Coordination of leaf development via regulation of KNOX1 genes

Class I KNOTTED1-LIKE HOMEOBOX (KNOX1) genes are expressed in the shoot apical meristem (SAM) to effect its formation and maintenance. KNOX1 genes are also involved in leaf shape control throughout angiosperm evolution. Leaves can be classified as either simple or compound, and KNOX1 expression patterns in leaf primordia are highly correlated with leaf shape; in most simple-leafed species, KNOX1 genes are expressed only in the SAM but not in leaf primordia, while in compound-leafed species they are expressed both in the SAM and leaf primordia. How can KNOX1 expression be maintained to a high degree in the SAM, but simultaneously be so variable in leaves? This dichotomy suggests that the processes of leaf and SAM development have been compartmentalized during evolution. Here, we introduce our findings regarding the regulation of expression of SHOOT MERISTEMLESS, a KNOX1 gene, together with a brief review of KNOX1 genes from an evolutionary viewpoint. We also present our findings regarding another aspect of KNOX1 regulation via a protein–protein interaction network involved in the natural variation in leaf shape. Both aspects of KNOX1 regulation could be utilized for fine-tuning leaf morphology during evolution without affecting the essential function of KNOX genes in the shoot.     

Leaving the meristem behind: regulation of KNOX genes

The mechanism by which the plant reserves some cells as pluripotent stem cells while partitioning others into differentiated leaf tissue is fundamental to plant development. New work in Arabidopsis elucidates the genetic circuitry that distinguishes meristem from leaf.     

LMI1-like and KNOX1 genes coordinately regulate plant leaf development in dicotyledons

Plant Molecular Biology - This report reveals that the LMI1-like and KNOX1 genes coordinately control the leaf development and different combinations of those genes which produce diverse leaf...     

Direct repression of KNOX loci by the ASYMMETRIC LEAVES1 complex of Arabidopsis

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.     

Reduced leaf complexity in tomato wiry mutants suggests a role for PHAN and KNOX genes in generating compound leaves

Recent work on species with simple leaves suggests that the juxtaposition of abaxial (lower) and adaxial (upper) cell fates (dorsiventrality) in leaf primordia is necessary for lamina outgrowth. However, how leaf dorsiventral symmetry affects leaflet formation in species with compound leaves is largely unknown. In four non-allelic dorsiventrality-defective mutants in tomato, wiry, wiry3, wiry4 and wiry6, partial or complete loss of ab-adaxiality was observed in leaves as well as in lateral organs in the flower, and the number of leaflets in leaves was reduced significantly. Morphological analyses and expression patterns of molecular markers for ab-adaxiality [LePHANTASTICA (LePHAN) and LeYABBY B (LeYAB B)] indicated that ab-adaxial cell fates were altered in mutant leaves. Reduction in expression of both LeT6 (a tomato KNOX gene) and LePHAN during post-primordial leaf development was correlated with a reduction in leaflet formation in the wiry mutants. LePHAN expression in LeT6 overexpression mutants suggests that LeT6 is a negative regulator of LePHAN. KNOX expression is known to be correlated with leaflet formation and we show that LeT6 requires LePHAN activity to form leaflets. These phenotypes and gene expression patterns suggest that the abaxial and adaxial domains of leaf primordia are important for leaflet primordia formation, and thus also important for compound leaf development. Furthermore, the regulatory relationship between LePHAN and KNOX genes is different from that proposed for simple-leafed species. We propose that this change in the regulatory relationship between KNOX genes and LePHAN plays a role in compound leaf development and is an important feature that distinguishes simple leaves from compound leaves.     

The cellular basis for synergy between RCO and KNOX1 homeobox genes in leaf shape diversity

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A simple nomenclature for a complex proton pump: VHA genes encode the vacuolar H(+)-ATPase

The vacuolar-type H(+)-ATPase acidifies intracellular compartments and is essential for many processes, including cotransport, guard cell movement, development, and tolerance to environmental stress. We have identified at least 26 genes encoding subunits of the vacuolar-type H(+)-ATPase in the Arabidopsis thaliana genome, although inconsistent nomenclature of these genes is confusing. The pump consists of subunits A through H of the peripheral V(1) complex, and subunits a, c, c" and d of the V(o) membrane sector. Most V(1) subunits are encoded by a single gene, whereas V(o) subunits are encoded by multiple genes found in duplicated segments of the genome. We propose to name these genes VHA-x, where x represents the letter code for each subunit. Applying a consistent nomenclature will help us to understand how the expression, assembly and activity of this pump are integrated with plant growth, signaling, development and adaptation.     

The ARP2/3 complex: an actin nucleator comes of age

The cellular functions of the actin cytoskeleton require precise regulation of both the initiation of actin polymerization and the organization of the resulting filaments. The actin-related protein-2/3 (ARP2/3) complex is a central player in this regulation. A decade of study has begun to shed light on the molecular mechanisms by which this powerful machine controls the polymerization, organization and recycling of actin-filament networks, both in vitro and in the living cell.     

