Julie Kang

Vascular patterning and leaf shape

Morphogenesis of leaf shape and formation of the major elements of leaf vasculature are temporally coordinated during leaf development. Current analyses of mutant phenotypes provide strong support for the role of auxin signaling in vascular pattern formation and indicate that leaf shape and vasculature are developmentally coupled. Two other mechanisms that may contribute to the regulation of these processes are a diffusion-reaction system and long-distance signaling of informational macromolecules.

Auxin patterns Solanum lycopersicum leaf morphogenesis.

One of the most striking aspects of plant diversity is variation in leaf shape. Much of this diversity is achieved by the modulation of leaf blade dissection to form lobes or leaflets. Here, we show that the phytohormone auxin is a crucial signal regulating the partitioned outgrowth necessary to develop a dissected leaf. In developing leaves, the asymmetric distribution of auxin, driven by active transport, delineates the initiation of lobes and leaflets and specifies differential laminar outgrowth. Furthermore, homologous members of the AUX/indole-3-acetic acid (IAA) gene family mediate the action of auxin in determining leaf shape by repressing outgrowth in areas of low auxin concentration during both simple and compound leaf development. These results provide molecular evidence that leaflets initiate in a process reminiscent of organogenesis at the shoot apical meristem, but that compound and simple leaves regulate marginal growth through an evolutionarily conserved mechanism, thus shedding light on the homology of compound and simple leaves.

Natural Variation in Leaf Morphology Results from Mutation of a Novel KNOX Gene

Striking diversity in size, arrangement, and complexity of leaves can sometimes be seen in closely related species. One such variation is found between wild tomato species collected by Charles Darwin from the Galapagos Islands [1-5]. Here, we show that a single-nucleotide deletion in the promoter of the PETROSELINUM (PTS) [3] gene upregulates the gene product in leaves and is responsible for the natural variation in leaf shape in the Galapagean tomatoes. PTS encodes a novel KNOTTED1-LIKE HOMEOBOX (KNOX) gene that lacks a homeodomain. We also showed that the tomato classical mutant bipinnata (bip) [6], which recapitulates the Pts phenotype, results from the loss of function of a BEL-LIKE HOMEODOMAIN (BELL) gene, BIP. We used bimolecular fluorescence complementation and two-hybrid competition assays to show that PTS represses KNOX1 protein interactions with BIP, as well as subsequent nuclear localization of this transcriptional complex. We suggest that natural variation in leaf shape can be created with a rheostat-like mechanism that alters the KNOX1 protein interaction network specifically during leaf development. This subtle change in interaction between transcription factors leaves essential KNOX1 function in the shoot apical meristem intact and appears to be a facile way to alter leaf morphology during evolution.

VEIN PATTERN DEVELOPMENT IN ADULT LEAVES OF ARABIDOPSIS THALIANA

Vein pattern development in the leaves of higher plants requires that a continuous system with hierarchical vein size classes and regular spacing be formed de novo from ground meristem precursors. In this study, we use a molecular marker of procambium identity, AtHB‐8::GUS, to investigate the procambial stage of vein pattern formation in adult rosette leaves of Arabidopsis thaliana and to compare the elaboration of AtHB‐8‐marked vein pattern with that expressed by xylem differentiation. While the well‐studied juvenile leaves of Arabidopsis have a relatively simple brochidodromous vein pattern with the simultaneous appearance of the looped secondary veins, adult leaves have a complex semicraspedodromous vein pattern, and a majority of the straight secondary veins develop progressively. Late‐formed secondary veins either arise in the basal portions of the leaf or are intercalated between earlier‐formed secondary veins. Smaller “connector” veins become enhanced during development to form the subterminal loops of the semicraspedodromous secondary vein pattern. Higher‐order veins, especially freely ending veinlets, are formed throughout leaf expansion, maintaining a stable vein density. These unique features of adult leaf vein pattern are strongly correlated with the presence of marginal serrations and a protracted period of leaf expansion. In early leaf development, AtHB‐8::GUS expression precedes any of the hallmark anatomical features of procambial cells in presumptive procambial strands, defining a “preprocambial” stage. In contrast, AtHB‐8::GUS expression was not detected during the late formation of higher‐order veins, indicating that functionally redundant mechanisms guide the development of leaf vascular pattern.

