Ectopic expression of a tobacco homeobox gene, NTH15, dramatically alters leaf morphology and hormone levels in transgenic tobacco

Shaping leaves through TALE homeodomain transcription factors

The first TALE homeodomain transcription factor gene to be described in plants was maize knotted1 (kn1). Dominant mutations in kn1 disrupt leaf development, with abnormal knots of tissue forming in the leaf blade. kn1 was found to be expressed in the shoot meristem but not in a peripheral region that gives rise to leaves. Furthermore, KN1 and closely related proteins were excluded from initiating and developing leaves. These findings were a prelude to a large body of work wherein TALE homeodomain proteins have been identified as vital regulators of meristem homeostasis and organ development in plants. KN1 homologues are widely represented across land plant taxa. Thus, studying the regulation and mechanistic action of this gene class has allowed investigations into the evolution of diverse plant morphologies. This review will focus on the function of TALE homeodomain transcription factors in leaf development in eudicots. Here, we discuss how TALE homeodomain proteins contribute to a spectrum of leaf forms, from the simple leaves of Arabidopsis thaliana to the compound leaves of Cardamine hirsuta and species beyond the Brassicaceae.

KNOTTED1-like homeobox (KNOX) transcription factors - Hubs in a plethora of networks: A review

KNOX (KNOTTED1-like HOMEOBOX) belongs to a class of important homeobox genes, which encode the homeodomain proteins binding to the specific element of target genes, and widely participate in plant development. Advancements in genetics and molecular biology research generate a large amount of information about KNOX genes in model and non-model plants, and their functions in different developmental backgrounds are gradually becoming clear. In this review, we summarize the known and presumed functions of the KNOX gene in plants, focusing on horticultural plants and crops. The classification and structural characteristics, expression characteristics and regulation, interacting protein factors, functions, and mechanisms of KNOX genes are systematically described. Further, the current research gaps and perspectives were discussed. These comprehensive data can provide a reference for the directional improvement of agronomic traits through KNOX gene regulation.

Up-regulation of LsKN1 promotes cytokinin and suppresses gibberellin biosynthesis to generate wavy leaves in lettuce

Lettuce (Lactuca sativa) is one of the most popular vegetables worldwide, and diverse leaf shapes, including wavy leaves, are important commercial traits. In this study, we examined the genetics of wavy leaves using an F2 segregating population, and identified a major QTL controlling wavy leaves. The candidate region contained LsKN1, which has previously been shown to be indispensable for leafy heads in lettuce. Complementation tests and knockout experiments verified the function of LsKN1 in producing wavy leaves. The LsKN1∇ allele, which has the insertion of a transposon and has previously been shown to control leafy heads, promoted wavy leaves in our population. Transposition of the CACTA transposon from LsKN1 compromised its function for wavy leaves. High expression of LsKN1 up-regulated several key genes associated with cytokinin (CK) to increase the content in the leaves, whereas it down-regulated the expression of genes in the gibberellin (GA) biosynthesis pathway to decrease the content. Application of CK to leaves enhanced the wavy phenotype, while application of GA dramatically flattened the leaves. We conclude that the changes in CK and GA contents that result from high expression of LsKN1 switch determinate cells to indeterminate, and consequently leads to the development of wavy leaves.

Trafficking and localization of KNOTTED1 related mRNAs in shoot meristems

Multicellular organisms use transcripts and proteins as signaling molecules for cell-to-cell communication. Maize KNOTTED1 (KN1) was the first homeodomain transcription factor identified in plants, and functions in maintaining shoot stem cells. KN1 acts non-cell autonomously, and both its messenger RNA (mRNA) and protein traffic between cells through intercellular nanochannels called plasmodesmata. KN1 protein and mRNA trafficking are regulated by a chaperonin subunit and a catalytic subunit of the RNA exosome, respectively. These studies suggest that the function of KN1 in stem cell regulation requires the cell-to-cell transport of both its protein and mRNA. However, in situ hybridization experiments published 25 years ago suggested that KN1 mRNA was missing from the epidermal (L1) layer of shoot meristems, suggesting that only the KN1 protein could traffic. Here, we show evidence that KN1 mRNA is present at a low level in L1 cells of maize meristems, supporting an idea that both KN1 protein and mRNA traffic to the L1 layer. We also summarize mRNA expression patterns of KN1 homologs in diverse angiosperm species, and discuss KN1 trafficking mechanisms.

Identification and responding to exogenous hormone of HB-KNOX family based on transcriptome data of Caucasian clover

Caucasian clover (Trifolium ambiguum M. Bieb.) is a strongly rhizomatous, low-crowned perennial leguminous and ground-covering grass. The species is resistant to cold, arid temperatures and grazing due to a well-developed underground rhizome system and a strong clonal reproduction capacity. KNOTTED1-LIKE HOMEOBOX (KNOX) genes are a family of plant-specific homeobox transcription factors with important roles in plant development. Preliminary transcriptome analysis enabled us to understand the gene expression in five different tissues, which helped us to screen the predetermined genes of the HB-KNOX family genes for the rhizome growth and development of Caucasian clover. The study identified 41 TaKNOX genes from the Caucasian clover transcriptome database. Gene length, MW and pl of TaKNOX family transcription factors varied, but the gene structure and motifs were relatively conserved in bioinformatics analysis. Phylogenetic analyses of Arabidopsis thaliana, soybean, Medicago truncatula and Caucasian clover were performed to study the evolutionary and functional relationships in various species. Prediction and verification of the subcellular localizations revealed the diverse subcellular localization of these 41 TaKNOX proteins. The expression profile of exogenous hormones showed that the TaKNOX gene showed multiple expression regulation patterns, and was involved in 6-BA, IAA and KT signaling pathways. Our results reveal the characteristics of the TaKNOX gene family, thus laying a foundation for further functional analysis of the KNOX family in Caucasian clover.

Plant Cellular and Molecular Biotechnology: Following Mariotti's Steps

This review is dedicated to the memory of Prof. Domenico Mariotti, who significantly contributed to establishing the Italian research community in Agricultural Genetics and carried out the first experiments of Agrobacterium-mediated plant genetic transformation and regeneration in Italy during the 1980s. Following his scientific interests as guiding principles, this review summarizes the recent advances obtained in plant biotechnology and fundamental research aiming to: (i) Exploit in vitro plant cell and tissue cultures to induce genetic variability and to produce useful metabolites; (ii) gain new insights into the biochemical function of Agrobacterium rhizogenes rol genes and their application to metabolite production, fruit tree transformation, and reverse genetics; (iii) improve genetic transformation in legume species, most of them recalcitrant to regeneration; (iv) untangle the potential of KNOTTED1-like homeobox (KNOX) transcription factors in plant morphogenesis as key regulators of hormonal homeostasis; and (v) elucidate the molecular mechanisms of the transition from juvenility to the adult phase in Prunus tree species.

