The formation of leaves

How dense can you be? New automatic measures of vein density in angiosperm leaves

Premise

Because of the trade-off between water loss and carbon dioxide assimilation, the conductivity of the transpiration path in a leaf is an important limit on photosynthesis. Closely packed veins correspond to short paths and high assimilation rates while widely spaced veins are associated with higher resistance to flow and lower maximum photosynthetic rates. Vein length per area (VLA) has become the standard metric for comparing leaves with different vein densities; its measurement typically utilizes digital image processing with varying amounts of human input.

Methods and Results

Here, we propose three new ways of measuring vein density using image analysis that improve on currently available procedures: (1) areole area distributions, (2) a sizing transform, and (3) a distance map. Each alternative has distinct practical, statistical, and biological limitations and advantages. In particular, we advocate the log-transformed modal distance map of a vein mask as an estimator to replace VLA as a standard metric for vein density.

Conclusions

These methods, for which open-source code appropriate for high-throughput automation is provided, improve on VLA by producing determinate measures of vein density as distributions rather than point estimates. Combined with advances in image quality and computational efficiency, these methods should help clarify the physiological and evolutionary significance of vein density.

Chloroplast differentiation in the growing leaves of Arabidopsis thaliana

Here, we describe the development of chloroplasts and the buildup of the thylakoid membranes in growing Arabidopsis leaves. Organelles were analyzed from three distinct positions, namely, at the tip, the upper leaf margin, and the base from leaves 1, 3, 5, and 7 of 14-day-old plants. Clear developmental gradients are described within a given leaf and between leaves of different age. Chloroplasts at the tip of every leaf are always the most matured within a given leaf, while already at the upper leaf margin a differentiation gradient can be observed from the edge of the leaf toward the midrib. The data presented here can serve as a standard for a subcellular phenotypic analysis in chloroplast biogenesis mutants.

Reading the leaves: A comparison of leaf rank and automated areole measurement for quantifying aspects of leaf venation

The reticulate venation that is characteristic of a dicot leaf has excited interest from systematists for more than a century, and from physiological and developmental botanists for decades. The tools of digital image acquisition and computer image analysis, however, are only now approaching the sophistication needed to quantify aspects of the venation network found in real leaves quickly, easily, accurately, and reliably enough to produce biologically meaningful data. In this paper, we examine 120 leaves distributed across vascular plants (representing 118 genera and 80 families) using two approaches: a semiquantitative scoring system called "leaf ranking," devised by the late Leo Hickey, and an automated image-analysis protocol. In the process of comparing these approaches, we review some methodological issues that arise in trying to quantify a vein network, and discuss the strengths and weaknesses of automatic data collection and human pattern recognition. We conclude that subjective leaf rank provides a relatively consistent, semiquantitative measure of areole size among other variables; that modal areole size is generally consistent across large sections of a leaf lamina; and that both approaches-semiquantitative, subjective scoring; and fully quantitative, automated measurement-have appropriate places in the study of leaf venation.

Correlation of mRNA and protein abundance in the developing maize leaf

To help understand regulation of maize leaf blade development, including sink-source transitions and induction of C4 photosynthesis, we compared large-scale quantitative proteome and transcriptomes collected at specific stages along the developmental maize leaf blade gradient. Proteome data were based on label-free shotgun proteomics (spectral counting) and transcript data were based on RNA-seq using the same source materials, and had been published previously (Nat Genet, 42, 2010, 1060-1067; The Plant Cell, 22, 2010, 3509-3542). Transcript and protein abundance followed near normal distributions, in contrast with several studies with other organisms. Protein observability correlated with transcript abundance following a 'lazy step function' similar to that in bacteria and yeast. mRNA and protein abundance showed significant positive correlations (up to 0.8) for log-transformed length-weighted normalized spectral abundance factor (NSAF) and reads per kilobase of exon model per million mapped reads (RPKM) and non-weighted abundances (NadjSPC and COV) in dependence of function and development. Correlations were much weaker in the leaf 'sink-source' transition zone, i.e. the zone with massive investments in leaf chloroplast biogenesis and build-up of photosynthetic capacity. Clustering analyses of gene-specific protein-mRNA ratios revealed co-ordinated shifts in control points in gene expression along the leaf blade developmental gradient. The highest protein-mRNA ratio for each gene generally corresponded to leaf developmental stages in which the protein function was most important, with the exception of the 80S ribosome. Specific examples are discussed in the context of C4 photosynthesis, leaf development and sink-source transitions. This large-scale mRNA-protein correlation analysis in plants (maize) using label-free spectral counting for protein quantification and RNA-seq for mRNA abundance will provide a template for future mRNA-protein correlation studies.

