The HD‐ZIP IV transcription factor OCL4 is necessary for trichome patterning and anther development in maize

Abstract

). † These authors contributed equally to this work. SUMMARYAmong the genes controlling the differentiation and maintenance of epidermal cell fate are members of the HD-ZIP IV class family of plant-specific transcription factors, most of which are specifically expressed in the epidermis of tissues. Here, we report the functional analysis of the maize HD-ZIP IV gene OCL4 (outer cell layer 4) via the phenotypic analysis of two insertional mutants, and of OCL4-RNAi transgenic plants. In all three materials, the macrohairs, one of the three types of trichomes present on adult maize leaf blades, developed ectopically at the margin of juvenile and adult leaves. Consistent with this phenotype, OCL4 is expressed in the epidermis of the leaf blade, with a maximum at the margin of young leaf primordia. Expression of OCL4 in the model plant Arabidopsis under the control of the GLABRA2 (GL2) promoter, a member of the Arabidopsis HD-ZIP IV family involved in trichome differentiation, did not complement the gl2-1 mutant, but instead aggravated its phenotype. The construct also caused a glabrous appearance of rosette leaves in transformed control plants of the Ler ecotype, suggesting that OCL4 inhibits trichome development both in maize and Arabidopsis. Furthermore, insertional mutants showed a partial male sterility that is likely to result from the presence of an extra subepidermal cell layer with endothecium characteristics in the anther wall. Interestingly, the epidermis-specific OCL4 expression in immature anthers was restricted to the region of the anther locule where the extra cell layer differentiated. Taken together these results suggest that OCL4 inhibits trichome development and influences division and/or differentiation of the anther cell wall.

Genome-Wide Characterization of the HD-ZIP IV Transcription Factor Family in Maize: Preferential Expression in the Epidermis

“…With the notable exception of ZmHDZIV11, which is expressed in mature but not in immature tassels, the expression of most ZmHDZIV genes is more abundant in immature compared with mature organs. This observation is consistent with the functional analysis of ZmHDZIV4_OCL4, which demonstrated that morphological defects in ocl4 mutant leaves and anthers become microscopically detectable at stages when ZmHDZIV4_OCL4 is no longer expressed, but these occur precisely in those tissues that express ZmHDZIV4_OCL4 earlier in development (Vernoud et al, 2009). Extrapolating these observations, ZmHDZIV genes may set the stage in immature organs for developmental events manifesting themselves in mature organs.…”

“…Previous findings in eudicots suggest that this is a general feature of transcription factor genes, which show significantly higher retention than genes fulfilling basic cellular functions (Tang et al, 2008). In addition, one may speculate that the specialization of epidermal functions in land plants necessitated additional control mechanisms, and it may not be a coincidence that all the HD-ZIP IV genes involved in trichome development, which are GL2, HDG11, and HDG12 from Arabidopsis , ZmHDZIV4_OCL4 from maize (Vernoud et al, 2009), and GaHOX1 from cotton (Guan et al, 2008), are members of clade Ib (data not shown for GaHOX1).…”

“…Total RNA from different maize organs (described by Vernoud et al, 2009) was extracted using TRIzol reagent according to the manufacturer's instructions (Invitrogen). Genomic DNA contaminants were removed from RNA samples using TURBO DNase from Ambion.…”

“…Single mutants in the remaining 13 HD-ZIP IV genes lack detectable phenotypes, but the atml1pdf2 double mutant has a very strong epidermal phenotype; it never forms an organized epidermal layer in the apical part of the proembryo and dies at the seedling stage under standard conditions (Abe et al, 2003). In maize, the ocl4 mutant is characterized by ectopic macrohairs (a class of maize trichomes) on the leaf margins and the formation of an additional, subepidermal cell layer in the anther wall (Vernoud et al, 2009), whereas a dominant negative form of OCL1 causes a strong delay in kernel development (Khaled et al, 2005). In tomato, a point mutation in Cutin deficient2 is likely responsible for the dramatically reduced cutin content in the tomato fruit (Isaacson et al, 2009).…”

Spatiotemporally dynamic, cell-type–dependent premeiotic and meiotic phasiRNAs in maize anthers

“…3A and Dataset S3) (20). OCL4 (OUTER CELL LAYER 4) is an epidermal-specific transcription factor repressing periclinal divisions in the adjacent subepidermal endothecial cells (21), presumably through a mobile signal (22). ocl4 anthers lack all 21-nt premeiotic phasiRNAs despite containing reduced but robust levels of the miR2118 trigger (Fig.…”