The vascular plants: open system of growth

What is fascinating in plants (true also in sessile animals such as corals and hydroids) is definitely their open and indeterminate growth, as a result of meristematic activity. Plants as well as animals are characterized by a multicellular organization, with which they share a common set of genes inherited from a common eukaryotic ancestor; nevertheless, circa 1.5 billion years of evolutionary history made the two kingdoms very different in their own developmental biology. Flowering plants, also known as angiosperms, arose during the Cretaceous Period (145-65 million years ago), and up to date, they count around 235,000 species, representing the largest and most diverse group within the plant kingdom. One of the foundations of their success relies on the plant-pollinator relationship, essentially unique to angiosperms that pushed large speciation in both plants and insects and on the presence of the carpel, the structure devoted to seed enclosure. A seed represents the main organ preserving the genetic information of a plant; during embryogenesis, the primary axis of development is established by two groups of pluripotent cells: the shoot apical meristem (SAM), responsible for gene rating all aboveground organs, and the root apical meristem (RAM), responsible for producing all underground organs. During postembryonic shoot development, axillary meristem (AM) initiation and outgrowth are responsible for producing all secondary axes of growth including inflorescence branches or flowers. The production of AMs is tightly linked to the production of leaves and their separation from SAM. As leaf primordia are formed on the flanks of the SAM, a region between the apex and the developing organ is established and referred to as boundary zone. Interaction between hormones and the gene network in the boundary zone is fundamental for AM initiation. AMs only develop at the adaxial base of the leaf; thus, AM initiation is also strictly associated with leaf polarity. AMs function as new SAMs: form axillary buds with a few leaves and then the buds can either stay dormant or develop into shoot branches to define a plant architecture, which in turn affects assimilate production and reproductive efficiency. Therefore, the radiation of angiosperms was accompanied by a huge diversification in growth forms that determine an enormous morphological plasticity helping plants to environmental changes. In this review, we focused on the developmental processes of AM initiation and outgrowth. In particular, we summarized the primary growth of SAM, the key role of positional signals for AM initiation, and the dissection of molecular players involved in AM initiation and outgrowth. Finally, the interaction between phytohormone signals and gene regulatory network controlling AM development was discussed.

Screening of Candidate Leaf Morphology Genes by Integration of QTL Mapping and RNA Sequencing Technologies in Oilseed Rape (Brassica napus L.)

Leaf size and shape play important roles in agronomic traits, such as yield, quality and stress responses. Wide variations in leaf morphological traits exist in cultivated varieties of many plant species. By now, the genetics of leaf shape and size have not been characterized in Brassica napus. In this study, a population of 172 recombinant inbred lines (RILs) was used for quantitative trait locus (QTL) analysis of leaf morphology traits. Furthermore, fresh young leaves of extreme lines with more leaf lobes (referred to as ‘A’) and extreme lines with fewer lobes (referred to as ‘B’) selected from the RIL population and leaves of dissected lines (referred to as ‘P’) were used for transcriptional analysis. A total of 31 QTLs for the leaf morphological traits tested in this study were identified on 12 chromosomes, explaining 5.32–39.34% of the phenotypic variation. There were 8, 6, 2, 5, 8, and 2 QTLs for PL (petiole length), PN (lobe number), LW (lamina width), LL (Lamina length), LL/LTL (the lamina size ratio) and LTL (leaf total length), respectively. In addition, 74, 1,166 and 1,272 differentially expressed genes (DEGs) were identified in ‘A vs B’, ‘A vs P’ and ‘B vs P’ comparisons, respectively. The Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases were used to predict the functions of these DEGs. Gene regulators of leaf shape and size, such as ASYMMETRIC LEAVES 2, gibberellin 20-oxidase 3, genes encoding gibberellin-regulated family protein, genes encoding growth-regulating factor and KNOTTED1-like homeobox were also detected in DEGs. After integrating the QTL mapping and RNA sequencing data, 33 genes, including a gene encoding auxin-responsive GH3 family protein and a gene encoding sphere organelles protein-related gene, were selected as candidates that may control leaf shape. Our findings should be valuable for studies of the genetic control of leaf morphological trait regulation in B. napus.

KNAT3/4/5-like class 2 KNOX transcription factors are involved in Medicago truncatula symbiotic nodule organ development

We investigated the role of KNOX genes in legume root nodule organogenesis. Class 1 KNOX homeodomain transcription factors (TFs) are involved in plant shoot development and leaf shape diversity. Class 2 KNOX genes are less characterized, even though an antagonistic function relative to class 1 KNOXs was recently proposed. In silico expression data and further experimental validation identified in the Medicago truncatula model legume three class 2 KNOX genes, belonging to the KNAT3/4/5-like subclass (Mt KNAT3/4/5-like), as expressed during nodulation from early stages. RNA interference (RNAi)-mediated silencing and overexpression studies were used to unravel a function for KNOX TFs in nodule development. Mt KNAT3/4/5-like genes encoded four highly homologous proteins showing overlapping expression patterns during nodule organogenesis, suggesting functional redundancy. Simultaneous reduction of Mt KNAT3/4/5-like genes indeed led to an increased formation of fused nodule organs, and decreased the expression of the MtEFD (Ethylene response Factor required for nodule Differentiation) TF and its direct target MtRR4, a cytokinin response gene. Class 2 KNOX TFs therefore regulate legume nodule development, potentially through the MtEFD/MtRR4 cytokinin-related regulatory module, and may control nodule organ boundaries and shape like class 2 KNOX function in leaf development.

Regulation, overexpression, and target gene identification of Potato Homeobox 15 (POTH15) - a class-I KNOX gene in potato

Potato Homeobox 15 (POTH15) is a KNOX-I (Knotted1-like homeobox) family gene in potato that is orthologous to Shoot Meristemless (STM) in Arabidopsis. Despite numerous reports on KNOX genes from different species, studies in potato are limited. Here, we describe photoperiodic regulation of POTH15, its overexpression phenotype, and identification of its potential targets in potato (Solanum tuberosum ssp. andigena). qRT-PCR analysis showed a higher abundance of POTH15 mRNA in shoot tips and stolons under tuber-inducing short-day conditions. POTH15 promoter activity was detected in apical and axillary meristems, stolon tips, tuber eyes, and meristems of tuber sprouts, indicating its role in meristem maintenance and leaf development. POTH15 overexpression altered multiple morphological traits including leaf and stem development, leaflet number, and number of nodes and branches. In particular, the rachis of the leaf was completely reduced and leaves appeared as a bouquet of leaflets. Comparative transcriptomic analysis of 35S::GUS and two POTH15 overexpression lines identified more than 6000 differentially expressed genes, including 2014 common genes between the two overexpression lines. Functional analysis of these genes revealed their involvement in responses to hormones, biotic/abiotic stresses, transcription regulation, and signal transduction. qRT-PCR of selected candidate target genes validated their differential expression in both overexpression lines. Out of 200 randomly chosen POTH15 targets, 173 were found to have at least one tandem TGAC core motif, characteristic of KNOX interaction, within 3.0kb in the upstream sequence of the transcription start site. Overall, this study provides insights to the role of POTH15 in controlling diverse developmental processes in potato.

