The PHANTASTICA gene encodes a MYB transcription factor involved in growth and dorsoventrality of lateral organs in Antirrhinum

A mutational analysis of leaf morphogenesis in Arabidopsis thaliana

As a contribution to a better understanding of the developmental processes that are specific to plants, we have begun a genetic analysis of leaf ontogeny in the model system Arabidopsis thaliana by performing a large-scale screening for mutants with abnormal leaves. After screening 46,159 M2 individuals, arising from 5770 M1 parental seeds exposed to EMS, we isolated 1926 M2 putative leaf mutants, 853 of which yielded viable M3 inbred progeny. Mutant phenotypes were transmitted with complete penetrance and small variations in expressivity in 255 lines. Most of them were inherited as recessive monogenic traits, belonging to 94 complementation groups, which suggests that we did not reach saturation of the genome. We discuss the nature of the processes presumably perturbed in the phenotypic classes defined among our mutants.

LEAF DEVELOPMENT IN ANGIOSPERMS

Leaves are produced in succession on the shoot apical meristem (SAM) of a plant. The three landmark stages in leaf morphogenesis include initiation, acquisition of suborgan identities, and tissue differentiation. The expression of various genes relative to these steps in leaf morphogenesis is described. KNOTTED-like homeobox (KNOX) genes, FLO/LFY, and floral homeotic genes may be involved in generation of leaf shape and complexity. The differences between compound leaves and simple leaves in gene expression characteristics and morphogenetic patterns are discussed.

Development of the leaf epidermis

The leaf epidermis is essential to plant survival not only because of its protective role at the interface with the plant's environment but also because of crucial developmental functions. The protoderm is set aside early in embryogenesis, possibly in the zygote. Epidermal identity is determined by the interactions of a complex set of factors, including developmental phase of the plant, regional identity within the leaf, and axiality. For the most part, these characteristics appear to be specified by internal tissues. On the other hand, the epidermis has a key role in regulating organ growth and expansion; thus interactions between the epidermis and internal tissues regulate the overall leaf architecture. Overlying this is the specification of different cell types within the epidermis. Some aspects of this appear to involve interactions with internal tissues but the patterning of many epidermal cell types seems to occur within the two-dimensional field of the epidermis itself and to require both cell signaling and cell lineage dependent mechanisms. Genetic analyses have provided much of the insight into the underlying principles that regulate epidermal development and a number of molecules important for various aspects of the process have been identified. Yet, for the most part, our understanding of the molecular basis for each component of epidermal development is still rudimentary and we have not yet scratched the surface of understanding how these pieces are integrated. The emerging technologies of functional genomics will provide powerful tools for solving these problems and the near future is likely to produce rapid progress.

Molecular genetics of gynoecium development in Arabidopsis

Carpels are the ovule-bearing structural units in angiosperms. In Arabidopsis, the specification of carpel identity is achieved by at least two separate pathways: a pathway mediated by the C class gene AG and an AG-independent pathway. Both pathways are negatively regulated by A class genes. Two genes, SPT and CRC, can promote differentiation of carpel tissue independently of AG and are thus components of the AG-independent pathway. CRC and SPT appear to act in a redundant manner to promote the differentiation of subsets of carpel tissues. The carpel primordium is subdivided into regional domains, both medial versus lateral and abaxial versus adaxial. Based on morphological and gene expression analyses, it appears likely that these domains define developmental compartments. The medial domain appears fated to differentiate into the marginal tissue types of the carpel (septum with transmitting tract and placenta with ovules), whereas the lateral domain gives rise to the ovary walls. The expression of ETT defines the abaxial domain, and this gene is involved in the abaxial-adaxial and, possibly, the apical-basal patterning of tissues in the carpel. Once regional domains have been established, the differentiation of tissue and cell types occurs. The MADS-box gene FUL and AGLI/5 are involved in the differentiation of specific tissue types in the valves and valve margins. Thus, the genes identified can be arranged in a functional hierarchy: specification of carpel identity, patterning of the carpel primordium and directing the differentiation of the specialized tissues of the carpel.

Shoot meristems: Intercellular signals keep the balance

Recent studies have identified a complex intercellular communication network which maintains the balance of indeterminate and determinate cells at the plant apical meristem. The widespread presence of homologous regulatory genes indicates that 'stemness' arose before the evolutionary split between plants and animals.

