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The liverwort Marchantia polymorpha has been utilized as a model for biological studies since the 18th century. In the past few decades, there has been a Renaissance in its utilization in genomic and genetic approaches to investigating physiological, developmental, and evolutionary aspects of land plant biology. The reasons for its adoption are similar to those of other genetic models, e.g. simple cultivation, ready access via its worldwide distribution, ease of crossing, facile genetics, and more recently, efficient transformation, genome editing, and genomic resources. The haploid gametophyte dominant life cycle of M. polymorpha is conducive to forward genetic approaches. The lack of ancient whole-genome duplications within liverworts facilitates reverse genetic approaches, and possibly related to this genomic stability, liverworts possess sex chromosomes that evolved in the ancestral liverwort. As a representative of one of the three bryophyte lineages, its phylogenetic position allows comparative approaches to provide insights into ancestral land plants. Given the karyotype and genome stability within liverworts, the resources developed for M. polymorpha have facilitated the development of related species as models for biological processes lacking in M. polymorpha. [ABSTRACT FROM AUTHOR]

A key question in developmental biology is how cells exchange positional information for proper patterning during organ development. In plant roots the radial tissue organization is highly conserved with a central vascular cylinder in which two water conducting cell types, protoxylem and metaxylem, are patterned centripetally. We show that this patterning occurs through crosstalk between the vascular cylinder and the surrounding endodermis mediated by cell-to-cell movement of a transcription factor in one direction and microRNAs in the other. SHORT ROOT, produced in the vascular cylinder, moves into the endodermis to activate SCARECROW. Together these transcription factors activate MIR165a and MIR166b. Endodermally produced microRNA165/6 then acts to degrade its target mRNAs encoding class III homeodomain-leucine zipper transcription factors in the endodermis and stele periphery. The resulting differential distribution of target mRNA in the vascular cylinder determines xylem cell types in a dosage-dependent manner., Organ development involves extensive communication between cells to orchestrate tissue specification and differentiation. This communication is often mediated by mobile molecules such as hormones, mRNAs, proteins and small RNAs (1). [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2007.04.004 Byline: John L. Bowman (1), Sandra K. Floyd (1), Keiko Sakakibara (1) Abstract: As more plant genome sequences become available, researchers are increasingly using comparative genomics to address some of the major questions in plant biology. Such questions include the evolution of photosynthesis and multicellularity, the developmental genetic changes responsible for alterations in body plan, and the origin of important plant innovations such as roots, leaves, and vascular tissue. Author Affiliation: (1) School of Biological Sciences, Monash University, Clayton Campus, Melbourne, Victoria 3800, Australia

Land plants underwent tremendous evolutionary change following the divergence of the ancestral lineage from algal relatives. Several important developmental innovations appeared as the embryophyte clade diversified, leading to the appearance of new organs and tissue types. To understand how these changes came about, we need to identify the fundamental genetic developmental programs that are responsible for growth, patterning, and differentiation and describe how these programs were modified and elaborated through time to produce novel morphologies. Class III homeodomain-leucine zipper (class III HD-Zip) genes, identified in the model plant Arabidopsis thaliana, provide good candidates for basic land plant patterning genes. We show that these genes may have evolved in a common ancestor of land planks and their algal sister group and that the gene family has diversified as land plant lineages have diversified. Phylogenetic analysis, expression data from nonflowering lineages, and evidence from Arabidopsis and other flowering plants indicate that class III HD-Zip genes acquired new functions in sporophyte apical growth, vascular patterning and differentiation, and leaf development. Modification of expression patterns that accompanied diversification of class III HD-Zip genes likely played an important role in the evolution of land plant form.

A study is conducted to provide insights into how miRNAs, likely acting through a complex containing an ARGONAUTE protein, regulate Class III HD-Zip gene expression in arabidopsis and maize. Many of the targets of miRNA-mediated gene regulation encode transcription factors with functions in development such as the class III HD-Zip gene family whose members direct polarity establishment in leaves and vasculature.

Author(s): Sarah J. Liljegren [1]; Gary S. Ditta [1]; Yuval Eshed [2]; Beth Savidge [1, 3]; John L. Bowman [2]; Martin F. Yanofsky (corresponding author) [1] The fruit, which mediates [...]

Polarity establishment in carpels of Arabidopsis are discussed. They are promoted by distinct mechanisms. Genetic enhancers of the crc mutant phenotype were isolated to shed light on the role of CRABS CLAW (CRC) in carpel development. Mutations representing genes that act redundantly with CRC in control of abaxial-adaxial polarity in the carpel were found.