Law and order in plants - the origin and functional relevance of phyllotaxis.

The regular arrangement of organs (phyllotaxis) in vegetative shoots and flowers is one of the most stunning features of plants. Spiral patterns characterized by Fibonacci numbers have attracted the particular interest of natural scientists and mathematicians. Numerous reviews have dealt with the molecular genetic mechanisms underlying phyllotaxis, and modeling studies have sought to recreate phyllotaxis according to mathematical, biochemical, or physical laws. However, what is the functional significance of regular plant architecture, and how did it evolve? We discuss the developmental constraints and selective forces that may have favored the selection of phyllotaxis, and we argue that a central driver of regular phyllotaxis may have been limitations in the allocation of founder cells and metabolic resources to the different tissues in the shoot apex.

Fusarium oxysporum infection activates the plastidial branch of the terpenoid biosynthesis pathway in flax, leading to increased ABA synthesis.

MAIN CONCLUSION: Upregulation of the terpenoid pathway and increased ABA content in flax upon Fusarium infection leads to activation of the early plant's response (PR genes, cell wall remodeling, and redox status). Plants have developed a number of defense strategies against the adverse effects of fungi such as Fusarium oxysporum. One such defense is the production of antioxidant secondary metabolites, which fall into two main groups: the phenylpropanoids and the terpenoids. While functions and biosynthesis of phenylpropanoids have been extensively studied, very little is known about the genes controlling the terpenoid synthesis pathway in flax. They can serve as antioxidants, but are also substrates for a plethora of different compounds, including those of regulatory functions, like ABA. ABA's function during pathogen attack remains obscure and often depends on the specific plant-pathogen interactions. In our study we showed that in flax the non-mevalonate pathway is strongly activated in the early hours of pathogen infection and that there is a redirection of metabolites towards ABA synthesis. The elevated synthesis of ABA correlates with flax resistance to F. oxysporum, thus we suggest ABA to be a positive regulator of the plant's early response to the infection.

Meristem development and activity in gametophytes of the model fern, Ceratopteris richardii.

Ceratopteris richardii is a model fern species widely used to analyze various developmental processes and their regulation in gametophytes. The form of mature C. richardii gametophytes depends on the activity of the marginal meristem, but knowledge on meristem formation and structure is limited. Therefore, we analyzed cellular events accompanying the development of gametophytes using cell lineage and proliferation analyses to explain the establishment and functioning of the marginal meristem. We show that: i) gametophytes are devoid of the apical initial cell or the apical cell-based meristem in the early developmental stages; ii) the cells that are predestined to form the marginal meristem divide according to a stable pattern; iii) only one transient initial cell is present in the marginal meristem, and the selection of a new functioning initial cell is related to a stable sequence of its divisions. Our results contribute to a better understanding of the developmental events underlying gametophyte growth and marginal meristem functioning in Ceratopteris. The principles, which were established in this study and enabled the identification of functioning initial cells, can be applied to analyze genetic and/or physiological mechanism(s) governing meristem maintenance in vascular plants, both in developmental and evolutionary contexts.

Impairment of Meristem Proliferation in Plants Lacking the Mitochondrial Protease AtFTSH4.

Shoot and root apical meristems (SAM and RAM, respectively) are crucial to provide cells for growth and organogenesis and therefore need to be maintained throughout the life of a plant. However, plants lacking the mitochondrial protease AtFTSH4 exhibit an intriguing phenotype of precocious cessation of growth at both the shoot and root apices when grown at elevated temperatures. This is due to the accumulation of internal oxidative stress and progressive mitochondria dysfunction. To explore the impacts of the internal oxidative stress on SAM and RAM functioning, we study the expression of selected meristem-specific (STM, CLV3, WOX5) and cell cycle-related (e.g., CYCB1, CYCD3;1) genes at the level of the promoter activity and/or transcript abundance in wild-type and loss-of-function ftsh4-1 mutant plants grown at 30 °C. In addition, we monitor cell cycle progression directly in apical meristems and analyze the responsiveness of SAM and RAM to plant hormones. We show that growth arrest in the ftsh4-1 mutant is caused by cell cycle dysregulation in addition to the loss of stem cell identity. Both the SAM and RAM gradually lose their proliferative activity, but with different timing relative to CYCB1 transcriptional activity (a marker of G2-M transition), which cannot be compensated by exogenous hormones.

The mitochondrial protease AtFTSH4 safeguards Arabidopsis shoot apical meristem function.

The shoot apical meristem (SAM) ensures continuous plant growth and organogenesis. In LD 30 °C, plants lacking AtFTSH4, an ATP-dependent mitochondrial protease that counteracts accumulation of internal oxidative stress, exhibit a puzzling phenotype of premature SAM termination. We aimed to elucidate the underlying cellular and molecular processes that link AtFTSH4 with SAM arrest. We studied AtFTSH4 expression, internal oxidative stress accumulation, and SAM morphology. Directly in the SAM we analysed H2O2 accumulation, mitochondria behaviour, and identity of stem cells using WUS/CLV3 expression. AtFTSH4 was expressed in proliferating tissues, particularly during the reproductive phase. In the mutant, SAM, in which internal oxidative stress accumulates predominantly at 30 °C, lost its meristematic fate. This process was progressive and stage-specific. Premature meristem termination was associated with an expansion in SAM area, where mitochondria lost their functionality. All these effects destabilised the identity of the stem cells. SAM termination in ftsh4 mutants is caused both by internal oxidative stress accumulation with time/age and by the tissue-specific role of AtFTSH4 around the flowering transition. Maintaining mitochondria functionality within the SAM, dependent on AtFTSH4, is vital to preserving stem cell activity throughout development.