Compound leaf development in the palm Chamaedorea elegans is KNOX-independent

PREMISE OF THE STUDY

How a leaf acquires its shape is a major and largely unresolved question in plant biology. This problem is particularly complex in the case of compound leaves, where the leaf blade is subdivided into leaflets. In many eudicots with compound leaves, class I KNOTTED1-LIKE HOMEOBOX (KNOX) genes are upregulated in the leaf primordium and promote leaflet initiation, while KNOX genes are restricted to the shoot apical meristem in simple-leaved plants. In monocots, however, little is known about the extent of KNOX contribution to compound leaf development, and we aimed to address this issue in the palm Chamaedorea elegans.

METHODS

We investigated the accumulation pattern of KNOX proteins in shoot apical meristems and leaf primordia of the palm C. elegans using immunolocalization experiments.

KEY RESULTS

KNOX proteins accumulated in vegetative and inflorescence apical meristems and in the subtending stem tissue, but not in the plicated regions of the leaf primordia. These plicated areas form during primary morphogenesis and are the only meristematic tissue in the developing primordium. In addition, KNOX proteins did not accumulate in any region of the developing leaf during secondary morphogenesis, when leaflets separate to create the final pinnately compound leaf.

CONCLUSIONS

The compound leaf character in palms, C. elegans in particular and likely other pinnately compound palms, does not depend on the activities of KNOX proteins.

The development and geometry of shape change in Arabidopsis thaliana cotyledon pavement cells

BACKGROUND

The leaf epidermis is an important architectural control element that influences the growth properties of underlying tissues and the overall form of the organ. In dicots, interdigitated pavement cells are the building blocks of the tissue, and their morphogenesis includes the assembly of specialized cell walls that surround the apical, basal, and lateral (anticlinal) cell surfaces. The microtubule and actin cytoskeletons are highly polarized along the cortex of the anticlinal wall; however, the relationships between these arrays and cell morphogenesis are unclear.

RESULTS

We developed new quantitative tools to compare population-level growth statistics with time-lapse imaging of cotyledon pavement cells in an intact tissue. The analysis revealed alternating waves of lobe initiation and a phase of lateral isotropic expansion that persisted for days. During lateral isotropic diffuse growth, microtubule organization varied greatly between cell surfaces. Parallel microtubule bundles were distributed unevenly along the anticlinal surface, with subsets marking stable cortical domains at cell indentations and others clearly populating the cortex within convex cell protrusions.

CONCLUSIONS

Pavement cell morphogenesis is discontinuous, and includes punctuated phases of lobe initiation and lateral isotropic expansion. In the epidermis, lateral isotropic growth is independent of pavement cell size and shape. Cortical microtubules along the upper cell surface and stable cortical patches of anticlinal microtubules may coordinate the growth behaviors of orthogonal cell walls. This work illustrates the importance of directly linking protein localization data to the growth behavior of leaf epidermal cells.

Structural and metabolic transitions of C4 leaf development and differentiation defined by microscopy and quantitative proteomics in maize

C(4) grasses, such as maize (Zea mays), have high photosynthetic efficiency through combined biochemical and structural adaptations. C(4) photosynthesis is established along the developmental axis of the leaf blade, leading from an undifferentiated leaf base just above the ligule into highly specialized mesophyll cells (MCs) and bundle sheath cells (BSCs) at the tip. To resolve the kinetics of maize leaf development and C(4) differentiation and to obtain a systems-level understanding of maize leaf formation, the accumulation profiles of proteomes of the leaf and the isolated BSCs with their vascular bundle along the developmental gradient were determined using large-scale mass spectrometry. This was complemented by extensive qualitative and quantitative microscopy analysis of structural features (e.g., Kranz anatomy, plasmodesmata, cell wall, and organelles). More than 4300 proteins were identified and functionally annotated. Developmental protein accumulation profiles and hierarchical cluster analysis then determined the kinetics of organelle biogenesis, formation of cellular structures, metabolism, and coexpression patterns. Two main expression clusters were observed, each divided in subclusters, suggesting that a limited number of developmental regulatory networks organize concerted protein accumulation along the leaf gradient. The coexpression with BSC and MC markers provided strong candidates for further analysis of C(4) specialization, in particular transporters and biogenesis factors. Based on the integrated information, we describe five developmental transitions that provide a conceptual and practical template for further analysis. An online protein expression viewer is provided through the Plant Proteome Database.

