Photoperiodic control of seasonal growth is mediated by ABA acting on cell-cell communication.

In temperate and boreal ecosystems, seasonal cycles of growth and dormancy allow perennial plants to adapt to winter conditions. We show, in hybrid aspen trees, that photoperiodic regulation of dormancy is mechanistically distinct from autumnal growth cessation. Dormancy sets in when symplastic intercellular communication through plasmodesmata is blocked by a process dependent on the phytohormone abscisic acid. The communication blockage prevents growth-promoting signals from accessing the meristem. Thus, precocious growth is disallowed during dormancy. The dormant period, which supports robust survival of the aspen tree in winter, is due to loss of access to growth-promoting signals.

Transcription factors PRE3 and WOX11 are involved in the formation of new lateral roots from secondary growth taproot in A. thaliana.

The spatial deployment of lateral roots determines the ability of a plant to interact with the surrounding environment for nutrition and anchorage. This paper shows that besides the pericycle, the vascular cambium becomes active in Arabidopsis thaliana taproot at a later stage of development and is also able to form new lateral roots. To demonstrate the above, we implemented a two-step approach in which the first step leads to development of a secondary structure in A. thaliana taproot, and the second applies a mechanical stress on the vascular cambium to initiate formation of a new lateral root primordium. GUS staining showed PRE3, DR5 and WOX11 signals in the cambial zone of the root during new lateral root formation. An advanced level of wood formation, characterized by the presence of medullar rays, was achieved. Preliminary investigations suggest the involvement of auxin and two transcription factors (PRE3/ATBS1/bHLH135/TMO7 and WOX11) in the transition of some vascular cambium initials from a role as producers of xylem/phloem mother cells to founder cells of a new lateral root primordium.

Stem cell function during plant vascular development.

While many regulatory mechanisms controlling the development and function of root and shoot apical meristems have been revealed, our knowledge of similar processes in lateral meristems, including the vascular cambium, is still limited. Our understanding of even the anatomy and development of lateral meristems (procambium or vascular cambium) is still relatively incomplete, let alone their genetic regulation. Research into this particular tissue type has been mostly hindered by a lack of suitable molecular markers, as well as the fact that thus far very few mutants affecting plant secondary development have been described. The development of suitable molecular markers is a high priority in order to help define the anatomy, especially the location and identity of cambial stem cells and the developmental phases and molecular regulatory mechanisms of the cambial zone. To date, most of the advances have been obtained by studying the role of the major plant hormones in vascular development. Thus far auxin, cytokinin, gibberellin and ethylene have been implicated in regulating the maintenance and activity of cambial stem cells; the most logical question in research would be how these hormones interact during the various phases of cambial development.

Cell signalling during vascular morphogenesis.

Vascular tissue in plants is unique due to its diverse and dynamic cellular patterns. Through research in several organisms, such as Arabidopsis, Populus and Zinnia, using biochemical, genetic and genomic approaches, significant progress has recently been made in revealing the molecular nature of several signals underlying the patterning of vascular tissue. These signals include ligands, receptors and transcriptional regulators. The future challenge is to understand how the identified signals work together to control vascular morphogenesis.

The genome of black cottonwood, Populus trichocarpa (Torr. & Gray).

We report the draft genome of the black cottonwood tree, Populus trichocarpa. Integration of shotgun sequence assembly with genetic mapping enabled chromosome-scale reconstruction of the genome. More than 45,000 putative protein-coding genes were identified. Analysis of the assembled genome revealed a whole-genome duplication event; about 8000 pairs of duplicated genes from that event survived in the Populus genome. A second, older duplication event is indistinguishably coincident with the divergence of the Populus and Arabidopsis lineages. Nucleotide substitution, tandem gene duplication, and gross chromosomal rearrangement appear to proceed substantially more slowly in Populus than in Arabidopsis. Populus has more protein-coding genes than Arabidopsis, ranging on average from 1.4 to 1.6 putative Populus homologs for each Arabidopsis gene. However, the relative frequency of protein domains in the two genomes is similar. Overrepresented exceptions in Populus include genes associated with lignocellulosic wall biosynthesis, meristem development, disease resistance, and metabolite transport.

