Evolution of the grass leaf by primordium extension and petiole-lamina remodeling.

The sheathing leaf found in grasses and other monocots is an evolutionary innovation, yet its origin has been a subject of long-standing debate. Here, we revisit the problem in the light of developmental genetics and computational modeling. We show that the sheathing leaf likely arose through WOX-gene-dependent extension of a primordial zone straddling concentric domains around the shoot apex. Patterned growth within this zone, oriented by two polarity fields, accounts for wild-type, mutant and mosaic grass leaf development, whereas zone contraction and growth remodeling accounts for eudicot leaf development. In contrast to the prevailing view, our results suggest that the sheath derives from petiole, whereas the blade derives from the lamina of the eudicot leaf, consistent with homologies proposed in the 19th century.

Opposing effects of pro- and anti-inflammatory cytokine gene polymorphisms on the risk for breast cancer in western Indian women: a pilot study.

In an earlier study, the genotypes associated with higher level of transforming growth factor-ß1 (TGF-ß1) were found to reduce the risk for breast cancer in western Indian women. This observation implied that gene polymorphisms affecting the levels of pro- and anti-inflammatory cytokines may influence the risk for breast cancer in this population. Hence, we performed genotyping for three more functional single-nucleotide polymorphisms (SNPs) responsible for variations in the levels of cytokines associated with inflammation. To that effect, polymorphisms in genes coding for IL-4 (IL-4 C-590T; rs2243250), IFN-γ (IFN-G A + 874T; rs2430561) and MCP-1 (MCP-1 A-2578G; rs1024611) were examined in premenopausal, healthy women (N = 239) and patients with breast cancer (N = 182) from western India. In carriers of the IL-4*590T allele, a reduced risk for the disease (dominant model; OR = 0.61, 95% CI 0.37-0.98) was seen similar to that seen in TGF-B1*10C carriers. An opposite trend was observed with respect to the alleles associated with higher expression of MCP-1 or IFN-γ. In individuals positive for three or more alleles associated with higher levels of either pro- or anti-inflammatory cytokines, an additive effect on the modulation of risk for the disease was evident (for TGF-B1 & IL-4, OR = 0.33, 95% CI 0.12-0.87; for IFN-G & MCP-1, OR = 2.29, 95% CI 0.95-5.51). In the context of contrasting observations in other populations, these results indicate a significant contribution of anti-inflammatory genotypes in the modulation of risk for breast cancer in western Indian women.

How to pattern a leaf.

Leaf development presents a tremendous resource for tackling the question of patterning in biology. Leaves can be simple or highly dissected. They may have elaborated parts such as the tendrils of a pea leaf or the rolled blade of a carnivorous pitcher plant. Despite the variation in size, shape, and function, all leaves initiate in the same manner: from the flanks of a meristem. The maize leaf is useful for analysis of patterning due to the wealth of mutants and the distinct tissues along the proximal distal axis. The blade is distal, the sheath is proximal, and the ligule forms at the blade/sheath boundary. Establishment of this boundary involves the transcription factors LIGULELESS1 and LIGULELESS2 and the kinase LIGULELESS NARROW. The meristem-specific protein KNOTTED1 (KN1) binds and modulates the lg2 gene. Given the localization of KN1 at the proximal end of the leaf from the time of inception, we hypothesize that KN1 has a role in establishing the very proximal end of the leaf, whereas an auxin maximum guides the growing distal tip.

Linking the epigenetic 'language' of covalent histone modifications to cancer.

Covalent modifications of histones, such as acetylation, methylation, and phosphorylation, and other epigenetic modulations of the chromatin, such as methylation of DNA and ATP-dependent chromatin reorganisation, can play a major part in the multistep process of carcinogenesis, with far-reaching implications for human biology and human health. This review focuses on how aberrant covalent histone modifications may contribute to the development of a variety of human cancers, and discusses the recent findings with regard to potential therapies.

