Direct Repression of KNOX Loci by the ASYMMETRIC LEAVES1 Complex of Arabidopsis[W][OA]

KNOTTED1-like homeobox (KNOX) genes promote stem cell activity and must be repressed to form determinate lateral organs. Stable KNOX gene silencing during organogenesis is known to involve the predicted DNA binding proteins ASYMMETRIC LEAVES1 (AS1) and AS2 as well as the chromatin-remodeling factor HIRA. However, the mechanism of silencing is unknown. Here, we show that AS1 and AS2 form a repressor complex that binds directly to the regulatory motifs CWGTTD and KMKTTGAHW present at two sites in the promoters of the KNOX genes BREVIPEDICELLUS (BP) and KNAT2. The two binding sites act nonredundantly, and interaction between AS1-AS2 complexes at these sites is required to repress BP. Promoter deletion analysis further indicates that enhancer elements required for BP expression in the leaf are located between the AS1-AS2 complex binding sites. We propose that AS1-AS2 complexes interact to create a loop in the KNOX promoter and, likely through recruitment of HIRA, form a repressive chromatin state that blocks enhancer activity during organogenesis. Our model for AS1-AS2–mediated KNOX gene silencing is conceptually similar to the action of an insulator. This regulatory mechanism may be conserved in simple leafed species of monocot and dicot lineages and constitutes a potential key determinant in the evolution of compound leaves

The 46th Annual Maize Genetics Conference. Unlocking the Secrets of the Maize Genome

For the first time in its history, the Annual Maize Genetics Conference was held in Mexico City, near the center of origin for many Zea species, including maize. Maize research has made many contributions to our understanding of plant physiology and development, the regulation of transposable elements and chromosome structure, and the epigenetic control of gene expression. In addition to the reported advances in these research fields, this year's meeting emphasized the tremendous genetic diversity present within maize races. Explorations of this variation in studies of domestication, population genetics, and crop improvement hinted at the tremendous potential that lies within the maize genome. Tapping into this potential will soon be made easier as highlighted in a workshop outlining advances in maize genomics. Remarkable progress has been made toward sequencing the genic regions of the maize genome, and strategies to anchor and finish the entire maize gene space by 2006 were presented and discussed. This report highlights the new developments made in these areas of maize biology

Signals and prepatterns: new insights into organ polarity in plants

The flattening of leaves results from the interaction between upper (adaxial) and lower (abaxial) domains in the developing primordium. These domains are specified by conserved, overlapping genetic pathways involving several distinct transcription factor families and small regulatory RNAs. Polarity determinants employ a series of antagonistic interactions to produce mutually exclusive cell fates whose positioning is likely refined by signaling across the adaxial–abaxial boundary. Signaling candidates include a mobile small RNA—the first positional signal described in adaxial–abaxial polarity. Possible mechanisms to polarize the incipient primordium are discussed, including meristem-derived signaling and a model in which a polarized organogenic zone prepatterns the adaxial–abaxial axis

Maize rough sheath2 and Its Arabidopsis Orthologue ASYMMETRIC LEAVES1 Interact with HIRA, a Predicted Histone Chaperone, to Maintain knox Gene Silencing and Determinacy during Organogenesis

Plant shoots are characterized by indeterminate growth resulting from the action of a population of stem cells in the shoot apical meristem (SAM). Indeterminacy within the SAM is specified in part by the class I knox homeobox genes. The myb domain proteins rough sheath2 (RS2) and ASYMMETRIC LEAVES1 (AS1) from maize (Zea mays) and Arabidopsis thaliana, respectively, are required to establish determinacy during leaf development. These proteins are part of a cellular memory system that in response to a stem cell–derived signal keeps knox genes in an off state during organogenesis. Here, we show that RS2/AS1 can form conserved protein complexes through interaction with the DNA binding factor ASYMMETRIC LEAVES2, a predicted RNA binding protein (RIK, for RS2-Interacting KH protein), and a homologue of the chromatin-remodeling protein HIRA. Partial loss of HIRA function in Arabidopsis results in developmental defects comparable to those of as1 and causes reactivation of knox genes in developing leaves, demonstrating a direct role for HIRA in knox gene repression and the establishment of determinacy during leaf formation. Our data suggest that RS2/AS1 and HIRA mediate the epigenetic silencing of knox genes, possibly by modulating chromatin structure. Components of this process are conserved in animals, suggesting the possibility that a similar epigenetic mechanism maintains determinacy during both plant and animal development

Two small regulatory RNAs establish opposing fates of a developmental axis

Small RNAs are important regulators of gene expression. In maize, adaxial/abaxial (dorsoventral) leaf polarity is established by an abaxial gradient of microRNA166 (miR166), which spatially restricts the expression domain of class III homeodomain leucine zipper (HD-ZIPIII) transcription factors that specify adaxial/upper fate. Here, we show that leafbladeless1 encodes a key component in the trans-acting small interfering RNA (ta-siRNA) biogenesis pathway that acts on the adaxial side of developing leaves and demarcates the domains of hd-zipIII and miR166 accumulation. Our findings indicate that tasiR-ARF, a ta-siRNA, and miR166 establish opposing domains along the adaxial–abaxial axis, thus revealing a novel mechanism of pattern formation

ragged seedling2 Encodes an ARGONAUTE7-Like Protein Required for Mediolateral Expansion, but Not Dorsiventrality, of Maize Leaves[C][W]

This work identifies ragged seedling2 as a maize homolog of Arabidopsis argonaute7 and focuses on the role of small RNA molecules in establishing leaf polarity

Pattern formation via small RNA mobility

MicroRNAs and trans-acting siRNAs (ta-siRNAs) have important regulatory roles in development. Unlike other developmentally important regulatory molecules, small RNAs are not known to act as mobile signals during development. Here, we show that low-abundant, conserved ta-siRNAs, termed tasiR-ARFs, move intercellularly from their defined source of biogenesis on the upper (adaxial) side of leaves to the lower (abaxial) side to create a gradient of small RNAs that patterns the abaxial determinant AUXIN RESPONSE FACTOR3. Our observations have important ramifications for the function of small RNAs and suggest they can serve as mobile, instructive signals during development