A novel two-component hybrid molecule regulates vascular morphogenesis of the Arabidopsis root.

The developmental ontogeny of the vascular system (consisting of xylem, phloem and [pro]cambium) is poorly understood despite its central role in plant physiology. We show that in the Arabidopsis root meristem, xylem cell lineages are specified early, whereas phloem and procambium are established through a set of asymmetric cell divisions. These divisions require the WOODEN LEG (WOL) gene. The WOL gene encodes a novel two-component signal transducer with an unusual tandem arrangement of two receiver domains. It is expressed specifically in the vasculature from the early stages of embryogenesis on, consistent with a role as a sensor for vascular morphogenesis.

Molecular analysis of the SCARECROW gene in maize reveals a common basis for radial patterning in diverse meristems.

Maize and Arabidopsis root apical meristems differ in several aspects of their radial organization and ontogeny. Despite the large evolutionary distance and differences in root radial patterning, analysis of the putative maize ortholog of the Arabidopsis patterning gene SCARECROW (SCR) revealed expression localized to the endodermis, which is similar to its expression in Arabidopsis. Expression in maize extends through the quiescent center, a population of mitotically inactive cells formerly thought to be undifferentiated and to lack radial pattern information. Zea mays SCARECROW (ZmSCR), the putative maize SCR ortholog, was used as a molecular marker to investigate radial patterning during regeneration of the root tip after either whole or partial excision. Analysis of the dynamic expression pattern of ZmSCR as well as other markers indicates the involvement of positional information as a primary determinant in regeneration of the root radial pattern.

The SHORT-ROOT gene controls radial patterning of the Arabidopsis root through radial signaling.

Asymmetric cell divisions play an important role in the establishment and propagation of the cellular pattern of plant tissues. The SHORT-ROOT (SHR) gene is required for the asymmetric cell division responsible for formation of ground tissue (endodermis and cortex) as well as specification of endodermis in the Arabidopsis root. We show that SHR encodes a putative transcription factor with homology to SCARECROW (SCR). From analyses of gene expression and cell identity in genetically stable and unstable alleles of shr, we conclude that SHR functions upstream of SCR and participates in a radial signaling pathway. Consistent with a regulatory role in radial patterning, ectopic expression of SHR results in supernumerary cell divisions and abnormal cell specification in the root meristem.

Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot.

Mutation of the SCARECROW (SCR) gene results in a radial pattern defect, loss of a ground tissue layer, in the root. Analysis of the shoot phenotype of scr mutants revealed that both hypocotyl and shoot inflorescence also have a radial pattern defect, loss of a normal starch sheath layer, and consequently are unable to sense gravity in the shoot. Analogous to its expression in the endodermis of the root, SCR is expressed in the starch sheath of the hypocotyl and inflorescence stem. The SCR expression pattern in leaf bundle sheath cells and root quiescent center cells led to the identification of additional phenotypic defects in these tissues. SCR expression in a pin-formed mutant background suggested the possible origins of the starch sheath in the shoot inflorescence. Analysis of SCR expression and the mutant phenotype from the earliest stages of embryogenesis revealed a tight correlation between defective cell divisions and SCR expression in cells that contribute to ground tissue radial patterning in both embryonic root and shoot. Our data provides evidence that the same molecular mechanism regulates the radial patterning of ground tissue in both root and shoot during embryogenesis as well as postembryonically.