Ectopic expression a tomato KNOX Gene Tkn4 affects the formation and the differentiation of meristems and vasculature

The KNOTTED-LIKE HOMEODOMAIN genes are involved in maintenance of the shoot apical meristem which produces the whole above-ground body of vascular plants. In this report, a tomato homolog gene, named as Tkn4 (a nucleus targeted transcription factor) was identified and characterized. By performing RT-PCR, the transcript level of Tkn4 was separately found in stem, root, stamen, stigma, fruit and sepal but hardly visible in the leaf. Besides, Tkn4 was induced by a series of plant hormones. Overexpression of Tkn4 gene in tomato resulted in dwarf phenotype and strongly repressed the formation of shoot apical meristem, lateral meristem and cambiums in transgenic lines. The transgenic lines had wrinkled leaves and anatomic analysis showed that there was no obvious palisade tissues in the leaves and the layer of cells changed in vascular tissue (xylem and phloem). To explore the regulation network of Tkn4, RNA-sequencing was performed in overexpression lines and wild type plants, by which many genes related to the synthesis and the signal transduction of cytokinin, auxin, gibberellin, ethylene, abscisic acid, and tracheary element differentiation or extracellular matrix synthesis were significantly regulated. Taken together, our results demonstrate that Tkn4 plays important roles in regulating the biosynthesis and signal transduction of diverse plant hormones, and the formation and differentiation of meristems and vasculature in tomato.

Reprogramming of plant cells induced by 6b oncoproteins from the plant pathogen Agrobacterium

Reprogramming of plant cells is an event characterized by dedifferentiation, reacquisition of totipotency, and enhanced cell proliferation, and is typically observed during formation of the callus, which is dependent on plant hormones. The callus-like cell mass, called a crown gall tumor, is induced at the sites of infection by Agrobacterium species through the expression of hormone-synthesizing genes encoded in the T-DNA region, which probably involves a similar reprogramming process. One of the T-DNA genes, 6b, can also by itself induce reprogramming of differentiated cells to generate tumors and is therefore recognized as an oncogene acting in plant cells. The 6b genes belong to a group of Agrobacterium T-DNA genes, which include rolB, rolC, and orf13. These genes encode proteins with weakly conserved sequences and may be derived from a common evolutionary origin. Most of these members can modify plant growth and morphogenesis in various ways, in most cases without affecting the levels of plant hormones. Recent studies have suggested that the molecular function of 6b might be to modify the patterns of transcription in the host nuclei, particularly by directly targeting the host transcription factors or by changing the epigenetic status of the host chromatin through intrinsic histone chaperone activity. In light of the recent findings on zygotic resetting of nucleosomal histone variants in Arabidopsis thaliana, one attractive idea is that acquisition of totipotency might be facilitated by global changes of epigenetic status, which might be induced by replacement of histone variants in the zygote after fertilization and in differentiated cells upon stimulation by plant hormones as well as by expression of the 6b gene.

The tomato leaf as a model system for organogenesis

Compound tomato leaves are composed of multiple leaflets that are generated gradually during leaf development, and each resembles a simple leaf. The elaboration of a compound leaf form requires the maintenance of transient organogenic activity at the leaf margin. The developmental window of organogenic activity is defined by the antagonistic activities of factors that promote maturation, such as TCP transcription factors, SFT and gibberellin, and factors that delay maturation, such as KNOX transcription factors and cytokinin. Leaflet initiation sites are specified spatially and temporally by spaced and specific activities of CUCs, auxin and ENTIRE, as well as additional factors. The partially indeterminate growth of the compound tomato leaf makes it a useful model to understand the balance between determinate and indeterminate growth, and the mechanisms of organogenesis, some of which are common to many developmental processes in plants.

The mRNA of a Knotted1-like transcription factor of potato is phloem mobile

Potato Homeobox1 (POTH1) is a Knotted1-like transcription factor from the Three Amino Acid Loop Extension (TALE) superfamily that is involved in numerous aspects of development in potato (Solanum tuberosum L). POTH1 interacts with its protein partner, StBEL5, to facilitate binding to specific target genes to modulate hormone levels, mediate leaf architecture, and enhance tuber formation. In this study, promoter analyses show that the upstream sequence of POTH1 drives β-glucuronidase activity in response to light and in association with phloem cells in both petioles and stems. Because POTH1 transcripts have previously been detected in phloem cells, long-distance movement of its mRNA was tested. Using RT-PCR and transgenic potato lines over-expressing POTH1, in vitro micrografts demonstrated unilateral movement of POTH1 RNA in a rootward direction. Movement across a graft union into leaves from newly arising axillary shoots and roots of wild type stocks was verified using soil-grown tobacco heterografts. Leaves from the wild type stock containing the mobile POTH1 RNA exhibited a reduction in leaf size relative to leaves from wild type grafts. Both untranslated regions of POTH1 when fused to an expression marker β-glucuronidase, repressed its translation in tobacco protoplasts. RNA/protein binding assays demonstrated that the UTRs of POTH1 bind to two RNA-binding proteins, a polypyrimidine tract-binding protein and an alba-domain type. Conserved glycerol-responsive elements (GRE), specific to alba-domain interaction, are duplicated in both the 5' and 3' untranslated regions of POTH1. These results suggest that POTH1 functions as a mobile signal in regulating development.

A strawberry KNOX gene regulates leaf, flower and meristem architecture

The KNOTTED-LIKE HOMEODOMAIN (KNOX) genes play a central role in maintenance of the shoot apical meristem. They also contribute to the morphology of simple and compound leaves. In this report we characterize the FaKNOX1 gene from strawberry (Fragaria spp.) and demonstrate its function in trasgenic plants. The FaKNOX1 cDNA was isolated from a cultivated strawberry (F.×ananassa) flower EST library. The sequence is most similar to Class I KNOX genes, and was mapped to linkage group VI of the diploid strawberry genome. Unlike most KNOX genes studied, steady-state transcript levels were highest in flowers and fruits. Transcripts were also detected in emerging leaf primordia and the apical dome. Transgenic strawberry plants suppressing or overexpressing FaKNOX1 exhibited conspicuous changes in plant form. The FaKNOX1 RNAi plants presented a dwarfed phenotype with deeply serrated leaflets and exaggerated petiolules. They also exhibited a high level of cellular disorganization of the shoot apical meristem and leaves. Overexpression of FaKNOX1 caused dwarfed stature with wrinkled leaves. These gain- and loss-of-function assays in strawberry functionally demonstrate the contributions of a KNOX domain protein in a rosaceous species.

