SUMMARYUnlike the situation in animals, the final morphology of the plant body is highly modulated by the environment. During Arabidopsis development, intrinsic factors provide the framework for basic patterning processes. CLASS III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIPIII) transcription factors are involved in embryo, shoot and root patterning. During vegetative growth HD-ZIPIII proteins control several polarity set-up processes such as in leaves and the vascular system. We have identified several direct target genes of the HD-ZIPIII transcription factor REVOLUTA (REV) using a chromatin immunoprecipitation/DNA sequencing (ChIP-Seq) approach. This analysis revealed that REV acts upstream of auxin biosynthesis and affects directly the expression of several class II HD-ZIP transcription factors that have been shown to act in the shadeavoidance response pathway. We show that, as well as involvement in basic patterning, HD-ZIPIII transcription factors have a critical role in the control of the elongation growth that is induced when plants experience shade. Leaf polarity is established by the opposed actions of HD-ZIPIII and KANADI transcription factors. Finally, our study reveals that the module that consists of HD-ZIPIII/KANADI transcription factors controls shade growth antagonistically and that this antagonism is manifested in the opposed regulation of shared target genes.

SUMMARYPlants sense the presence of competing neighboring vegetation as a change in light quality: i.e. they sense the reduced ratio of red light to far-red light. The responses to shade are generally referred to as the shade avoidance syndrome (SAS), and involve various developmental changes intended to outgrow or outcompete the neighboring plants. Here, we analyze the function of ATHB4, a gene encoding a homeodomain-leucine zipper (HD-Zip) class-II transcription factor from Arabidopsis thaliana, the expression of which is rapidly and directly upregulated after proximity perception by the phytochrome photoreceptors. ATHB4 acts redundantly with other members of the HD-Zip class-II transcription factors. The expression of these genes is regulated by other members of the same class, forming a small transcriptional network of factors in which homeostasis is mutually controlled. Our results suggest that some members of this small gene subfamily can modulate SAS responses by controlling auxin, brassinosteroid and gibberellin molecular and/or physiological responsiveness. In particular, we propose ATHB4 as a new shade signaling component that participates in integrating shade perception and hormone-mediated growth.

In response to plant proximity or canopy shade, plants can react by altering elongation growth and development. Several members of the class II homeodomain-leucine zipper (HD-ZIPII) transcription factor family have been shown to play an instrumental role in the responses to shade. HD-ZIP members of the class III (HD-ZIPIII), by contrast, are involved in basic patterning processes. We recently showed that REVOLUTA (REV), a member of the HD-ZIPIII family, directly and positively regulates the expression of several genes involved in shade-induced growth, such as those encoding HD-ZIPII factors HAT2, HAT3, ATHB2/HAT4 and ATHB4, and of the components of the auxin biosynthesis pathway YUCCA5 and TAA1. Furthermore, we could demonstrate a novel role for HD-ZIPIII in shade-induced promotion of growth. Here we show that besides responding to shade, ATHB4 and HAT3 have a critical role in establishing the dorso-ventral axis in cotyledons and developing leaves. Loss-of-function mutations in these two HD-ZIPII genes (athb4 hat3) results in severely abaxialized, entirely radialized leaves. Conversely, overexpression of HAT3 results in adaxialized leaf development. Taken together, our findings unravel a so far unappreciated role for an HD-ZIPII/HD-ZIPIII module required for dorso-ventral patterning of leaves. The finding that HD-ZIPII/HD-ZIPIII also function in shade avoidance suggests that this module is at the nexus of patterning and growth promotion.

SummaryShade perception involves altered hormone synthesis and sensitivity. Here, we showed that several shade regulators act as positive and negative modulators of the hypocotyl auxin and/or brassinosteroid-induced elongation.

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