GEG participates in the regulation of cell and organ shape during corolla and carpel development in gerbera hybrida

The molecular mechanisms that control organ shape during flower development are largely unknown. By using differential hybridization techniques, a cDNA designated GEG (for Gerbera hybrida homolog of the gibberellin [GA]-stimulated transcript 1 [GAST1] from tomato) was isolated from a library representing late stages of corolla development in Gerbera. GEG expression was detected in corollas and carpels, with expression spatiotemporally coinciding with flower opening. In corollas and styles, GEG expression is temporally correlated with the cessation of longitudinal cell expansion. In plants constitutively expressing GEG, reduced corolla lengths and carpels with shortened and radially expanded stylar parts were found, with concomitant reduction of longitudinal cell expansion in these organs. In addition, in styles, an increase in radial cell expansion was detected. Taken together, these observations indicate a regulatory role for the GEG gene product in determining the shape of the corolla and carpel. The deduced amino acid sequence of the GEG gene product shares high similarity with previously characterized putative cell wall proteins encoded by GA-inducible genes, namely, GAST1, GIP (for GA-induced gene of petunia), and the GASA (for GA-stimulated in Arabidopsis) gene family. Our studies suggest that GEG, the expression of which can also be induced by application of GA3, plays a role in phytohormone-mediated cell expansion.

Organ identity genes and modified patterns of flower development in Gerbera hybrida (Asteraceae)

We have used Gerbera hybrida (the cultivated ornamental, gerera) to investigate the molecular basis of flower development in Asteraceae, a family of flowering plants that have heteromorphic flowers and specialized floral organs. Flowers of the same genotype may differ in a number of parameters, including sex expression, symmetry, sympetaly and pigmentation. In order to study the role of organ identity determination in these phenomena we isolated and functionally analysed six MADS box genes from gerbera; these were shown by phylogenetic analysis to be orthologous to well characterized regulatory genes described from Arabidopsis and Antirrhinum. Expression analysis suggests that the two gerbera agamous orthologues, the globosa orthologue and one of the deficiens orthologues may have functional equivalency to their counterparts, participating in the C and B functions, respectively. However, the function of a second deficiens orthologue appears unrelated to the B function, and that of a squamosa orthologue seems distinct from squamosa as well as from the A function. The induction patterns of gerbera MADS box genes conform spatiotemporally to the multi-flowered, head-like inflorescence typical of Asteraceae. Furthermore, gerbera plants transgenic for the newly isolated MADS box genes shed light onto the mechanistic basis for some floral characteristics that are typical for Asteraceae. We can conclude, therefore, that the pappus bristles are sepals highly modified for seed dispersal, and that organ abortion in the female marginal flowers is dependent upon organ identity and not organ position when position is homeotically altered.

A bHLH transcription factor mediates organ, region and flower type specific signals on dihydroflavonol-4-reductase (dfr) gene expression in the inflorescence of Gerbera hybrida (Asteraceae).

The angiosperm family Asteraceae is characterized by composite inflorescences, which are highly organized structures consisting of different types of flowers. In order to approach the control of floral organ differentiation in Asteraceae at molecular level, we are studying regulation of flavonoid biosynthesis in Gerbera hybrida. Dihydroflavonol-4-reductase (dfr) expression is regulated according to anthocyanin pigmentation patterns in all tested gerbera varieties at several anatomical levels. We have isolated a promoter for one of the dfr genes, Pgdfr2. Gerbera plants transgenic for a Pgdfr2-uidA construct reveal that the activity of the Pgdfr2 promoter from one variety follows the pigmentation in other varieties which have different color patterns. It is thus evident that the observed complex regulation of dfr expression occurs in trans. In order to identify the trans-acting regulators, we isolated a cDNA (gmyc1) homologous to the previously characterized genes encoding bHLH-type regulators of the anthocyanin pathway in plants. The expression of gmyc1 in different varieties suggests that it has a major role in regulating dfr activity in corolla and carpel, but not in pappus and stamen. Specifically in gerbera, the identical patterns of gmyc1 and dfr expression in corolla tissue suggest that GMYC1 also regulates dfr expression in a region and flower type specific manner. Our studies show that in gerbera GMYC1-dfr interaction is part of several developmental processes characteristic for Asteraceae (such as specification of flower types across the composite inflorescence), whereas in other processes (such as differentiation of sepal as pappus) other regulators control dfr expression to determine the spatial specificity.