Dichotomous branching: the plant form and integrity upon the apical meristem bifurcation.

The division of the apical meristem into two independently functioning axes is defined as dichotomous branching. This type of branching typically occurs in non-vascular and non-seed vascular plants, whereas in seed plants it presents a primary growth form only in several taxa. Dichotomy is a complex process, which requires a re-organization of the meristem structure and causes changes in the apex geometry and activity. However, the mechanisms governing the repetitive apex divisions are hardly known. Here, an overview of dichotomous branching is presented, occurring in structurally different apices of phylogenetically distant plants, and in various organs (e.g., shoots, roots, rhizophores). Additionally, morphogenetic effects of dichotomy are reviewed, including its impact on organogenesis and mechanical constraints. At the end, the hormonal and genetic regulation of the dichotomous branching is discussed.

Changes in accumulation of heteroplasmic mitochondrial DNA and frequency of recombination via short repeats during plant lifetime in Phaseolus vulgaris.

Recombination via short repeats in plant mitochondrial genomes results in sublimons--DNA molecules with a copy number much lower compared to the main mitochondrial genome. Coexistence of stoichiometrically different mitotypes, called heteroplasmy, plays an important evolutionary role, since sublimons occasionally replace the main genome resulting in a new plant phenotype. It is not clear, how frequency of recombination and sublimon production is regulated and how it is related to changes in the quantity of the main genome and sublimons. We analyzed the accumulation of two recombining main genome sequences and two resulting sublimons in apical meristems, undifferentiated tissues and leaves of different age of Phaseolus vulgaris. Copy numbers of the main genome sequences varied greatly depending on tissue type and organ age while accumulation of sublimons remained much more stable. Although the overall accumulation of plant mtDNA decreased with the leaf age, the quantity of sublimons increased relative to the main genome indicating a higher frequency of recombination via the short 314 bp repeat. Recombination was symmetrical in young developing leaves while in senescent tissues it shifted towards asymmetric events resulting in overrepresentation of one product. We propose that during plant lifetime replication and recombination frequencies change oppositely sustaining heteroplasmic compositions of the genome, which are favorable for inheritance and maintenance of complex plant mtDNA.

The lack of mitochondrial AtFtsH4 protease alters Arabidopsis leaf morphology at the late stage of rosette development under short-day photoperiod.

AtFtsH4 is one of four inner membrane-bound mitochondrial ATP-dependent metalloproteases in Arabidopsis thaliana, called AAA proteases, whose catalytic site is exposed to the intermembrane space. In the present study, we used a reverse-genetic approach to investigate the physiological role of the AtFtsH4 protease. We found that loss of AtFtsH4 did not significantly affect Arabidopsis growth under optimal conditions (long days); however, severe morphological and developmental abnormalities in late rosette development occurred under short-day conditions. The asymmetric shape and irregular serration of expanding leaf blades were the most striking features of the ftsh4 mutant phenotype. The severe abnormal morphology of the leaf blades was accompanied by ultrastructural changes in mitochondria and chloroplasts. These abnormalities correlated with elevated levels of reactive oxygen species and carbonylated mitochondrial proteins. We found that two classes of molecular chaperones, Hsp70 and prohibitins, were over-expressed in ftsh4 mutants during late vegetative growth under both short- and long-day conditions. Taken together, our data indicate that lack of AtFtsH4 results in impairment of organelle development and Arabidopsis leaf morphology under short-day conditions.

Vascular architecture in shoots of early divergent vascular plants, Lycopodium clavatum and Lycopodium annotinum.

Lycopodium represents a phylogenetically distinct clade of basal vascular plants with anatomical characters that have no parallel in other lineages. Thus, knowledge of lycopod structure and development may reveal important information about the common ancestors of all vascular plants. Here we report the unique architecture of the conducting system in Lycopodium annotinum and Lycopodium clavatum. Based on multiple series of anatomical sections, we reconstructed spatial relationships between microphylls and the stelar system. Analysis revealed that protoxylem ribs (PXR) were vertical, regardless of type of phyllotaxis, and their numbers were variable. Microphyll traces (MTr) were randomly distributed between ribs, resulting in the absence of defined sympodia and varied lengths of MTr. Dichotomous branching contributed to additional features, for example occurrence of mesarch protoxylem, affecting stele structure and PXR numbers. Our data showed limited interrelationships between lycopod vasculature and microphyll phyllotaxis. This may suggest that both systems developed independently, then evolved together to form the integrated supply system. Thus vasculature in extant lycophytes may be less functionally efficient than in seed plants, where consistent leaf-trace lengths guarantee predictable energy utilization during ontogeny. Differences may result from the phylogenetically different origin of microphylls, and the level of vascular complexity.