Overexpression of the Epidermis-Specific Homeodomain-Leucine Zipper IV Transcription Factor OUTER CELL LAYER1 in Maize Identifies Target Genes Involved in Lipid Metabolism and Cuticle Biosynthesis    

Abstract

Transcription factors of the homeodomain-leucine zipper IV (HD-ZIP IV) family play crucial roles in epidermis-related processes. To gain further insight into the molecular function of OUTER CELL LAYER1 (OCL1), 14 target genes up- or down-regulated in transgenic maize (Zea mays) plants overexpressing OCL1 were identified. The 14 genes all showed partial coexpression with OCL1 in maize organs, and several of them shared preferential expression in the epidermis with OCL1. They encoded proteins involved in lipid metabolism, defense, envelope-related functions, or cuticle biosynthesis and include ZmWBC11a (for white brown complex 11a), an ortholog of AtWBC11 involved in the transport of wax and cutin molecules. In support of the annotations, OCL1-overexpressing plants showed quantitative and qualitative changes of cuticular wax compounds in comparison with wild-type plants. An increase in C24 to C28 alcohols was correlated with the transcriptional up-regulation of ZmFAR1, coding for a fatty acyl-coenzyme A reductase. Transcriptional activation of ZmWBC11a by OCL1 was likely direct, since transactivation in transiently transformed maize kernels was abolished by a deletion of the activation domain in OCL1 or mutations in the L1 box, a cis-element bound by HD-ZIP IV transcription factors. Our data demonstrate that, in addition to AP2/EREBP and MYB-type transcription factors, members of the HD-ZIP IV family contribute to the transcriptional regulation of genes involved in cuticle biosynthesis.

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

“…4B). As observed previously (Javelle et al, 2010), ZmHDZIV1_OCL1 transcripts accumulated preferentially but not exclusively in epidermal cells of juvenile leaves. Similar results were obtained for ZmHDZIV3_ OCL3 and ZmHDZIV10.…”

“…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). In addition, numerous direct or indirect targets of the maize HD-ZIP IV transcription factors are predicted to function in epidermal processes, which further supports the proposed link between this transcription factor family and epidermis-related functions (Abe et al, 2003;Javelle et al, 2010). In particular, OCL1 activates genes involved in lipid metabolism and cuticle biosynthesis in maize (Javelle et al, 2010).…”

The Formation and Function of Plant Cuticles

“…A more general role of HD-ZIP IV proteins in regulating cutin synthesis is further suggested by the homologous tomato gene CD2, which is required for the biosynthesis of cutin in the fruit and other organs Nadakuduti et al, 2012). In maize (Zea mays), the HD-ZIP IV gene OUTER CELL LAYER1 (OCL1) was shown to be an epidermisspecific positive regulator of wax biosynthesis, although cutin was not quantified in plants overexpressing this gene (Javelle et al, 2010). Interestingly, HD-ZIP IV proteins have also been implicated in regulating other epidermis-specific processes, such as trichome differentiation and the formation of root hairs and stomatal guard cells (Masucci et al, 1996;Nakamura et al, 2006;Takada et al, 2013).…”

Duplicate Maize Wrinkled1 Transcription Factors Activate Target Genes Involved in Seed Oil Biosynthesis

“…Total RNA was extracted with the TRIzol reagent, treated with DNase, and reverse transcribed as described previously (Javelle et al, 2010). Real-time PCR was carried out with the Platinum SYBR Green qPCR SuperMix UDG (Invitrogen) on a StepOne Real-Time PCR System (Applied Biosystems).…”