Genetic networks regulated by ASYMMETRIC LEAVES1 (AS1) and AS2 in leaf development in Arabidopsis thaliana: KNOX genes control five morphological events

The asymmetric leaves 1 ( as1 ) and as2 mutants of Arabidopsis thaliana exhibit pleiotropic phenotypes. Expression of a number of genes, including three class-1 KNOTTED -like homeobox ( KNOX ) genes ( BP , KNAT2 and KNAT6 ) and ETTIN / ARF3 , is enhanced in these mutants. In the present study, we attempted to identify the phenotypic features of as1 and as2 mutants that were generated by ectopic expression of KNOX genes, using multiple loss-of-function mutations of KNOX genes as well as as1 and as2 . Our results revealed that the ectopic expression of class-1 KNOX genes resulted in reductions in the sizes of leaves, reductions in the size of sepals and petals, the formation of a less prominent midvein, the repression of adventitious root formation and late flowering. Our results also revealed that the reduction in leaf size and late flowering were caused by the repression, by KNOX genes, of a gibberellin (GA) pathway in as1 and as2 plants. The formation of a less prominent midvein and the repression of adventitious root formation were not, however, related to the GA pathway. The asymmetric formation of leaf lobes, the lower complexity of higher-ordered veins, and the elevated frequency of adventitious shoot formation on leaves of as1 and as2 plants were not rescued by multiple mutations in KNOX genes. These features must, therefore, be controlled by other genes in which expression is enhanced in the as1 and as2 mutants.     

Cytokinins in Populus x robusta Schneid: A complex in leaves

Summary At least seven cytokinins have been detected in mature leaves of Populus x robusta Schneid after chromatography on Sephadex LH-20. Two of these have similar elution volumes to zeatin and zeatin riboside. A third appears to be a cytokinin glucoside. A fourth is a new, unidentified cytokinin, susceptible to mild oxidation, and yielding two cytokinin active products after acid hydrolysis. This cytokinin complex has been found in fully expanded leaves, a tissue in which cell division is completed.     

A simple method for two-dimensional color analyses of plant leaves

Coloration and color pattern are basic information for plant physiology, ecology, and taxonomy. Until recently, such color analysis of biological materials required expensive equipment. Here we report a convenient method for analyzing color of plant leaves with ImageJ, free software. Color digital images of croton leaves were divided with ImageJ into Red, Green, and Blue images, and then exported as two-dimensional numeric data. After processing images with Adobe Photoshop Elements and Microsoft Excel, color patterns were obtained as histograms. A comparison of the histograms evaluated characteristics of croton leaf variegation. The method successfully quantified color information of the plant leaves with an inexpensive PC set. Apart from taxonomic applications, the simple algorithms allow application of the present methodology to various types of color and color pattern analyses. Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 1, pp. 139–147. The text was submitted by the authors in English.     

The web and the rock: cell adhesion and the ARP2/3 complex

Cell locomotion entails functional and structural cooperation between cell surface adhesion and the actin cytoskeleton. A new paper by DeMali et al. provides new insights into the link between actin assembly and integrin adhesion at the leading edges of migrating cells.     

Plant meristems: the interplay of KNOX and gibberellins

KNOX proteins are required for meristem activity in plants. Recent studies have identified potential targets of KNOX proteins, and in doing so suggested a possible mechanism for the generation of different leaf morphologies.     

A simple stochastic model of spatially complex neurons

A method for studying the coding properties of a multicompartmental integrate-and-fire neuron of arbitrary geometry is presented. Depolarization at each compartment evolves like a leaky integrator with an after-firing reset imposed only at the trigger zone. The frequency of firing at the steady-state regime is related to the properties of the multidimensional input. The decreasing variability of subthreshold depolarization from the dendritic tree to the trigger zone is shown for an input that is corrupted by a white noise. The role of a Poissonian noise is also investigated. The proposed method gives an estimate of the mean interspike interval that can be used to study the input output transfer function of the system. Both types of the stochastic inputs result in broadening the transfer function with respect to the deterministic case.     

A reconciliation of simple and complex models of age-dependent predation

Predation intensity often varies with prey age or development stage. Several theoretical studies have considered what effect such age-dependent predation has on the stability of predator-prey interactions. The simple models suggest its effect is typically stabilizing; more complex models suggest otherwise. Current theory is therefore divided on this issue. Here I show that the conclusion of previous studies of simple models is due to biologically extreme restrictions on certain parameter values. When these are relaxed, age-dependent predation becomes equally capable of a stabilizing or destabilizing effect, just as in more complex models. Specific conditions favoring each effect are identified and discussed.