-Y) transcription factors (also known as Heme-associated proteins [HAPs] and CCAAT box binding factors [CBFs]) are rapidly emerging as important regulators of numerous plant developmental and stress-induced responses. NF-Ys are sequence-specific transcription factors with histone-like subunits, with the unique characteristic that they bind DNA at CCAAT sites as heterotrimeric complexes composed of single subunits from each of three protein families: NF-YA, NF-YB, and NF-YC. In the plant lineage, each subunit type is encoded by a family of ∼10 genes, both in dicots and monocots, which differentiates them from animal systems, where there is typically only one or two genes encoding each subunit type. The expansion of NF-Y families in plants, combined with their heterotrimeric nature, means that many possible NF-Y complexes can form. This leads to the formation of a flexible, combinatorial system of transcription factors that may allow subtle adjustments to many different environmental conditions.
Because of overlapping functionality in the plant NF-Y families, our knowledge with respect to their functions has lagged somewhat compared with animal and fungal systems. Nevertheless, in recent years, numerous reports have emerged demonstrating the roles of individual subunits in many important processes. In this review, we comprehensively examine progress in understanding NF-Y functions in the plant lineage, providing an entrée for the nonexpert and a broad review for the aficionados. Additionally, to avoid further confusion with acronyms and the related, but functionally distinct, histone fold domain (HFD) proteins, we propose a reclassification of the plant NF-Y genes. For additional information/perspectives, we recommend the recent NF-Y review by Laloum et al. (2012).