Establishment of the embryonic shoot apical meristem in Arabidopsis thaliana

Establishment of the Embryonic Shoot Meristem Involves Activation of Two Classes of Genes with Opposing Functions for Meristem Activities

The shoot meristem, a stem-cell-containing tissue initiated during plant embryogenesis, is responsible for continuous shoot organ production in postembryonic development. Although key regulatory factors including KNOX genes are responsible for stem cell maintenance in the shoot meristem, how the onset of such factors is regulated during embryogenesis is elusive. Here, we present evidence that the two KNOX genes STM and KNAT6 together with the two other regulatory genes BLR and LAS are functionally important downstream genes of CUC1 and CUC2, which are a redundant pair of genes that specify the embryonic shoot organ boundary. Combined expression of STM with any of KNAT6, BLR, and LAS can efficiently rescue the defects of shoot meristem formation and/or separation of cotyledons in cuc1cuc2 double mutants. In addition, CUC1 and CUC2 are also required for the activation of KLU, a cytochrome P450-encoding gene known to restrict organ production, and KLU counteracts STM in the promotion of meristem activity, providing a possible balancing mechanism for shoot meristem maintenance. Together, these results establish the roles for CUC1 and CUC2 in coordinating the activation of two classes of genes with opposite effects on shoot meristem activity.

Evolution of unusual morphologies in Lentibulariaceae (bladderworts and allies) and Podostemaceae (river-weeds): a pictorial report at the interface of developmental biology and morphological diversification

BACKGROUND

Various groups of flowering plants reveal profound ('saltational') changes of their bauplans (architectural rules) as compared with related taxa. These plants are known as morphological misfits that appear as rather large morphological deviations from the norm. Some of them emerged as morphological key innovations (perhaps 'hopeful monsters') that gave rise to new evolutionary lines of organisms, based on (major) genetic changes.

SCOPE

This pictorial report places emphasis on released bauplans as typical for bladderworts (Utricularia, approx. 230 secies, Lentibulariaceae) and river-weeds (Podostemaceae, three subfamilies, approx. 54 genera, approx. 310 species). Bladderworts (Utricularia) are carnivorous, possessing sucking traps. They live as submerged aquatics (except for their flowers), as humid terrestrials or as epiphytes. Most Podostemaceae are restricted to rocks in tropical river-rapids and waterfalls. They survive as submerged haptophytes in these extreme habitats during the rainy season, emerging with their flowers afterwards. The recent scientific progress in developmental biology and evolutionary history of both Lentibulariaceae and Podostemaceae is summarized.

CONCLUSIONS

Lentibulariaceae and Podostemaceae follow structural rules that are different from but related to those of more typical flowering plants. The roots, stems and leaves - as still distinguishable in related flowering plants - are blurred ('fuzzy'). However, both families have stable floral bauplans. The developmental switches to unusual vegetative morphologies facilitated rather than prevented the evolution of species diversity in both families. The lack of one-to-one correspondence between structural categories and gene expression may have arisen from the re-use of existing genetic resources in novel contexts. Understanding what developmental patterns are followed in Lentibulariaceae and Podostemaceae is a necessary prerequisite to discover the genetic alterations that led to the evolution of these atypical plants. Future molecular genetic work on morphological misfits such as bladderworts and river-weeds will provide insight into developmental and evolutionary aspects of more typical vascular plants.

BLADE-ON-PETIOLE genes: setting boundaries in development and defense

BLADE-ON-PETIOLE (BOP) genes encode an ancient and conserved subclade of BTB-ankryin transcriptional co-activators, divergent in the NPR1 family of plant defense regulators. Arabidopsis BOP1/2 were originally characterized as regulators of leaf and floral patterning. Recent investigation of BOP activity in a variety of land plants provides a more complete picture of their conserved functions at lateral organ boundaries in the determination of leaf, flower, inflorescence, and root nodule architecture. BOPs exert their function in part through promotion of lateral organ boundary genes including ASYMMETRIC LEAVES2, KNOTTED1-LIKE FROM ARABIDOPSIS6, and ARABIDOPSIS THALIANA HOMEOBOX GENE1 whose products restrict growth, promote differentiation, and antagonize meristem activity in various developmental contexts. Mutually antagonistic interactions between BOP and meristem factors are important in maintaining a border between meristem-organ compartments and in controlling irreversible transitions in cell fate associated with differentiation. We also examine intriguing new evidence for BOP function in plant defense. Comparisons to NPR1 highlight previously unexplored mechanisms for co-ordination of development and defense in land plants.

Axis formation in Arabidopsis - transcription factors tell their side of the story

Apical-to-basal auxin flux is a defining feature of land plants and determines their main body axis. How is the axis first set up in the embryo? Recent studies reveal that the establishment of embryonic polarity with the asymmetric first division as well as the separation of shoot and root fates within the proembryo depend on transcriptional regulation in the zygote and early embryo. Although the functional connections need to be better defined, this transcriptional network likely provides the positional information required for initiating the machinery capable of processing the systemic signal auxin in a context-dependent manner.

CUP-SHAPED COTYLEDON1 transcription factor activates the expression of LSH4 and LSH3, two members of the ALOG gene family, in shoot organ boundary cells

The establishment of organ boundaries is a fundamental process for proper morphogenesis in multicellular organisms. In plants, the shoot meristem repetitively forms organ primordia from its periphery, and boundary cells are generated between them to separate their cellular fates. The genes CUP-SHAPED COTYLEDON1 (CUC1) and CUC2, which encode plant-specific NAC transcription factors, play central roles in establishment of the shoot organ boundaries in Arabidopsis thaliana. Here we show that CUC1 protein activates expression of LIGHT-DEPENDENT SHORT HYPOCOTYLS 4 (LSH4) and its homolog LSH3 in shoot organ boundary cells. Both genes encode nuclear proteins of the Arabidopsis LSH1 and Oryza G1 (ALOG) family, the members of which are widely conserved in land plants. Expression of LSH4 and LSH3 is detected in the boundary cells of various shoot organs, such as cotyledons, leaves and floral organs, and requires the activity of CUC1 and CUC2. Experiments using the glucocorticoid receptor system indicate that transcription of LSH4 and LSH3 is directly up-regulated by CUC1. Constitutive expression of LSH4 in the shoot apex causes inhibition of leaf growth in the vegetative phase, and formation of extra shoots or shoot organs within a flower in the reproductive phase. Together, our results indicate that CUC1 directly activates transcription of the nuclear factor genes LSH4 and LSH3, which may suppress organ differentiation in the boundary region.