Interspecific RNA Interference of SHOOT MERISTEMLESS-Like Disrupts Cuscuta pentagona Plant Parasitism

The authors demonstrate that parasite gene-specific silencing signals originating from a transgenic host are transferred into the invading parasite, leading to reduced parasite yield, stature, and infectivity. This article also refreshes the debate on the origin of haustoria as the authors use morphological and molecular evidence to show that haustoria have both stem and root characteristics. Infection of crop species by parasitic plants is a major agricultural hindrance resulting in substantial crop losses worldwide. Parasitic plants establish vascular connections with the host plant via structures termed haustoria, which allow acquisition of water and nutrients, often to the detriment of the infected host. Despite the agricultural impact of parasitic plants, the molecular and developmental processes by which host/parasitic interactions are established are not well understood. Here, we examine the development and subsequent establishment of haustorial connections by the parasite dodder (Cuscuta pentagona) on tobacco (Nicotiana tabacum) plants. Formation of haustoria in dodder is accompanied by upregulation of dodder KNOTTED-like homeobox transcription factors, including SHOOT MERISTEMLESS-like (STM). We demonstrate interspecific silencing of a STM gene in dodder driven by a vascular-specific promoter in transgenic host plants and find that this silencing disrupts dodder growth. The reduced efficacy of dodder infection on STM RNA interference transgenics results from defects in haustorial connection, development, and establishment. Identification of transgene-specific small RNAs in the parasite, coupled with reduced parasite fecundity and increased growth of the infected host, demonstrates the efficacy of interspecific small RNA–mediated silencing of parasite genes. This technology has the potential to be an effective method of biological control of plant parasite infection.

Modification of cell proliferation patterns alters leaf vein architecture in Arabidopsis thaliana

Formation of leaf vascular pattern requires regulation of a number of cellular processes, including cell proliferation. To assess the role of cell proliferation during vein order formation, leaf development in genetic lines exhibiting aberrant cell proliferation patterns due to altered expression patterns of ANT and ICK1 genes was analyzed. Modification of cell proliferation patterns alters the number of higher order veins and the number of minor tertiary veins remodeled as intersecondary veins in Arabidopsis rosette leaves. Minor vein complexity, as indicated by branch point and freely ending veinlet number, is highly correlated with a decrease or increase in cell proliferation. Observations of procambial strand formation in modified cell proliferation pattern lines showed that vein pattern is specified early in leaf development and that formation of freely ending veinlets is temporally correlated with the expansion of ground meristem when cell proliferation is terminated prematurely. Taken together, our observations indicate that: (1) genes that modulate cell proliferation play a key role in regulating the meristematic competence of ground meristem cells to form procambium and vein pattern during leaf development, and (2) ANT is a crucial part of this regulation.

The development of enantiostyly

Enantiostyly, the deflection of the style either to the left (left-styled) or right (right-styled) side of the floral axis, has evolved in at least ten angiosperm families. Two types of enantiostyly occur: monomorphic enantiostyly, in which individuals exhibit both stylar orientations, and dimorphic enantiostyly, in which the two stylar orientations occur on separate plants. To evaluate architectural or developmental constraints on the evolution of both forms of enantiostyly, we examined inflorescence structure and floral development among unrelated enantiostylous species. We investigated relations between the position of left- and right-styled flowers and inflorescence architecture in four monomorphic enantiostylous species, and we examined the development of enantiostyly in nine monomorphic and dimorphic enantiostylous species from five unrelated lineages. The location of left- and right-styled flowers within inflorescences ranged from highly predictable (in Solanum rostratum) to random (in Heteranthera mexicana). There were striking differences among taxa in the timing of stylar bending. In Wachendorfia paniculata, Dilatris corymbosa, and Philydrum lanuginosum, the style deflected in the bud, whereas in Heteranthera spp., Monochoria australasica, Cyanella lutea, and Solanum rostratum, stylar bending occurred at the beginning of anthesis. Comparisons of organ initiation and development indicated that asymmetries along the left-right axis were expressed very late in development, despite the early initiation of a dorsiventral asymmetry. We suggest that the evolution of dimorphic enantiostyly from monomorphic enantiostyly may be constrained by a lack of left-right positional information in the bud.

KNOX1 genes regulate lignin deposition and composition in monocots and dicots.

Plant secondary cell walls are deposited mostly in vascular tissues such as xylem vessels, tracheids, and fibers. These cell walls are composed of a complex matrix of compounds including cellulose, hemicellulose, and lignin. Lignin functions primarily to maintain the structural and mechanical integrity of both the transport vessel and the entire plant itself. Since lignin has been identified as a major source of biomass for biofuels, regulation of secondary cell wall biosynthesis has been a topic of much recent investigation. Biosynthesis and patterning of lignin involves many developmental and environmental cues including evolutionarily conserved transcriptional regulatory modules and hormonal signals. Here, we investigate the role of the class I Knotted1-like-homeobox (KNOX) genes and gibberellic acid in the lignin biosynthetic pathway in a representative monocot and a representative eudicot. Knotted1 overexpressing mutant plants showed a reduction in lignin content in both maize and tobacco. Expression of four key lignin biosynthesis genes was analyzed and revealed that KNOX1 genes regulate at least two steps in the lignin biosynthesis pathway. The negative regulation of lignin both in a monocot and a eudicot by the maize Kn1 gene suggests that lignin biosynthesis may be preserved across large phylogenetic distances. The evolutionary implications of regulation of lignification across divergent species are discussed.

Regulation of leaf shape

The arrangement and shape of leaves of flowering plants are some of the most diverse characteristics…