Ectopic expression of the Arabidopsis MINI ZINC FINGER1 and MIF3 genes induces shoot meristems on leaf margins

A leaf undergoes determinate growth from a primordium on flank of the shoot apical meristem. Several intrinsic pathways restrict meristematic activity in the leaf of Arabidopsis; however, other factors remain to be defined. We report here that the overexpression of MINI ZINC FINGER1 (MIF1) or MIF3 disrupted the leaf determinate growth by inducing ectopic shoot meristems on leaf margins. These ectopic meristems occurred along margins of late rosette leaves at serration sinuses in an ERECTA-dependent manner. Expression of STM was activated in these ectopic meristems but not other leaf regions. The formation of ectopic meristems was independent of the BP gene but suppressed by exogenous gibberellic acid. In addition, reduced auxin response along leaf margins and subsequent response peak in the sinus were correlated with the occurrence of ectopic meristems. Our results suggest that the sinus of leaf serration is a quiescent domain possessing the potential for meristem formation. MIF1- or MIF3-overexpressing transgenic plants may provide a new genetic system for dissecting the molecular mechanism that maintains leaf determinate growth, and for understanding the interactions between hormone actions and meristematic activity.

A branched-chain aminotransferase may regulate hormone levels by affecting KNOX genes in plants

Branched-chain amino acid transaminases (BCATs) play a crucial role in the metabolic pathway of leucine, isoleucine and valine by catalyzing the last step of synthesis and/or the initial step of degradation of these amino acids. In this study, we characterized a new BCAT from Nicotiana benthamiana (NbBCAT, GeneBank accession No. EU194916), the deduced amino acid sequence of which exhibits a very high percentage of identity to the homologous enzymes from Solanum tuberosum (StBCAT-2, 91.5%) and Arabidopsis thaliana (AtBCAT1-6, 56.4-68.6%). Complementation experiment using a Deltabat1/Deltabat2 double knockout yeast strain system demonstrated enzymatic activities for NbBCAT. Ectopically expressed NbBCAT::green fluorescence fusion protein was targeted predominantly to the chloroplasts in tobacco protoplasts. The highest levels of NbBCAT transcripts were found in open flowers as well as in young leaves. Virus-induced gene silencing of NbBCAT resulted in abnormal leaf development and loss of apical dominance. In NbBCAT-silenced plants, two KNOTTED1-type genes, NTH15 and NTH23, were upregulated. This was accompanied by various hormone changes, as a result of transcriptional regulation of gibberellin 20-oxidase (Ntc12) and adenosine phosphate isopentenyltransferase. The transcript levels of NbBCAT could also be repressed by hormone treatment. These results suggest that NbBCAT, an enzyme in the branched-chain amino acid metabolic pathway, may be involved in the regulation of endogenous hormones by its effect on KNOX genes.

Advances in selectable marker genes for plant transformation

Plant transformation systems for creating transgenics require separate process for introducing cloned DNA into living plant cells. Identification or selection of those cells that have integrated DNA into appropriate plant genome is a vital step to regenerate fully developed plants from the transformed cells. Selectable marker genes are pivotal for the development of plant transformation technologies because marker genes allow researchers to identify or isolate the cells that are expressing the cloned DNA, to monitor and select the transformed progeny. As only a very small portion of cells are transformed in most experiments, the chances of recovering transgenic lines without selection are usually low. Since the selectable marker gene is expected to function in a range of cell types it is usually constructed as a chimeric gene using regulatory sequences that ensure constitutive expression throughout the plant. Advent of recombinant DNA technology and progress in plant molecular biology had led to a desire to introduce several genes into single transgenic plant line, necessitating the development of various types of selectable markers. This review article describes the developments made in the recent past on plant transformation systems using different selection methods adding a note on their importance as marker genes in transgenic crop plants.

Expression of class I knotted1-like homeobox genes in the storage roots of sweetpotato (Ipomoea batatas)

As a first step in clarifying the involvement of class I knotted1-like homeobox (KNOXI) genes in the storage root development of sweetpotato (Ipomoea batatas), we isolated three KNOXI genes, named Ibkn1, Ibkn2 and Ibkn3, expressed in the storage roots. Phylogenetic analysis showed that Ibkn1 was homologous to the SHOOT MERISTEMLESS (STM) gene of Arabidopsis, while Ibkn2 and Ibkn3 were homologous to the BREVIPEDICELLUS (BP) gene. Of these, expression of Ibkn1 and Ibkn2 were upregulated in developing and mature storage roots compared with fibrous roots. Ibkn1 and Ibkn2 showed different expression patterns in the storage roots. Ibkn1 was preferentially expressed at the proximal end and around the primary vascular cambium, while Ibkn2 expression was highest in the thickest part and lower in both the proximal and distal ends. In contrast to Ibkn1 and Ibkn2, expression of Ibkn3 in roots was not consistent among sweetpotato cultivars. The distribution of endogenous trans-zeatin riboside (t-ZR) in sweetpotato roots showed a similarity to the expression pattern of KNOXI genes, supporting the idea that KNOXI genes control cytokinin levels in the storage roots. The physiological functions of these KNOXI genes in storage root development are discussed.

Characterization of KNOX genes in Medicago truncatula

We isolated three class I and three class II KNOX genes in Medicago truncatula. The predicted amino acid sequences suggested a possible orthology to the Arabidopsis homeodomain proteins STM, KNAT1/BP, KNAT3 and KNAT7 that was confirmed by phylogenetic and conserved structural domain analyses. Moreover, the STM-like MtKNOX1 and MtKNOX6 proteins were shown to retain the capability to interact with the Arabidopsis BELL protein partners of STM and KNAT1/BP. Amino acid residues that characterize the different classes of KNOX proteins were identified. Gene expression studies revealed organ-specificity, possible cytokinin-dependent transcriptional activation of two MtKNOXs and expression of one STM-like and a BP/KNAT1-like MtKNOX in roots. Interestingly, mRNA localization studies carried out on class I MtKNOX genes revealed important differences with previously characterised legume KNOXs. M. truncatula transcripts were not down-regulated in leaf primordia and early stages of leaf development, features shared with the more distant compound-leaved species Solanum lycopersicum.