Linking the epigenetic 'language' of covalent histone modifications to cancer.

Covalent modifications of histones, such as acetylation, methylation, and phosphorylation, and other epigenetic modulations of the chromatin, such as methylation of DNA and ATP-dependent chromatin reorganisation, can play a major part in the multistep process of carcinogenesis, with far-reaching implications for human biology and human health. This review focuses on how aberrant covalent histone modifications may contribute to the development of a variety of human cancers, and discusses the recent findings with regard to potential therapies.

The fasciated ear2 gene encodes a leucine-rich repeat receptor-like protein that regulates shoot meristem proliferation in maize.

The ability to initiate organs throughout the lifecycle is a unique feature of plant development that is executed by groups of stem cells called meristems. The balance between stem cell proliferation and organ initiation is carefully regulated and ensures that organs can be initiated in regular geometric patterns. To understand how this regulation is achieved, we isolated a novel mutant of maize, fasciated ear2 (fea2), which causes a massive overproliferation of the ear inflorescence meristem and a more modest effect on floral meristem size and organ number. We cloned the fea2 gene using transposon tagging, and it encodes a membrane localized leucine-rich repeat receptor-like protein that is most closely related to CLAVATA2 from Arabidopsis. These findings provide evidence that the CLAVATA pathway for regulation of meristem size is functionally conserved throughout the angiosperms. A possible connection of fea2 to the control of crop yields is discussed.

barren inflorescence2 regulates axillary meristem development in the maize inflorescence.

Organogenesis in plants is controlled by meristems. Shoot apical meristems form at the apex of the plant and produce leaf primordia on their flanks. Axillary meristems, which form in the axils of leaf primordia, give rise to branches and flowers and therefore play a critical role in plant architecture and reproduction. To understand how axillary meristems are initiated and maintained, we characterized the barren inflorescence2 mutant, which affects axillary meristems in the maize inflorescence. Scanning electron microscopy, histology and RNA in situ hybridization using knotted1 as a marker for meristematic tissue show that barren inflorescence2 mutants make fewer branches owing to a defect in branch meristem initiation. The construction of the double mutant between barren inflorescence2 and tasselsheath reveals that the function of barren inflorescence2 is specific to the formation of branch meristems rather than bract leaf primordia. Normal maize inflorescences sequentially produce three types of axillary meristem: branch meristem, spikelet meristem and floral meristem. Introgression of the barren inflorescence2 mutant into genetic backgrounds in which the phenotype was weaker illustrates additional roles of barren inflorescence2 in these axillary meristems. Branch, spikelet and floral meristems that form in these lines are defective, resulting in the production of fewer floral structures. Because the defects involve the number of organs produced at each stage of development, we conclude that barren inflorescence2 is required for maintenance of all types of axillary meristem in the inflorescence. This defect allows us to infer the sequence of events that takes place during maize inflorescence development. Furthermore, the defect in branch meristem formation provides insight into the role of knotted1 and barren inflorescence2 in axillary meristem initiation.

The role of floral meristems in patterning.

The flower is one of the most complex and varied structures found in plants. Over the past decade, we have begun to understand how floral patterning is established in a handful of model species. Recent studies have identified the presence of several potential pathways for organ patterning. Many genes that are involved in these pathways have been cloned, providing opportunities for further fruitful investigations into the genetic components of flower development.

Transcription factors on the move.

We think of transcription factors as being confined to the cell where they are transcribed, but recent work shows they are able to move from cell to cell in plants. Plant cells are thought to communicate through membrane-lined channels called plasmodesmata. Although the size exclusion limit for these pores is small, microinjection experiments indicate that macromolecular trafficking through plasmodesmata occurs and can be regulated. But can functional plant proteins in their normal expression domain also move between cells? A recent paper by Sessions et al. elegantly addresses this question using meristem chimeras. Intriguingly, only one of the two proteins studied moved, thereby setting the stage for analysis of the mechanism and selectivity of protein trafficking between cells.

