Yrjö Helariutta

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.

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.

New pathway to polyketides in plants

The repertoire of secondary metabolism (involving the production of compounds not essential for growth) in the plant kingdom is enormous, but the genetic and functional basis for this diversity is hard to analyse as many of the biosynthetic enzymes are unknown. We have now identified a key enzyme in the ornamental plant Gerbera hybrida (Asteraceae) that participates in the biosynthesis of compounds that contribute to insect and pathogen resistance. Plants transformed with an antisense construct of gchs2, a complementary DNA encoding a previously unknown function,, completely lack the pyrone derivatives gerberin and parasorboside. The recombinant plant protein catalyses the principal reaction in the biosynthesis of these derivatives: GCHS2 is a polyketide synthase that uses acetyl-CoA and two condensation reactions with malonyl-CoA to form the pyrone backbone of thenatural products. The enzyme also accepts benzoyl-CoA to synthesize the backbone of substances that have become of interest as inhibitors of the HIV-1 protease. GCHS2 is related to chalcone synthase (CHS) and its properties define a new class of function in the protein superfamily. It appears that CHS-related enzymes are involved in the biosynthesis of a much larger range of plant products than was previously realized.

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.

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.

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.

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.

Transformation of antisense constructs of the chalcone synthase gene superfamily into Gerbera hybrida: differential effect on the expression of family members

Suppression of gene expression using antisense technology has been successful in various applications. In this paper we report differential inhibition of gene expression of the chalcone synthase (chs) gene superfamily members in transgenic Gerbera hybrida (Asteraceae) plants. We have transformed two different cDNAs of the chs gene family, gchs 1 [4] and gchs2, in antisense orientation under control of the CaMV 35S promoter into gerbera. Gchs1 codes for an enzyme with chalcone synthase activity while gchs2 is a more diverged member of the gene family having distinct structure and expression pattern. Furthermore, gchs2 is evidently not involved in anthocyanin synthesis and encodes an enzyme with novel catalytic properties. In both cases effective blocking of the resident sense gene expression was detected. In addition, the transformation affected differentially the expression of other members of the chs gene family. The degree of inhibition appeared to depend on the sequence homology between the antisense and the target genes. In the unevenly coloured inflorescences detected among anti-gchs1 transformants during their growth, relaxation of the antisense effect was here shown to start from the most distant member of the gene family, further demonstrating the influence of sequence homology in the stability of antisense inhibition.

Integration of hormonal signaling networks and mobile microRNAs is required for vascular patterning in Arabidopsis roots

Significance The vascular tissues form a continuous network providing the long-distance transport of water and nutrients in all higher plants (tracheophytes). To incorporate separate organs into this network, it is essential that the position of different vascular cell types is tightly regulated. Several factors required for root vascular patterning (including hormones and gene products) have previously been identified in the model plant Arabidopsis. We have now established a mathematical model formulizing the interaction between these factors, allowing us to identify a minimal regulatory network capable of maintaining a stable vascular pattern in Arabidopsis roots. We envisage that this model will help future researchers understand how similar regulatory units can be applied to create alternative patterns in other species. As multicellular organisms grow, positional information is continually needed to regulate the pattern in which cells are arranged. In the Arabidopsis root, most cell types are organized in a radially symmetric pattern; however, a symmetry-breaking event generates bisymmetric auxin and cytokinin signaling domains in the stele. Bidirectional cross-talk between the stele and the surrounding tissues involving a mobile transcription factor, SHORT ROOT (SHR), and mobile microRNA species also determines vascular pattern, but it is currently unclear how these signals integrate. We use a multicellular model to determine a minimal set of components necessary for maintaining a stable vascular pattern. Simulations perturbing the signaling network show that, in addition to the mutually inhibitory interaction between auxin and cytokinin, signaling through SHR, microRNA165/6, and PHABULOSA is required to maintain a stable bisymmetric pattern. We have verified this prediction by observing loss of bisymmetry in shr mutants. The model reveals the importance of several features of the network, namely the mutual degradation of microRNA165/6 and PHABULOSA and the existence of an additional negative regulator of cytokinin signaling. These components form a plausible mechanism capable of patterning vascular tissues in the absence of positional inputs provided by the transport of hormones from the shoot.

The formation of wood and its control

Wood continues to increase in importance as a sustainable source of energy and shelter. Wood formation is a dynamic process derived from plant secondary (radial) growth. Several experimental systems have been employed to study wood formation and its regulation. The use of genetic manipulation approaches and genome-wide analyses in model plants have significantly advanced our understanding of wood formation. In this review, we provide an update of our knowledge of the genetic and hormonal regulation of wood formation based on research in different plants systems, as well as considering the subject from an evo-devo perspective.