Arabidopsis HD-Zip II transcription factors control apical embryo development and meristem function

Abstract

SUMMARYThe Arabidopsis genome encodes ten Homeodomain-Leucine zipper (HD-Zip) II proteins. ARABIDOPSIS THALIANA HOMEOBOX 2 (ATHB2), HOMEOBOX ARABIDOPSIS THALIANA 1 (HAT1), HAT2, HAT3 and ATHB4 are regulated by changes in the red/far red light ratio that induce shade avoidance in most of the angiosperms. Here, we show that progressive loss of HAT3, ATHB4 and ATHB2 activity causes developmental defects from embryogenesis onwards in white light. Cotyledon development and number are altered in hat3 athb4 embryos, and these defects correlate with changes in auxin distribution and response. athb2 gain-of-function mutation and ATHB2 expression driven by its promoter in hat3 athb4 result in significant attenuation of phenotypes, thus demonstrating that ATHB2 is functionally redundant to HAT3 and ATHB4. In analogy to loss-of-function mutations in HD-Zip III genes, loss of HAT3 and ATHB4 results in organ polarity defects, whereas triple hat3 athb4 athb2 mutants develop one or two radialized cotyledons and lack an active shoot apical meristem (SAM). Consistent with overlapping expression pattern of HD-Zip II and HD-Zip III gene family members, bilateral symmetry and SAM defects are enhanced when hat3 athb4 is combined with mutations in PHABULOSA (PHB), PHAVOLUTA (PHV) or REVOLUTA (REV). Finally, we show that ATHB2 is part of a complex regulatory circuit directly involving both HD-Zip II and HD-Zip III proteins. Taken together, our study provides evidence that a genetic system consisting of HD-Zip II and HDZip III genes cooperates in establishing bilateral symmetry and patterning along the adaxial-abaxial axis in the embryo as well as in controlling SAM activity.

Regulation of MIR165/166 by class II and class III homeodomain leucine zipper proteins establishes leaf polarity

“…Previous studies have shown that the HD-ZIPII genes HAT3 and ATHB4 play an essential role in establishing leaf polarity by promoting adaxial cell fate (5,6). Although HAT3 and ATHB4 are known to be downstream targets of the HD-ZIPIII transcription factor REVOLUTA (14), we further investigated the relationship between these genes by monitoring the expression of REV in the hat3 athb4 double mutant (6) using the functional fluorescent reporter pREV::REV-2xYPET.…”

“…To gauge the relative importance of MIR165/166 regulation to overall HD-ZIPII protein function, we transformed a miR165/166-resistant REV reporter (pREV::REVr-2xVENUS), as well as a miRsensitive REV reporter (pREV::REV-2xVENUS), into hat3 athb4 and hat3 athb4 athb2 plants (6) and assessed the degree of phenotypic rescue by REVr compared with the control. Compared with the miR-sensitive REV reporter ( (Fig.…”

“…In particular, the correct patterning of adaxialabaxial tissues in plant organs such as leaves is critical for the generation of a lamina shape and the formation of a polar vascular system (1)(2)(3)(4). Adaxial-abaxial cell-type patterning in turn depends on the restricted expression of several genes known to specify these cell types, including the class III homeodomain leucine zipper genes (HD-ZIPIIIs), KANADI genes, HD-ZIPIIs, and microRNA (MIR)165/166 (1,2,(4)(5)(6)(7)(8)(9)(10)(11). In general, genetic analyses have indicated that adaxial and abaxial factors act oppositely in organ patterning (1,2,4,(8)(9)(10)(11).…”

Auxin depletion from leaf primordia contributes to organ patterning

“…In addition, AS2 also directly suppresses ETT expression in the adaxial domain (13). However, HD-ZIPIII directly activates the expression of HD-ZIPII genes in the adaxial domain (14,15). It is conceivable that incipient leaf primordia may be prepatterned into adaxial and abaxial domains (16), because REV and KAN1 are restricted very early to the adaxial domain and the abaxial domain of incipient floral meristem, respectively (17,18).…”

Transcription factors involved in brassinosteroid repressed gene expression and their regulation by BIN2 kinase

“…23 The HD-ZIP II family members are involved in various biological processes such as shade avoidance, meristem determination, leaf polarity, gynoecium and fruit development, auxin and other hormone responses. 23,[25][26][27][28][29] The fact that HAT1 and its close homologs, HAT2, HAT3, and HAT4 (ATHB-2), are BES1/BZR1 direct target genes, 3,4 prompt us to examine the function of the family transcription factors in BR responses. HAT1 and HAT3 are positive regulators for BR-regulated growth as double mutants of hat1 hat3 have reduced BR response in hypocotyl elongation assays and overexpression of HAT1 leads to increased stem elongation at the early stage of vegetative growth.…”