ragged seedling2Encodes an ARGONAUTE7-Like Protein Required for Mediolateral Expansion, but Not Dorsiventrality, of Maize Leaves  

Abstract

Leaves arise from the flank of the shoot apical meristem and are asymmetrical along the adaxial/abaxial plane from inception. Mutations perturbing dorsiventral cell fate acquisition in a variety of species can result in unifacial (radially symmetrical) leaves lacking adaxial/abaxial polarity. However, mutations in maize (Zea mays) ragged seedling2 (rgd2) condition cylindrical leaves that maintain dorsiventral polarity. Positional cloning reveals that rgd2 encodes an ARGONAUTE7 (AGO7)-like protein required to produce ta-siARF, a trans-acting small interfering RNA that targets abaxially located auxin response factor3a (arf3a) transcripts for degradation. Previous studies implicated ta-siARF in dorsiventral patterning of monocot leaves. Here, we show that arf3a transcripts hyperaccumulate but remain abaxialized in rgd2 mutant apices, revealing that ta-siARF function is not required for arf3a polarization. RGD2 also regulates miR390 accumulation and localization in maize shoot apices. Similar to the abaxialized maize mutant leafbladeless1 (lbl1), rgd2 mutants exhibit ectopic accumulation of the abaxial identity factor miR166 in adaxial domains. Thus, hyperaccumulation of arf3a and ectopic accumulation of miR166 are insufficient to condition abaxialized leaf phenotypes in maize. Finally, transcripts of a maize ago1 paralog overaccumulate in lbl1 but not in rgd2 mutants, suggesting that upregulation of ago1 combined with ectopic accumulation of miR166 contribute to abaxialized leaf formation in lbl1. We present a revised model for the role of small RNAs in dorsiventral patterning of maize leaves.

Transcriptomic Analyses Indicate That Maize Ligule Development Recapitulates Gene Expression Patterns That Occur during Lateral Organ Initiation

“…leaf primordia (Supplemental Figures 2B and 2C). According to a model proposed by Waites and Hudson (1995), the juxtaposition of adaxial (upper leaf) and abaxial (lower leaf) developmental fields generates a new leaf axis that promotes laminar outgrowth (Waites and Hudson, 1995;Timmermans et al, 1998;Nogueira et al, 2007;Candela et al, 2008;Douglas et al, 2010). This mechanism may have been co-opted to generate new growth axes in leaves with nonplanar morphologies, such as the maize prophyll (Johnston et al, 2010).…”

PUNCTATE VASCULAR EXPRESSION1Is a Novel Maize Gene Required for Leaf Pattern Formation That Functions Downstream of the Trans-Acting Small Interfering RNA Pathway

“…In wild-type maize, ARF3A accumulates in abaxial regions of leaf primordia. Surprisingly, ARF3A still accumulates abaxially in rgd2 and lbl1 mutants, which are deficient in ta-siARF biosynthesis (Douglas et al, 2010). These findings suggest that ARF3A transcripts are polarized independent of the ta-siARF biogenesis pathway.…”

“…As reported previously (Henderson et al, 2005;Douglas et al, 2010), plants homozygous for the null mutation rgd2-R displayed a variable range of mutant shoot phenotypes (Fig. 1A).…”

“…RGD2 encodes an ARGONAUTE7 (AGO7)-like protein that is required for biogenesis of a transacting small interfering RNA termed trans-acting small interfering RNA (ta-siARF; Douglas et al, 2010). The production of ta-siARF is a plant-specific process that utilizes both microRNA and siRNA biogenesis components in a single pathway (Allen et al, 2005;Yoshikawa et al, 2005).…”

“…The production of ta-siARF is a plant-specific process that utilizes both microRNA and siRNA biogenesis components in a single pathway (Allen et al, 2005;Yoshikawa et al, 2005). ta-siARF production begins with the cleavage of the nonprotein coding TAS3 transcripts by an RNA-induced silencing complex (RISC) composed of miR390 and RGD2/AGO7 (Allen et al, 2005;Adenot et al, 2006;Fahlgren et al, 2006;Douglas et al, 2010). Rather than being degraded, as would happen to most transcripts cleaved by a RISC, the cleavage product is stabilized and converted into double-stranded RNA in a process that requires LEAFBLADELESS1 (LBL1)/SUPPRESSOR-OF-GENE-SILENCING3, and RNA-DEPENDANT RNA POLYMERASE6.…”