The SCARECROW gene regulates an asymmetric cell division that is essential for generating the radial organization of the Arabidopsis root.

In the Arabidopsis root meristem, initial cells undergo asymmetric divisions to generate the cell lineages of the root. The scarecrow mutation results in roots that are missing one cell layer owing to the disruption of an asymmetric division that normally generates cortex and endodermis. Tissue-specific markers indicate that a heterogeneous cell type is formed in the mutant. The deduced amino acid sequence of SCARECROW (SCR) suggests that it is a member of a novel family of putative transcription factors. SCR is expressed in the cortex/endodermal initial cells and in the endodermal cell lineage. Tissue-specific expression is regulated at the transcriptional level. These results indicate a key role for SCR in regulating the radial organization of the root.

Duplication and functional divergence in the chalcone synthase gene family of Asteraceae: evolution with substrate change and catalytic simplification.

Plant-specific polyketide synthase genes constitute a gene superfamily, including universal chalcone synthase [CHS; malonyl-CoA:4-coumaroyl-CoA malonyltransferase (cyclizing) (EC 2.3.1.74)] genes, sporadically distributed stilbene synthase (SS) genes, and atypical, as-yet-uncharacterized CHS-like genes. We have recently isolated from Gerbera hybrida (Asteraceae) an unusual CHS-like gene, GCHS2, which codes for an enzyme with structural and enzymatic properties as well as ontogenetic distribution distinct from both CHS and SS. Here, we show that the GCHS2-like function is encoded in the Gerbera genome by a family of at least three transcriptionally active genes. Conservation within the GCHS2 family was exploited with selective PCR to study the occurrence of GCHS2-like genes in other Asteraceae. Parsimony analysis of the amplified sequences together with CHS-like genes isolated from other taxa of angiosperm subclass Asteridae suggests that GCHS2 has evolved from CHS via a gene duplication event that occurred before the diversification of the Asteraceae. Enzyme activity analysis of proteins produced in vitro indicates that the GCHS2 reaction is a non-SS variant of the CHS reaction, with both different substrate specificity (to benzoyl-CoA) and a truncated catalytic profile. Together with the recent results of Durbin et al. [Durbin, M. L., Learn, G. H., Jr., Huttley, G. A. & Clegg, M. T. (1995) Proc. Natl. Acad. Sci. USA 92, 3338-3342], our study confirms a gene duplication-based model that explains how various related functions have arisen from CHS during plant evolution.

Transgene inactivation in Petunia hybrida is influenced by the properties of the foreign gene.

Petunia mutant RL01 was transformed with maize A1 and gerbera gdfr cDNAs, which both encode dihydroflavonol-4-reductase (DFR) activity. The same Agrobacterium vector and the same version of the CaMV 35S promoter were used in both experiments. Transformation with the cDNAs resulted in production of pelargonidin pigments in the transformants. However, the A1 and gdfr transformants showed clearly different phenotypes. The flowers of the primary A1 transformants were pale and showed variability in pigmentation during their growth, while the flowers of the gdfr transformants showed intense and highly stable coloration. The color difference in the primary transformants was reflected in the expression levels of the transgenes as well as in the levels of anthocyanin pigment. As previously reported by others, the instability in pigmentation in the A1 transformants was more often detected in clones with multiple copies of the transgene and was associated with methylation of the 35S promoter and of the transgene cDNA itself. In the gdfr transformants, the most intense pigmentation was observed in plants with multiple transgenes in their genome. Only rarely was partial methylation of the 35S promoter detected, while the gdfr cDNA always remained in an unmethylated state. We conclude that the properties of the transgene itself strongly influence the inactivation process. The dicotyledonous gdfr cDNA with a lower GC content and fewer possible methylation sites is more 'compatible' the genomic organization of petunia and this prevents it being recognized as a foreign gene and hence silenced by methylation.

Gerbera hybrida (Asteraceae) imposes regulation at several anatomical levels during inflorescence development on the gene for dihydroflavonol-4-reductase.

