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 (alfa-delta). All the gamma (ARABIDOPSIS THALIANA HOMEOBOX 2 [ATHB2], HOMEOBOX ARABIDOPSIS THALIANA 1 [HAT1], HAT2) and delta (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 II gamma and HD-Zip II delta 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 PHYTO CHROM E INT ERA CTIN G FACTOR 4 (PIF4) and PIF5. Recent evidence demonstrate that, in addition to their function in shade avoidance, HD-Zip II gamma and HD-Zip II delta proteins play an essential role in plant development from embryogenesis onwards in a white light environment. © 2013 Landes Bioscience.