Homeodomain-Leucine Zipper II family of transcription factors to the limelight: central regulators of plant development.

The Arabidopsis genome encodes 10 Homeodomain-Leucine Zipper (HD-Zip) II transcription factors that can be subdivided into 4 clades (α-δ). All the γ (ARABIDOPSIS THALIANA HOMEOBOX 2 [ATHB2], HOMEOBOX ARABIDOPSIS THALIANA 1 [HAT1], HAT2) and δ (HAT3, ATHB4) genes are regulated by light quality changes (Low Red [R]/Far-Red [FR]) that induce the shade avoidance response in most of the angiosperms. HD-Zip IIγ and HD-Zip IIδ transcription factors function as positive regulators of shade avoidance, and there is evidence that at least ATHB2 is directly positively regulated by the basic Helix-Loop-Helix (bHLH) proteins PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5. Recent evidence demonstrate that, in addition to their function in shade avoidance, HD-Zip IIγ and HD-Zip IIδ proteins play an essential role in plant development from embryogenesis onwards in a white light environment.