Compound leaf development and evolution in the legumes

Across vascular plants, Class 1 KNOTTED1-like (KNOX1) genes appear to play a critical role in the development of compound leaves. An exception to this trend is found in the Fabaceae, where pea (Pisum sativum) uses UNIFOLIATA, an ortholog of the floral regulators FLORICAULA (FLO) and LEAFY (LFY), in place of KNOX1 genes to regulate compound leaf development. To assess the phylogenetic distribution of KNOX1-independent compound leaf development, a survey of KNOX1 protein expression across the Fabaceae was undertaken. The majority of compound-leafed Fabaceae have expression of KNOX1 proteins associated with developing compound leaves. However, in a large subclade of the Fabaceae, the inverted repeat-lacking clade (IRLC), of which pea is a member, KNOX1 expression is not associated with compound leaves. These data suggest that the FLO/LFY gene may function in place of KNOX1 genes in generating compound leaves throughout the IRLC. The contribution of FLO/LFY to leaf complexity in a member of the Fabaceae outside of the IRLC was examined by reducing expression of FLO/LFY orthologs in transgenic soybean (Glycine max). Transgenic plants with reduced FLO/LFY expression showed only slight reductions in leaflet number. Overexpression of a KNOX1 gene in alfalfa (Medicago sativa), a member of the IRLC, resulted in an increase in leaflet number. This implies that KNOX1 targets, which promote compound leaf development, are present in alfalfa and are still sensitive to KNOX1 regulation. These data suggest that KNOX1 genes and the FLO/LFY gene may have played partially overlapping roles in compound leaf development in ancestral Fabaceae but that the FLO/LFY gene took over this role in the IRLC.

Shoot formation from root tip region: a developmental alteration by WUS in transgenic tobacco

We examined the effect of ectopic expression of WUS on the morphology of tobacco seedlings and the segments in vitro. WUS was amplified from Arabidopsis cDNA and introduced into the tobacco genome under the transcriptional control of the beta-estradiol-inducible expression system. When 1-week-old transgenic seedlings were cultured in the presence of beta-estradiol, only the root tip region developed bulbous tissues followed by shoot formation and plant regeneration, suggesting its applicability for improving the strategy of micropropagation in recalcitrant species. Evident abnormality was not observed in the cotyledons, hypocotyl nor root except for the tip. However, ectopic WUS seemed to be functional in those parts through the observation of gene expression and the behavior of cultured segments. Small root segments with a root tip treated with beta-estradiol also showed bulbing but no shoots unless exogenous cytokinin was supplied. These findings suggest the existence of unknown factors regulating ectopic WUS function in the seedling.

Heterologous expression of the BABY BOOM AP2/ERF transcription factor enhances the regeneration capacity of tobacco (Nicotiana tabacum L.)

Gain-of-function studies have shown that ectopic expression of the BABY BOOM (BBM) AP2/ERF domain transcription factor is sufficient to induce spontaneous somatic embryogenesis in Arabidopsis (Arabidopsis thaliana (L.) Heynh) and Brassica napus (B. napus L.) seedlings. Here we examined the effect of ectopic BBM expression on the development and regenerative capacity of tobacco (Nicotiana tabacum L.) through heterologous expression of Arabidopsis and B. napus BBM genes. 35S::BBM tobacco lines exhibited a number of the phenotypes previously observed in 35S::BBM Arabidopsis and B. napus transgenics, including callus formation, leaf rumpling, and sterility, but they did not undergo spontaneous somatic embryogenesis. 35S::BBM plants with severe ectopic expression phenotypes could not be assessed for enhanced regeneration at the seedling stage due to complete male and female sterility of the primary transformants, therefore fertile BBM ectopic expression lines with strong misexpression phenotypes were generated by expressing a steroid-inducible, post-translationally controlled BBM fusion protein (BBM:GR) under the control of a 35S promoter. These lines exhibited spontaneous shoot and root formation, while somatic embryogenesis could be induced from in-vitro germinated seedling hypocotyls cultured on media supplemented with cytokinin. Together these results suggest that ectopic BBM expression in transgenic tobacco also activates cell proliferation pathways, but differences exist between Arabidopsis/B. napus and N. tabacum with respect to their competence to respond to the BBM signalling molecule.

Involvement of hormones and KNOXI genes in early Arabidopsis seedling development

Plant hormones control plant development by modulating the expression of regulatory genes, including homeobox-containing KNOXI genes. However, much remains to be elucidated about the interactions involved. Therefore, hormonal regulation of KNOXI gene expression was investigated using hormone applications and an inducible transgenic ipt expression system to increase endogenous cytokinin (CK) levels. Treatments with auxin, abscisic acid (ABA), cytokinins, ethylene, and gibberellin (GA) did not result in ectopic expression of the BP (BREVIPEDICELLUS) gene. However, BP expression was strongly reduced by ABA, increased by auxin treatment (correlating with the initiation of lateral root meristems, which strongly express BP), and did not significantly respond to short-term treatments with the other hormones in whole seedlings. Following short-term ipt activation, organ-specific differential regulation of KNOXI gene expression was observed. While several KNOXI genes were transiently up-regulated to low levels, STM was selectively repressed (especially at low light) in hypocotyls. In cotyledons, activation of CK-responsive genes preceded ipt induction, suggesting that CKs are transported more rapidly than the inducing agent (dexamethasone). Long-term increases in CK levels induced raised levels of several KNOXI transcripts in hypocotyls, correlating with the radial expansion of vascular tissues, the main domains of KNOXI gene expression, suggesting that CKs had little effect on KNOXI promoter activity. No alterations in hormone sensitivity were observed in a bp null mutant. Constitutive BP overexpression caused reductions in the length and number of lateral roots, while the primary root remained unaffected. The transgenic seedlings displayed weak, but significant, alterations in sensitivity to ABA, CK, and 1-amino-cyclopropane-1-carboxylic acid.

Shoot apical development during in vitro embryogenesisThis review is one of a selection of papers published on the Special Theme of Shoot Apical Meristems

Formation of the shoot apical meristem (SAM) has been extensively investigated in zygotic embryos of flowering plants, where it follows a prolonged and dynamic developmental pattern underlined by precise temporal and spatial changes in gene expression. Studies conducted on the plant model system Arabidopsis have revealed that SAM formation is controlled by a genetic network and involves the participation of several regulatory genes expressed at different stages of development. As a general rule apical meristem development in vivo occurs very early; at the globular stage of development in flowering plants and in club-stage embryos of conifers. Once formed, meristems of zygotic embryos are stable structures that become reactivated at the onset of germination. Shoot apical meristem formation during in vitro embryogenesis is demarked by structural events similar to those described for zygotic embryos, although differences can be observed during the late phases of development, where cellular differentiation and formation of intercellular spaces disrupt the architecture of SAMs produced in culture. These events, which denote the “unstable” nature of SAMs of somatic embryos, often result in poor conversion frequency and reduced plant regeneration. By using Picea glauca (Moench) Voss (white spruce) somatic embryos and microspore-derived embryos of Brassica napus L. (canola) as model systems, this review provides methods for improving SAM formation through manipulations of the culture medium which alter the cellular redox status. Meristem marker genes from Arabidopsis, such as WUSCHEL (which is required for the acquisition of stem fate identity), represent a valuable tool for estimating the quality of SAM produced by microspore-derived embryos of canola. In spruce, the identification of two novel meristem marker genes, HBK1 and PgAGO, will allow similar studies in conifers.