Mechanisms that control knox gene expression in the Arabidopsis shoot.

Knotted1-like homeobox (knox) genes are expressed in specific patterns within shoot meristems and play an important role in meristem maintenance. Misexpression of the knox genes, KNAT1 or KNAT2, in Arabidopsis produces a variety of phenotypes, including lobed leaves and ectopic stipules and meristems in the sinus, the region between lobes. We sought to determine the mechanisms that control knox gene expression in the shoot by examining recessive mutants that share phenotypic characteristics with 35S::KNAT1 plants. Double mutants of serrate (se) with either asymmetric1 (as1) or asymmetric2 (as2) showed lobed leaves, ectopic stipules in the sinuses and defects in the timely elongation of sepals, petals and stamens, similar to 35S::KNAT1 plants. Ectopic stipules and in rare cases, ectopic meristems, were detected in the sinuses on plants that were mutant for pickle and either as1 or as2. KNAT1 and KNAT2 were misexpressed in the leaves and flowers of single as1 and as2 mutants and in the sinuses of leaves of the different double mutants, but not in se or pickle single mutants. These results suggest that AS1 and AS2 promote leaf differentiation through repression of knox expression in leaves, and that SE and PKL globally restrict the competence to respond to genes that promote morphogenesis.

CIITA leucine-rich repeats control nuclear localization, in vivo recruitment to the major histocompatibility complex (MHC) class II enhanceosome, and MHC class II gene transactivation.

The major histocompatibility complex (MHC) class II transactivator CIITA plays a pivotal role in the control of the cellular immune response through the quantitative regulation of MHC class II expression. We have analyzed a region of CIITA with similarity to leucine-rich repeats (LRRs). CIITA LRR alanine mutations abolish both the transactivation capacity of full-length CIITA and the dominant-negative phenotype of CIITA mutants with N-terminal deletions. We demonstrate direct interaction of CIITA with the MHC class II promoter binding protein RFX5 and could also detect novel interactions with RFXANK, NF-YB, and -YC. However, none of these interactions is influenced by CIITA LRR mutagenesis. On the other hand, chromatin immunoprecipitation shows that in vivo binding of CIITA to the MHC class II promoter is dependent on LRR integrity. LRR mutations lead to an impaired nuclear localization of CIITA, indicating that a major function of the CIITA LRRs is in nucleocytoplasmic translocation. There is, however, evidence that the CIITA LRRs are also involved more directly in MHC class II gene transactivation. CIITA interacts with a novel protein of 33 kDa in a manner sensitive to LRR mutagenesis. CIITA is therefore imported into the nucleus by an LRR-dependent mechanism, where it activates transcription through multiple protein-protein interactions with the MHC class II promoter binding complex.

Shoot meristem size is dependent on inbred background and presence of the maize homeobox gene, knotted1.

The knotted1 (kn1) gene of maize is expressed in meristems and is absent from leaves, including the site of leaf initiation within the meristem. Recessive mutations of kn1 have been described that limit the capacity to make branches and result in extra carpels. Dominant mutations suggest that kn1 function plays a role in maintaining cells in an undifferentiated state. We took advantage of a Ds-induced dominant allele in order to screen for additional recessive alleles resulting from mobilization of the Ds element. Analysis of one such allele revealed a novel embryonic shoot phenotype in which the shoot initiated zero to few organs after the cotyledon was made, resulting in plants that arrested as seedlings. We refer to this phenotype as a limited shoot. The limited shoot phenotype reflected loss of kn1 function, but its penetrance was background dependent. We examined meristem size and found that plants lacking kn1 function had shorter meristems than non-mutant siblings. Furthermore, meristems of restrictive inbreds were significantly shorter than meristems of permissive inbreds, implying a correlation between meristem height and kn1 gene function in the embryo. Analysis of limited shoot plants during embryogenesis indicated a role for kn1 in shoot meristem maintenance. We discuss a model for kn1 in maintenance of the morphogenetic zone of the shoot apical meristem.