The ULTRACURVATA2 gene of Arabidopsis encodes an FK506-binding protein involved in auxin and brassinosteroid signaling

The dwarf ucu (ultracurvata) mutants of Arabidopsis display vegetative leaves that are spirally rolled downwards and show reduced expansion along the longitudinal axis. We have previously determined that the UCU1 gene encodes a SHAGGY/GSK3-like kinase that participates in the signaling pathways of auxins and brassinosteroids. Here, we describe four recessive alleles of the UCU2 gene, whose homozygotes display helical rotation of several organs in addition to other phenotypic traits shared with ucu1 mutants. Following a map-based strategy, we identified the UCU2 gene, which was found to encode a peptidyl-prolyl cis/trans-isomerase of the FK506-binding protein family, whose homologs in metazoans are involved in cell signaling and protein trafficking. Physiological and double mutant analyses suggest that UCU2 is required for growth and development and participates in auxin and brassinosteroid signaling.

The polar auxin transport inhibitor N-1-naphthylphthalamic acid disrupts leaf initiation, KNOX protein regulation, and formation of leaf margins in maize

Maize (Zea mays) leaves develop basipetally (tip to base); the upper blade emerges from the shoot apical meristem (SAM) before the expansion of the lower sheath. Founder cells, leaf initials located in the periphery of the SAM, are distinguished from the SAM proper by the differential accumulation of KNOX proteins. KNOX proteins accumulate in the SAM, but are excluded from maize leaf primordia and leaf founder cells. As in Arabidopsis and tomato (Lycopersicon esculentum), maize shoots failed to initiate new leaves when cultured in the polar auxin transport inhibitor N-1-naphthylphthalamic acid (NPA). We demonstrate that NPA-induced arrest of leaf initiation in maize is correlated with the failure to down-regulate KNOX accumulation in the SAM. In addition, NPA-cultured shoots formed abnormal tubular leaf bases in which the margins failed to separate in the lower leaf zone. The tubular leaf bases always formed in the fourth leaf from the arrested meristem. Moreover, the unseparated margin domains of these tubular leaf bases accumulated ectopic KNOX protein(s). Transfer of NPA-cultured apices to NPA-free media resulted in the resumption of leaf initiation from the SAM and the restoration of normal patterns of KNOX down-regulation, accordingly. These data suggest that the lower sheath margins emerge from the leaf base late in maize leaf development and that the separation of these leaf margin domains is correlated with auxin transport and down-regulation of KNOX proteins. In addition, these results suggest that the down-regulation of KNOX accumulation in maize apices is not upstream of polar auxin transport, although a more complicated feedback network may exist. A model for L1-derived margin development in maize leaves is presented.

Genetic analysis of natural variations in the architecture of Arabidopsis thaliana vegetative leaves

To ascertain whether intraspecific variability might be a source of information as regards the genetic controls underlying plant leaf morphogenesis, we analyzed variations in the architecture of vegetative leaves in a large sample of Arabidopsis thaliana natural races. A total of 188 accessions from the Arabidopsis Information Service collection were grown and qualitatively classified into 14 phenotypic classes, which were defined according to petiole length, marginal configuration, and overall lamina shape. Accessions displaying extreme and opposite variations in the above-mentioned leaf architectural traits were crossed and their F(2) progeny was found to be not classifiable into discrete phenotypic classes. Furthermore, the leaf trait-based classification was not correlated with estimates on the genetic distances between the accessions being crossed, calculated after determining variations in repeat number at 22 microsatellite loci. Since these results suggested that intraspecific variability in A. thaliana leaf morphology arises from an accumulation of mutations at quantitative trait loci (QTL), we studied a mapping population of recombinant inbred lines (RILs) derived from a Landsberg erecta-0 x Columbia-4 cross. A total of 100 RILs were grown and the third and seventh leaves of 15 individuals from each RIL were collected and morphometrically analyzed. We identified a total of 16 and 13 QTL harboring naturally occurring alleles that contribute to natural variations in the architecture of juvenile and adult leaves, respectively. Our QTL mapping results confirmed the multifactorial nature of the observed natural variations in leaf architecture.