Leaf senescence is delayed in tobacco plants expressing the maize knotted1 gene under the control of a wound-inducible promoter

To extend the shelf life of freshly harvested vegetables and cut flowers, a maize homeobox gene Knotted1 (kn1) was placed under the control of a wound-inducible promoter win3.12 from hybrid poplar (Populus trichocarpa x P. deltoides) and introduced into tobacco plants (Nicotiana tabacum cv. Xanthi). Transgenic win3.12::kn1 plants were morphologically normal. A leaf-detachment assay demonstrated that senescence in win3.12::kn1 leaves could be delayed by at least 2 weeks compared with wild type leaves. Furthermore, all leaves of win3.12::kn1 shoots remained green and healthy 3 weeks after excision and incubation in water, while older leaves of control shoots senesced under the same conditions. Additionally, a number of adventitious roots produced at the cut ends of wild type shoots after a 3-week incubation, but much a less number of adventitious roots appeared in win3.12::kn1 shoots. The delay in senescence was also confirmed by a higher total chlorophyll (a + b) content in win3.12::kn1 leaves relative to that of the control plants. RT-PCR analysis showed that the kn1 transcript was detected in win3.12::kn1 leaves with wounding treatment, but otherwise was not observed in leaves of wild type and unwounded transgenic plants. The results presented here indicate that expression of kn1 gene driven by the wound-inducible promoter win3.12 is potentially useful to delay senescence of vegetable crops and commercial horticulture after harvest.

Ectopic expression of KNOTTED1-like homeobox protein induces expression of cytokinin biosynthesis genes in rice

Some phytohormones such as gibberellins (GAs) and cytokinins (CKs) are potential targets of the KNOTTED1-like homeobox (KNOX) protein. To enhance our understanding of KNOX protein function in plant development, we identified rice (Oryza sativa) genes for adenosine phosphate isopentenyltransferase (IPT), which catalyzes the rate-limiting step of CK biosynthesis. Molecular and biochemical studies revealed that there are eight IPT genes, OsIPT1 to OsIPT8, in the rice genome, including a pseudogene, OsIPT6. Overexpression of OsIPTs in transgenic rice inhibited root development and promoted axillary bud growth, indicating that OsIPTs are functional in vivo. Phenotypes of OsIPT overexpressers resembled those of KNOX-overproducing transgenic rice, although OsIPT overexpressers did not form roots or ectopic meristems, both of which are observed in KNOX overproducers. Expression of two OsIPT genes, OsIPT2 and OsIPT3, was up-regulated in response to the induction of KNOX protein function with similar kinetics to those of down-regulation of GA 20-oxidase genes, target genes of KNOX proteins in dicots. However, expression of these two OsIPT genes was not regulated in a feedback manner. These results suggest that OsIPT2 and OsIPT3 have unique roles in the developmental process, which is controlled by KNOX proteins, rather than in the maintenance of bioactive CK levels in rice. On the basis of these findings, we concluded that KNOX protein simultaneously decreases GA biosynthesis and increases de novo CK biosynthesis through the induction of OsIPT2 and OsIPT3 expression, and the resulting high-CK and low-GA condition is required for formation and maintenance of the meristem.

L1 division and differentiation patterns influence shoot apical meristem maintenance

Plant development requires regulation of both cell division and differentiation. The class 1 KNOTTED1-like homeobox (KNOX) genes such as knotted1 (kn1) in maize (Zea mays) and SHOOTMERISTEMLESS in Arabidopsis (Arabidopsis thaliana) play a role in maintaining shoot apical meristem indeterminacy, and their misexpression is sufficient to induce cell division and meristem formation. KNOX overexpression experiments have shown that these genes interact with the cytokinin, auxin, and gibberellin pathways. The L1 layer has been shown to be necessary for the maintenance of indeterminacy in the underlying meristem layers. This work explores the possibility that the L1 affects meristem function by disrupting hormone transport pathways. The semidominant Extra cell layers1 (Xcl1) mutation in maize leads to the production of multiple epidermal layers by overproduction of a normal gene product. Meristem size is reduced in mutant plants and more cells are incorporated into the incipient leaf primordium. Thus, Xcl1 may provide a link between L1 division patterns, hormonal pathways, and meristem maintenance. We used double mutants between Xcl1 and dominant KNOX mutants and showed that Xcl1 suppresses the Kn1 phenotype but has a synergistic interaction with gnarley1 and rough sheath1, possibly correlated with changes in gibberellin and auxin signaling. In addition, double mutants between Xcl1 and crinkly4 had defects in shoot meristem maintenance. Thus, proper L1 development is essential for meristem function, and XCL1 may act to coordinate hormonal effects with KNOX gene function at the shoot apex.

Oncogene 6b from Agrobacterium tumefaciens induces abaxial cell division at late stages of leaf development and modifies vascular development in petioles

The 6b gene in the T-DNA region of the Ti plasmids of Agrobacterium tumefaciens and A. vitis is able to generate shooty calli in phytohormone-free culture of leaf sections of tobacco transformed with 6b. In the present study, we report characteristic morphological abnormalities of the leaves of transgenic tobacco and Arabidopsis that express 6b from pTiAKE10 (AK-6b), and altered expression of genes related to cell division and meristem formation in the transgenic plants. Cotyledons and leaves of both transgenic tobacco and Arabidopsis exhibited various abnormalities including upward curling of leaf blades, and transgenic tobacco leaves produced leaf-like outgrowths from the abaxial side. Transcripts of some class 1 KNOX homeobox genes, which are thought to be related to meristem functions, and cell cycle regulating genes were ectopically accumulated in mature leaves. M phase-specific genes were also ectopically expressed at the abaxial sides of mature leaves. These results suggest that the AK-6b gene stimulates the cellular potential for division and meristematic functions preferentially in the abaxial side of leaves and that the leaf phenotypes generated by AK-6b are at least in part due to such biased cell division during polar development of leaves. The results of the present experiments with a fusion gene between the AK-6b gene and the glucocorticoid receptor gene showed that nuclear import of the AK-6b protein was essential for upward curling of leaves and hormone-free callus formation, suggesting a role for AK-6b in nuclear events.

The maize Knotted1 gene is an effective positive selectable marker gene for Agrobacterium-mediated tobacco transformation

We have assessed the use of a homeobox gene knotted1 (kn1) from maize as a selectable marker gene for plant transformation. The kn1 gene under the control of cauliflower mosaic virus 35S promoter (35S::kn1) was introduced into Nicotiana tabacum cv. Xanthi via Agrobacterium-mediated transformation. Under nonselective conditions (without antibiotic selection) on a hormone-free medium (MS), a large number of transgenic calli and shoots were obtained from explants that were infected with Agrobacterium tumefaciens LBA4404 harboring the 35S::kn1 gene. On the other hand, no calli or shoots were produced from explants that were infected with an Agrobacterium strain harboring pBI121 (nptII selection) or from uninfected controls cultured under identical conditions. Relative to kanamycin selection conferred by nptII, the use of kn1 resulted in a 3-fold increase in transformation efficiency. The transgenic status of shoots obtained was confirmed by both histochemical detection of GUS activity and molecular analysis. The results presented here suggest that kn1 gene could be used as an effective alternative selection marker with a potential to enhance plant transformation efficiency in many plant species. With kn1 gene as a selection marker gene, no antibiotic-resistance or herbicide-resistance genes are needed so that potential risks associated with the use of these traditional selection marker genes can be eliminated.