Knots in the family tree: evolutionary relationships and functions of knox homeobox genes.

Knotted-like homeobox (knox) genes constitute a gene family in plants. Class I knox genes are expressed in shoot apical meristems, and (with notable exceptions) not in lateral organ primordia. Class II genes have more diverse expression patterns. Loss and gain of function mutations indicate that knox genes are important regulators of meristem function. Gene duplication has contributed to the evolution of families of homeodomain proteins in metazoans. We believe that similar mechanisms have contributed to the diversity of knox gene function in plants. Knox genes may have contributed to the evolution of compound leaves in tomato and could be involved in the evolution of morphological traits in other species. Alterations in cis-regulatory regions in some knox genes correlate with novel patterns of gene expression and distinctive morphologies. Preliminary data from the analysis of class I knox gene expression illustrates the evolution of complex patterns of knox expression is likely to have occurred through loss and gain of domains of gene expression.

Gnarley1 is a dominant mutation in the knox4 homeobox gene affecting cell shape and identity.

Maize leaves have a stereotypical pattern of cell types organized into discrete domains. These domains are altered by mutations in knotted1 (kn1) and knox (for kn1-like homeobox) genes. Gnarley (Gn1) is a dominant maize mutant that exhibits many of the phenotypic characteristics of the kn1 family of mutants. Gn1 is unique because it changes parameters of cell growth in the basal-most region of the leaf, the sheath, resulting in dramatically altered sheath morphology. The strongly expressive allele Gn1-R also gives rise to a floral phenotype in which ectopic carpels form. Introgression studies showed that the severity of the Gn1-conferred phenotype is strongly influenced by genetic background. Gn1 maps to knox4, and knox4 is ectopically expressed in plants with the Gn1-conferred phenotype. Immunolocalization experiments showed that the KNOX protein accumulates at the base of Gn1 leaves in a pattern that is spatially and temporally correlated with appearance of the mutant phenotype. We further demonstrate that Gn1 is knox4 by correlating loss of the mutant phenotype with insertion of a Mutator transposon into knox4.

Leaf senescence is delayed in tobacco plants expressing the maize homeobox gene knotted1 under the control of a senescence-activated promoter.

Leaf senescence is an active process involving remobilization of nutrients from senescing leaves to other parts of the plant. Whereas senescence is accompanied by a decline in leaf cytokinin content, supplemental cytokinin delays senescence. Plants that overexpress isopentenyl transferase (ipt), a cytokinin-producing gene, or knotted1 (kn1), a homeobox gene, have many phenotypes in common. Many of these phenotypes are characteristic of altered cytokinin physiology. The effect of kn1 on leaf senescence was tested by driving its expression using the promoter of the senescence-associated gene SAG12. SAG:kn1 tobacco plants showed a marked delay in leaf senescence but otherwise developed normally. The delay in senescence was revealed by an increase in chlorophyll content in SAG:kn1 leaves relative to leaves of the control plants and by a decrease in the number of dead leaves. Senescence was also delayed in detached leaves of SAG:kn1 plants. Delayed senescence was accompanied by increased leaf cytokinin content in older leaves expressing kn1. These experiments extend the current understanding of kn1 function and suggest that in addition to mediating meristem maintenance, kn1 is capable of regulating the onset of senescence in leaves.

Mosaic analysis of the dominant mutant, Gnarley1-R, reveals distinct lateral and transverse signaling pathways during maize leaf development.