In the ornamental cut flower plant Gerbera hybrida the spatial distribution of regulatory molecules characteristic of differentiation of the composite inflorescence is visualized as the various patterns of anthocyanin pigmentation of different varieties. In order to identify genes that the plant can regulate according to these anatomical patterns, we have analysed gene expression affecting two enzymatic steps, chalcone synthase (CHS) and dihydroflavonol-4-reductase (DFR), in five gerbera varieties with spatially restricted anthocyanin pigmentation patterns. The dfr expression profiles vary at the levels of floral organ, flower type and region within corolla during inflorescence development according to the anthocyanin pigmentation of the cultivars. In contrast, chs expression, although regulated in a tissue-specific manner during inflorescence development, varies only occasionally. The variation in the dfr expression profiles between the varieties reveals spatially specific gene regulation that senses the differentiation events characteristic of the composite inflorescence.

Chalcone synthase-like genes active during corolla development are differentially expressed and encode enzymes with different catalytic properties in Gerbera hybrida (Asteraceae).

Recent studies on chalcone synthase (CHS) and the related stilbene synthase (STS) suggest that the structure of chs-like genes in plants has evolved into different forms, whose members have both different regulation and capacity to code for different but related enzymatic activities. We have studied the diversity of chs-like genes by analysing the structure, expression patterns and catalytic properties of the corresponding enzymes of three genes that are active during corolla development in Gerbera hybrida. The expression patterns demonstrate that chs-like genes are representatives of three distinct genetic programmes that are active during organ differentiation in gerbera. Gchs1 and gchs3 code for typical CHS enzymes, and their gene expression pattern temporally correlates with flavonol (gchs1, gchs3) and anthocyanin (gchs1) synthesis during corolla development. Gchs2 is different. The expression pattern does not correlate with the pigmentation pattern, the amino acid sequence deviates considerably from the consensus of typical CHSs, and the catalytic properties are different. The data indicate that it represents a new member in the large superfamily of chs and chs-related genes.

A corolla- and carpel-abundant, non-specific lipid transfer protein gene is expressed in the epidermis and parenchyma of Gerbera hybrida var. Regina (Compositae).

We are examining the floral organ differentiation in Compositae by isolating and characterizing corolla abundant genes. Differential screening of a cDNA library made from the ray floret corolla of Gerbera hybrida var. Regina revealed an abundant cDNA clone which is expressed in the corolla but not in leaves. This cDNA (gltp1) codes for a polypeptide similar to non-specific lipid transfer proteins of the plants. The gltp1 gene is expressed only in the corolla and carpels and is developmentally regulated during corolla development. The gltp1 mRNA accumulates both in epidermal cell layers and in the mesophyll of the corolla. In the stylar part of the carpel, the gltp1 mRNA can be detected in the epidermal and in parenchymal cells but not in the transmitting tissue. Analogous patterns of gltp1 expression in the corolla and carpel may indicate that similar genetic programmes operates during the development of these two tissues.

Cloning of cDNA coding for dihydroflavonol-4-reductase (DFR) and characterization of dfr expression in the corollas of Gerbera hybrida var. Regina (Compositae).

We are approaching corolla differentiation in Compositae by studying the regulation of flavonoid pathway genes during inflorescence development in gerbera. We have cloned a dfr cDNA from a ray floret corolla cDNA library of Gerbera hybrida var. Regina by a PCR technique based on homologies found in genes isolated from other plant species. The functionality of the clone was tested in vivo by complementing the dihydrokaempferol accumulating petunia mutant line RL01. By Southern blot analysis, G. hybrida var. Regina was shown to harbour a small family of dfr genes, one member of which was deduced to be mainly responsible for the DFR activity in corolla. Dfr expression in corolla correlates with the anthocyanin accumulation pattern: it is basipetally induced, epidermally specific and restricted to the ligular part of corolla. By comparing the dfr expression in different floret types during inflorescence development, we could see that dfr expression reflects developmental schemes of the outermost ray and trans florets, contrasted with that of the disc florets.