FILAMENTOUS FLOWER, a meristem and organ identity gene of Arabidopsis, encodes a protein with a zinc finger and HMG-related domains

Distinctive from that of the animal system, the basic plan of the plant body is the continuous formation of a structural unit, composed of a stem with a meristem at the top and lateral organs continuously forming at the meristem. Therefore, mechanisms controlling the formation, maintenance, and development of a meristem will be a key to understanding the body plan of higher plants. Genetic analyses of filamentous flower (fil) mutants have indicated that FIL is required for the maintenance and growth of inflorescence and floral meristems, and of floral organs of Arabidopsis thaliana. FIL encodes a protein carrying a zinc finger and a HMG box-like domain, which is known to work as a transcription regulator. As expected, the FIL protein was shown to have a nuclear location. In situ hybridization clearly demonstrated that FIL is expressed only at the abaxial side of primordia of leaves and floral organs. Transgenic plants, ectopically expressing FIL, formed filament-like leaves with randomly arranged cells at the leaf margin. Our results indicate that cells at the abaxial side of the lateral organs are responsible for the normal development of the organs as well as for maintaining the activity of meristems.

ROUGH SHEATH2: a Myb protein that represses knox homeobox genes in maize lateral organ primordia

The regulation of members of the knotted1-like homeobox (knox) gene family is required for the normal initiation and development of lateral organs. The maize rough sheath2 (rs2) gene, which encodes a Myb-domain protein, is expressed in lateral organ primordia and their initials. Mutations in the rs2 gene permit ectopic expression of knox genes in leaf and floral primordia, causing a variety of developmental defects. Ectopic KNOX protein accumulation in rs2 mutants occurs in a subset of the normal rs2-expressing cells. This variegated accumulation of KNOX proteins in rs2 mutants suggests that rs2 represses knox expression through epigenetic means.

The maize rough sheath2 gene and leaf development programs in monocot and dicot plants

Leaves of higher plants develop in a sequential manner from the shoot apical meristem. Previously it was determined that perturbed leaf development in maize rough sheath2 (rs2) mutant plants results from ectopic expression of knotted1-like (knox) homeobox genes. Here, the rs2 gene sequence was found to be similar to the Antirrhinum PHANTASTICA (PHAN) gene sequence, which encodes a Myb-like transcription factor. RS2 and PHAN are both required to prevent the accumulation of knox gene products in maize and Antirrhinum leaves, respectively. However, rs2 and phan mutant phenotypes differ, highlighting fundamental differences in monocot and dicot leaf development programs.

Axioms and axes in leaf formation?

Formation of leaves and floral organs involves down-regulation of meristem-specific homeobox genes, and de novo expression of genes for organ identity, growth and patterning. Genes required for all these aspects of organ formation have been identified. The challenge now is to establish how they interact to direct organogenesis.

Shoot meristem formation and maintenance

The shoot apical meristem of higher plants is a self-maintaining stem cell system which gives rise to the entire above-ground part of a plant. In the past year, genetic and molecular studies have provided increasing insight into the processes of shoot meristem formation and maintenance, as well as into the relation between the apical meristem and its products.

Cellular differentiation in the leaf

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

LAM1 is required for dorsoventrality and lateral growth of the leaf blade in Nicotiana

The role of LAM1 in dorsoventrality and lateral growth of the leaf blade was investigated in the 'bladeless' lam1 mutant of Nicotiana sylvestris and in periclinal chimeras with lam1 and wild-type (N. glauca) cell layers. Mutant lam1 primordia show normal dorsoventrality at emergence, but produce blade tissue that lacks dorsal cell types and fails to expand in the lateral plane. In leaves of a lam1-glauca-glauca (L1-L2-L3) chimera, we observed restoration of dorsal identity in the lam1 upper epidermis, suggesting non-cell-autonomous movement of a dorsalizing factor between cell layers of the blade. A lam1-lam1-glauca chimera generated a leaf blade with lam1 cells in the L1-derived epidermis and the L2-derived upper and lower mesophyll. An in situ lineage analysis revealed that N. glauca cells in the L3-derived middle mesophyll restore palisade differentiation in the adjoining lam1 upper mesophyll. Movement of dorsalizing information appears short-range, however, having no effect on the upper lam1 epidermis in lam1-lam1-glauca. Clusters of lam1 mesophyll in distal or proximal positions show a localized default to radial growth, indicating that the LAM1 function is required for dorsoventrality and lateral growth throughout blade expansion.

Plant development: the making of a leaf

A transcription factor of the Myb family has been found to couple dorsoventral patterning and proximodistal outgrowth during leaf development.