Maize leaves are organized into two major domains along the proximal-distal axis: a broad flat blade at the distal end of the leaf, and a narrow, thickened sheath that encircles the stem. Between the blade and sheath are two wedge-shaped tissues called auricles, and the ligule, an epidermally derived fringe. Members of the Knotted1 (Kn1) family of mutations change the shape and position of both ligule and auricle, thus disturbing the overall pattern of the leaf. Here we present the results of a mosaic analysis of Gnarley1-R (Gn1-R), which like members of the Kn1 family, affects the ligule and auricle. Gn1-R is distinct, however, in altering the dimensions of cells that make up sheath tissue. To gain insight into the Gn1-R phenotype, we performed a mosaic analysis using X-ray induced chromosome breakage to generate wild-type (gn1+/-) sectors in otherwise Gn1-R leaves. These sectors allowed us to determine whether Gn1-R acts non-autonomously to influence adjacent cells. Most aspects of the Gn1-R phenotype, such as ligule position, inhibition of auricle development, and sheath thickness showed autonomy in the lateral dimension (leaf width). In contrast, all aspects of the Gn1-R phenotype were non-autonomous in the transverse dimension (leaf thickness), suggesting that signaling occurs between cell layers in the leaf. These results support a model for distinct signaling pathways along lateral versus transverse axes of a developing leaf.

Control of phyllotaxy in maize by the abphyl1 gene.

Organogenesis in plants occurs at the shoot apical meristem, a group of indeterminate stem cells that are organized during embryogenesis. Regulated initiation of leaves or flowers from the shoot meristem gives rise to the familiar geometric patterns observed throughout the plant kingdom. The mechanism by which these patterns, termed phyllotaxies, are generated, remains unclear. Maize plants initiate leaves singly, alternating from one side to the other in a regular pattern. Here we describe a recessive maize mutant, abphyl1, that initiates leaves in opposite pairs, in a pattern termed decussate phyllotaxy. The decussate shoot meristems are larger than normal throughout development, though the general structure and organization of the meristem is not altered. abph1 mutants are first distinguished during embryogenesis, prior to true leaf initiation, by a larger shoot meristem and coincident larger expression domain of the homeobox gene knotted1. Therefore, the abph1 gene regulates morphogenesis in the embryo, and plays a role in determining the phyllotaxy of the shoot.

The control of maize spikelet meristem fate by the APETALA2-like gene indeterminate spikelet1.

The orderly production of meristems with specific fates is crucial for the proper elaboration of plant architecture. The maize inflorescence meristem branches several times to produce lateral meristems with determinate fates. The first meristem formed, the spikelet pair meristem, produces two spikelet meristems, each of which produces two floral meristems. We have identified a gene called indeterminate spikelet1 (ids1) that specifies a determinate spikelet meristem fate and thereby limits the number of floral meristems produced. In the absence of ids1 gene function, the spikelet meristem becomes indeterminate and produces additional florets. Members of the grass family vary in the number of florets within their spikelets, suggesting that ids1 may play a role in inflorescence architecture in other grass species. ids1 is a member of the APETALA2 (AP2) gene family of transcription factors that has been implicated in a wide range of plant development roles. Expression of ids1 was detected in many types of lateral organ primordia as well as spikelet meristems. Our analysis of the ids1 mutant phenotype and expression pattern indicates that ids1 specifies determinate fates by suppressing indeterminate growth within the spikelet meristem.

Regulation of leaf initiation by the terminal ear 1 gene of maize.

Higher plants elaborate much of their architecture post-embryonically through development initiated at the tips of shoots. During vegetative growth, leaf primordia arise at predictable sites to give characteristic leaf arrangements, or phyllotaxies. How these sites are determined is a long-standing question that bears on the nature of pattern-formation mechanisms in plants. Fate-mapping studies in several species indicate that each leaf primordium becomes organized from a group of 100-200 cells on the flank of the shoot apex. Although molecular studies indicate that the regulated expression of specific homeobox genes plays some part in this determination process, mechanisms that regulate the timing and position of leaf initiation are less well understood. Here we describe a gene from maize, terminal ear 1. Patterns of expression of this gene in the shoot and phenotypes of mutants indicate a role for terminal ear 1 in regulating leaf initiation. The tel gene product contains conserved RNA-binding motifs, indicating that it may function through an RNA-binding activity.