Zeatin accumulation and misexpression of a class I knox gene are intimately linked in the epiphyllous response of the interspecific hybrid EMB-2 (Helianthus annuus x H. tuberosus)

Epiphylly, occurring in a somaclonal variant (EMB-2) of the interspecific hybrid Helianthus annuus x H. tuberosus, was used to investigate molecular and cyto-physiological mechanisms that underlie cellular fate change. EMB-2 plants are characterized by profuse proliferation of shoot- and embryo-like structures on some leaves. We addressed the putative relationship between cytokinins and knox genes in EMB-2 plants. A class I knox gene, HtKNOT1, was isolated from H. tuberosus. A high level of HtKNOT1 transcripts was detected in EMB-2 epiphyllous leaves compared to non-epiphyllous (NEP) ones. In addition, epiphylly was related to a localized increases in zeatin and N-glycosylated cytokinins. As ectopic morphogenesis proceeded, HtKNOT1 transcripts and zeatin co-localized and showed different patterns in ectopic shoot compared with embryo-like structures, consistent with the differential role of both cytokinin and knox genes in the two morphogenetic events. Notably, a massive shoot/embryo regeneration was induced in EMB-2 NEP leaves by in vitro zeatin treatment. These results clearly indicate that localized cytokinin accumulation and ectopic expression of HtKNOT1 are closely linked in the epiphylly of EMB-2 plants.

Leaf initiation: the integration of growth and cell division

The shoot apical meristem of higher plants is characterized by a conserved pattern of cell division, the functional significance of which is unclear. Although a causal role for cell division frequency and orientation in morphogenesis has been suggested, supporting data are limited. An alternative interpretation laying stress on the control of growth vector and its integration with networks of transcription factors and hormonal signals is discussed in this review.

Cell differentiation and organ initiation at the shoot apical meristem

Plants continuously generate organs at the flanks of their shoot apical meristems (SAMs). The patterns in which these organs are initiated, also called patterns of phyllotaxis, are highly stereotypic and characteristic for a particular species or developmental stage. This stable, predictable behaviour of the meristem has led to the idea that organ initiation must be based on simple and robust mechanisms. This conclusion is less evident, however, if we consider the very dynamic behaviour of the individual cells. How dynamic cellular events are coordinated and how they are linked to the regular patterns of organ initiation is a major issue in plant developmental biology.

Integrating hormone signaling and patterning mechanisms in plant development

Plant growth and development are driven by the bustling integration of a vast number of signals, among which plant hormones dominate. Understanding the role of hormones in particular developmental events requires their integration with developmental regulators known to be specific to those events. Using the increasing number of tools that can be utilized to probe hormone biosynthesis, transport and response, several recent studies have taken such an integrative approach, and in so doing have contributed to a clearer picture of precisely how hormones control plant development.

Overexpression in transgenic tobacco reveals different roles for the rice homeodomain gene OsBIHD1 in biotic and abiotic stress responses

The rice OsBIHD1 gene encodes a transcriptional factor belonging to the homeodomain class. It had previously been shown to be activated by treatment with benzothiadiazole, a chemical inducer of disease resistance, and in an incompatible interaction between rice and the blast fungus. To allow a better understanding of the function of OsBIHD1 in plant disease resistance response, the OsBIHD1 gene in tobacco was overexpressed by Agrobacterium-mediated leaf disc transformation with a construct containing the OsBIHD1 ORF under control of the 35S promoter. Overexpression of the rice OsBIHD1 gene in some of the transgenic tobacco lines led to some morphological abnormalities in the top buds and roots. The transgenic tobacco plants showed an elevated level of defence-related PR-1 gene expression and enhanced disease resistance against infection by tomato mosaic virus, tobacco mosaic virus, and Phytophthora parasitica var. nicotianae. However, the transgenic tobacco plants overexpressing OsBIHD1 showed enhanced sensitivity to salt and oxidative stress as compared with the wild-type plants. The results suggested that the OsBIHD1 protein may be positively involved in activating expression of the defence-related genes in disease resistance responses, and is also important in rice development and abiotic stress tolerance.

Up-regulation of OsBIHD1, a rice gene encoding BELL homeodomain transcriptional factor, in disease resistance responses

In the present study, we cloned and identified a full-length cDNA of a rice gene, OsBIHD1, encoding a homeodomain type transcriptional factor. OsBIHD1 is predicted to encode a 642 amino acid protein and the deduced protein sequence of OsBIHD1 contains all conserved domains, a homeodomain, a BELL domain, a SKY box, and a VSLTLGL box, which are characteristics of the BELL type homedomain proteins. The recombinant OsBIHD1 protein expressed in Escherichia coli bound to the TGTCA motif that is the characteristic cis-element DNA sequence of the homeodomain transcriptional factors. Subcellular localization analysis revealed that the OsBIHD1 protein localized in the nucleus of the plant cells. The OsBIHD1 gene was mapped to chromosome 3 of the rice genome and is a single-copy gene with four exons and three introns. Northern blot analysis showed that expression of OsBIHD1 was activated upon treatment with benzothiadiazole (BTH), which is capable of inducing disease resistance. Expression of OsBIHD1 was also up-regulated rapidly during the first 6 h after inoculation with Magnaporthe grisea in BTH-treated rice seedlings and during the incompatible interaction between M. grisea and a resistant genotype. These results suggest that OsBIHD1 is a BELL type of homeodomain transcription factor present in the nucleus, whose induction is associated with resistance response in rice.

KNOX homeobox genes potentially have similar function in both diploid unicellular and multicellular meristems, but not in haploid meristems

Members of the class 1 knotted-like homeobox (KNOX) gene family are important regulators of shoot apical meristem development in angiosperms. To determine whether they function similarly in seedless plants, three KNOX genes (two class 1 genes and one class 2 gene) from the fern Ceratopteris richardii were characterized. Expression of both class 1 genes was detected in the shoot apical cell, leaf primordia, marginal part of the leaves, and vascular bundles by in situ hybridization, a pattern that closely resembles that of class 1 KNOX genes in angiosperms with compound leaves. The fern class 2 gene was expressed in all sporophyte tissues examined, which is characteristic of class 2 gene expression in angiosperms. All three CRKNOX genes were not detected in gametophyte tissues by RNA gel blot analysis. Arabidopsis plants overexpressing the fern class 1 genes resembled plants that overexpress seed plant class 1 KNOX genes in leaf morphology. Ectopic expression of the class 2 gene in Arabidopsis did not result in any unusual phenotypes. Taken together with phylogenetic analysis, our results suggest that (a) the class 1 and 2 KNOX genes diverged prior to the divergence of fern and seed plant lineages, (b) the class 1 KNOX genes function similarly in seed plant and fern sporophyte meristem development despite their differences in structure, (c) KNOX gene expression is not required for the development of the fern gametophyte, and (d) the sporophyte and gametophyte meristems of ferns are not regulated by the same developmental mechanisms at the molecular level.