The UCU1 Arabidopsis gene encodes a SHAGGY/GSK3-like kinase required for cell expansion along the proximodistal axis

Most signal transduction pathways central to development are not shared by plants and animals. Such is the case of the Wingless/Wnt signaling pathway, whose components play key roles in metazoan pattern formation and tumorigenesis, but are absent in plants, with the exception of SHAGGY/GSK3, a cytoplasmic protein kinase represented in the genome of Arabidopsis thaliana by a family of 10 AtSK genes for which mutational evidence is scarce. Here, we describe the characterization of mutant alleles of the Arabidopsis ULTRACURVATA1 (UCU1) gene, the two strongest of which dramatically reduce cell expansion along the proximodistal axis, dwarfing the mutant plants, whose cells expand properly across but not along most organs. Proximodistal expansion of adaxial (dorsal) and abaxial (ventral) leaf cells exhibits a differential dependence on UCU1 function, as suggested by the leaves of ucu1 mutants, which are rolled spirally downward in a circinate manner. We have positionally cloned the UCU1 gene, which encodes an AtSK protein involved in the cross-talk between auxin and brassinosteroid signaling pathways, as indicated by the responses of ucu1 mutants to plant hormones and the phenotypes of double mutants involving ucu1 alleles.

Genetic analysis of incurvata mutants reveals three independent genetic operations at work in Arabidopsis leaf morphogenesis

In an attempt to identify genes involved in the control of leaf morphogenesis, we have studied 13 Arabidopsis thaliana mutants with curled, involute leaves, a phenotype herein referred to as Incurvata (Icu), which were isolated by G. Röbbelen and belong to the Arabidopsis Information Service Form Mutants collection. The Icu phenotype was inherited as a single recessive trait in 10 mutants, with semidominance in 2 mutants and with complete dominance in the remaining 1. Complementation analyses indicated that the studied mutations correspond to five genes, representative alleles of which were mapped relative to polymorphic microsatellites. Although most double-mutant combinations displayed additivity of the Icu phenotypes, those of icu1 icu2 and icu3 icu4 double mutants were interpreted as synergistic, which suggests that the five genes studied represent three independent genetic operations that are at work for the leaf to acquire its final form at full expansion. We have shown that icu1 mutations are alleles of the Polycomb group gene CURLY LEAF (CLF) and that the leaf phenotype of the icu2 mutant is suppressed in an agamous background, as is known for clf mutants. In addition, we have tested by means of multiplex RT-PCR the transcription of several floral genes in Icu leaves. Ectopic expression of AGAMOUS and APETALA3 was observed in clf and icu2, but not in icu3, icu4, and icu5 mutants. Taken together, these results suggest that CLF and ICU2 play related roles, the latter being a candidate to belong to the Polycomb group of regulatory genes. We propose that, as flowers evolved, a new major class of genes, including CLF and ICU2, may have been recruited to prevent the expression of floral homeotic genes in the leaves.

Disruption of auxin transport is associated with aberrant leaf development in maize

Despite recent progress, the mechanisms governing shoot morphogenesis in higher plants are only partially understood. Classical physiological studies have suggested that gradients of the plant growth regulator auxin may play a role in controlling tissue differentiation in shoots. More recent molecular genetic studies have also identified knotted1 like homeobox (knox) genes as important regulators of shoot development. The maize (Zea mays L.) mutant rough sheath2 (rs2) displays ectopic expression of at least three knox genes and consequently conditions a range of shoot and leaf phenotypes, including aberrant vascular development, ligular displacements, and dwarfism (R. Schneeberger, M. Tsiantis, M. Freeling, J.A. Langdale [1998] Development 125: 2857-2865). In this report, we show that rs2 mutants also display decreased polar auxin transport in the shoot. We also demonstrate that germination of wild-type maize seedlings on agents known to inhibit polar auxin transport mimics aspects of the rs2 mutant phenotype. The phenotype elaborated in inhibitor-treated plants is not correlated with ectopic KNOX protein accumulation.

Cellular differentiation in the leaf

The past year has seen significant advances in our understanding of the mechanisms that regulate cellular differentiation in the leaf. It has been suggested that a common developmental pathway involving MYB-like transcription factors is responsible for distinguishing between cellular identities in the epidermis and that nuclear-cytoplasmic partitioning of the GLABRA2 homeodomain protein plays a role in determining trichome cell fate. With respect to the differentiation of subepidermal cell types, molecular links have been made between auxin physiology and vascular development, and between plastid function and photosynthetic cell type development.