New pOp/LhG4 vectors for stringent glucocorticoid-dependent transgene expression in Arabidopsis

To facilitate glucocorticoid-inducible transgene expression from the pOp promoter in Arabidopsis the ligand-binding domain of a rat glucocorticoid receptor (GR LBD) was fused to the amino terminus of the synthetic transcription factor LhG4 to generate LhGR-N. Fusions bearing the GR LBD at other positions in LhG4 exhibited incomplete repression or inefficient induction. LhGR-N was stringently repressed in the absence of exogenous glucocorticoid but was fully activated by addition of 2 microm dexamethasone which resulted in 1000-fold increase in GUS reporter activity. Half maximal induction was achieved with 0.2 microm dexamethasone. Reporter transcripts were detectable within 2 h of dexamethasone application and peaked 4-10 h later. Neither LhGR-N nor dexamethasone affected seedling development although ethanol retarded development when used as a solvent for dexamethasone. The efficiency of the pOp target promoter was improved 10- to 20-fold by incorporating six copies of the ideal lac operator with sufficient inter-operator spacing to allow simultaneous occupancy. Introduction of the TMV Omega sequence into the 5'UTR resulted in a further 10-fold increase in dexamethasone-inducible reporter activity and an increase in the induction factor to 10(4). Although promoters containing the TMV Omega sequence exhibited slightly increased basal expression levels in the absence of dexamethasone, stringent regulation of the cytokinin biosynthetic gene ipt was achieved with all promoters. Despite the severity of the induced ipt phenotypes, transcripts for the KNOX homoeodomain transcription factors BREVIPEDICELLUS and SHOOTMERISTEMLESS were not significantly increased within 48 h of dexamethasone application to seedlings.

Tall tales from sly dwarves: novel functions of gibberellins in plant development

Gibberellins (GAs) are endogenous hormones controlling numerous aspects of plant growth and development. Our present understanding of GA physiology is based largely on genetic analysis in model plants such as Arabidopsis. In spite of the success of this approach, the discovery of additional physiological roles for GAs in seed development, pollen tube growth and meristem development indicates that the existing collection of GA-related mutants (identified partially or entirely on the basis of vegetative phenotypes) has failed to uncover all aspects of plant development that are controlled by GAs. The continued use of ever improving forward and reverse genetic techniques is expected to lead to the discovery of further novel roles for GAs in plant development.

The role of knox genes in plant development

knox genes encode homeodomain-containing transcription factors that are required for meristem maintenance and proper patterning of organ initiation. In plants with simple leaves, knox genes are expressed exclusively in the meristem and stem, but in dissected leaves, they are also expressed in leaf primordia, suggesting that they may play a role in the diversity of leaf form. This hypothesis is supported by the intriguing phenotypes found in gain-of-function mutations where knox gene misexpression affects leaf and petal shape. Similar phenotypes are also found in recessive mutations of genes that function to negatively regulate knox genes. KNOX proteins function as heterodimers with other homeodomains in the TALE superclass. The gibberellin and lignin biosynthetic pathways are known to be negatively regulated by KNOX proteins, which results in indeterminate cell fates.

Genetics of barley hooded suppression

The molecular basis of the barley dominant Hooded (K) mutant is a duplication of 305 bp in intron IV of the homeobox gene Bkn3. A chemical mutagenesis screen was carried out to identify genetical factors that participate in Bkn3 intron-mediated gene regulation. Plants from recurrently mutagenized KK seeds were examined for the suppression of the hooded awn phenotype induced by the K allele and, in total, 41 suK (suppressor of K) recessive mutants were identified. Complementation tests established the existence of five suK loci, and alleles suKB-4, suKC-33, suKD-25, suKE-74, and suKF-76 were studied in detail. All K-suppressed mutants showed a short-awn phenotype. The suK loci have been mapped by bulked segregant analysis nested in a standard mapping procedure based on AFLP markers. K suppressor loci suKB, B, E, and F all map in a short interval of chromosome 7H, while the locus suKD is assigned to chromosome 5H. A complementation test between the four suK mutants mapping on chromosome 7H and the short-awn mutant lks2, located nearby, excluded the allelism between suK loci and lks2. The last experiment made clear that the short-awn phenotype of suK mutants is due to a specific dominant function of the K allele, a function that is independent from the control on hood formation. The suK loci are discussed as candidate participants in the regulation of Bkn3 expression.

The shooty callus induced by suppression of tobacco CHRK1 receptor-like kinase is a phenocopy of the tobacco genetic tumor

CHRK1 encodes a tobacco receptor-like kinase that contains a chitinase-like sequence in the extracellular domain. In a previous study, CHRK1-suppressed transgenic tobacco plants exhibited pleiotropic developmental abnormalities including spontaneous growth of shooty callus from emerging embryos in the absence of any exogenous hormones. In this study, we show that the CHRK1 shooty callus mimics tobacco genetic tumors in its morphology, physiology, and gene expression profiles. Similar to CHRK1 shooty callus, tobacco genetic tumors exhibit shooty callus morphology and hormone-independent shoot organogenesis. Both the CHRK1 callus and genetic tumors constitutively expressed KNOTTED1-type homeobox genes at the high levels, consistent with their vigorous shoot formation. These two types of calli exhibited cell death phenotypes, accompanied by high H2O2 production, increased ion leakage, and callose accumulation. Consistently, both types of calli constitutively expressed high levels of defense genes induced during pathogen-mediated HR cell death. These results, together with previous reports that both the CHRK1 shooty callus and tobacco genetic tumor contained high levels of cytokinin, indicate that CHRK1 shooty callus is a phenocopy of tobacco genetic tumor. CHRK1-mediated signal transduction may play a role in the formation of the genetic tumor in tobacco.

Compound leaves: equal to the sum of their parts?

The leaves of seed plants can be classified as being either simple or compound according to their shape. Two hypotheses address the homology between simple and compound leaves, which equate either individual leaflets of compound leaves with simple leaves or the entire compound leaf with a simple leaf. Here we discuss the genes that function in simple and compound leaf development, such as KNOX1 genes, including how they interact with growth hormones to link growth regulation and development to cause changes in leaf complexity. Studies of transcription factors that control leaf development, their downstream targets, and how these targets are regulated are areas of inquiry that should increase our understanding of how leaf complexity is regulated and how it evolved through time.