Mechanics of the Meristems

Mechanics of reproductive differentiation in the land plants: a paradigm shift?

This article addresses the physical mechanics of gametogenesis in vascular plants. The earliest events that initiate reproductive differentiation in the land plants are not well understood. How are the few cells that initiate reproductive differentiation specified and how is that information translated into action at the cellular level? In this article I propose a physical mechanism that resolves the problem of spatial targeting without invoking dependence on diffusible morphogens or other external signals. I suggest that the initiation of archesporial differentiation can instead be attributed to the confluence of organ geometry, surficial topography, and the physical mechanics of sporangial growth, resulting in the spontaneous emergence of an isotropic singularity that locates and precipitates archesporial differentiation. In discussing the logic of single-cell target selection and the limits of stochastic molecular signaling I propose that the sporangium would be better understood as a pressurized stress-mechanical lens that focuses turgor-generated growth forces on a central location, generating a physical singularity that locates and specifies the cell or cells that become the archesporium and initiates their transition from somatic proliferation to reproductive differentiation.

How Do Trees Grow in Girth? Controversy on the Role of Cellular Events in the Vascular Cambium

Radial growth has long been a subject of interest in tree biology research. Recent studies have brought a significant change in the understanding of some basic processes characteristic to the vascular cambium, a meristem that produces secondary vascular tissues (phloem and xylem) in woody plants. A new hypothesis regarding the mechanism of intrusive growth of the cambial initials, which has been ratified by studies of the arrangement of cambial cells, negates the influence of this apical cell growth on the expansion of the cambial circumference. Instead, it suggests that the tip of the elongating cambial initial intrudes between the tangential (periclinal) walls, rather than the radial (anticlinal) walls, of the initial(s) and its(their) derivative(s) lying ahead of the elongating cell tip. The new concept also explains the hitherto obscure mechanism of the cell event called 'elimination of initials'. This article evaluates these new concepts of the cambial cell dynamics and offers a new interpretation for some curious events occurring in the cambial meristem in relation to the radial growth in woody plants.

What do we know about growth of vessel elements of secondary xylem in woody plants?

Despite extensive knowledge about vessel element growth and the determination of the axial course of vessels, these processes are still not fully understood. They are usually explained as resulting primarily from hormonal regulation in stems. This review focuses on an increasingly discussed aspect - mechanical conditions in the vascular cambium. Mechanical conditions in cambial tissue are important for the growth of vessel elements, as well as other cambial derivatives. In relation to the type of stress acting on cambial cells (compressive versus tensile stress) we: (i) discuss the shape of the enlarging vessel elements observed in anatomical sections; (ii) present hypotheses regarding the location of intrusive growth of vessel elements and cambial initials; (iii) explain the relationship between the growth of vessel elements and fibres; and (iv) consider the effect of mechanical stress in determining the course of a vessel. We also highlight the relationship between mechanical stress and transport of the most extensively studied plant hormone - auxin. We conclude that the integration of a biomechanical factor with the commonly acknowledged hormonal regulation could significantly enhance the analysis of the formation of vessel elements as well as entire vessels, which transport water and minerals in numerous plant species.

Disentangling phylogenetic and functional components of shape variation among shoot apical meristems of a wide range of herbaceous angiosperms

PREMISE

The shoot apical meristem (SAM) is the basic determinant of plant body organization, but interspecific variation in SAM shape and its relationship to stem and leaf morphological traits is not well known. Here we tested the hypothesis that different SAM shapes are associated with specific shoot traits of the plant body and examined the phylogenetic conservatism of these relationships.

METHODS

We used geometric morphometrics of SAM outlines for a phylogenetically representative set of 110 herbaceous angiosperms and examined their relationship to a number of shoot traits.

RESULTS

We found large variations in SAM shapes across angiosperm lineages, but covering only a subset of geometrically possible shapes. Part of this variation was allometric (due to SAM size), but the dominant shape variation (dome-shaped vs. flat surface) was size-independent and strongly phylogenetically conserved. SAM shapes were largely independent of their cell size and therefore of the number of cells involved. Different patterns in shape variation of outer and inner SAM boundaries were associated with stem thickness, leaf area, and leafiness of the stem.

CONCLUSIONS

The findings show that geometric interdependence of meristem zones gives rise to correlations among organ numbers, sizes, and their proportions. Phylogenetic conservatism in these correlations indicates conservatism in regulatory processes that underlie the correlations, or the individual traits, that give rise to plant architecture.

COMPARISON OF EMPIRICAL RULES DESCRIBING CELL PLATE FORMATION IN 2D COMPUTER SIMULATIONS OF APICAL MERISTEM IN PLANTS

There are two families of lines describing the cell wall pattern in root and shoot apices; periclines and anticlines. The lines of these two families are mutually orthogonal and steady during apex growth. They approximate orientation of cell walls in the apices. These lines are preserved in the growing organ. The direction of periclines and anticlines is regulated at the organ level. This paper focuses on the question: how is the pattern of periclines and anticlines maintained? There are a number of rules, which deal with the problem of the orientation of a new cell wall. We test three of them: Errera rule (the smallest possible area of a new cell wall that divides the mother cell into equal portions is chosen), Sachs rule (the new wall is perpendicular to the nearest wall from the geometric center where it is inserted) and Hejnowicz postulate (the new cell wall is perpendicular to one of the principal directions of growth rate). We tested these rules in the computer simulation of the organ growth and cell divisions.

Form matters: morphological aspects of lateral root development

BACKGROUND

The crucial role of roots in plant nutrition, and consequently in plant productivity, is a strong motivation to study the growth and functioning of various aspects of the root system. Numerous studies on lateral roots, as a major determinant of the root system architecture, mostly focus on the physiological and molecular bases of developmental processes. Unfortunately, little attention is paid either to the morphological changes accompanying the formation of a lateral root or to morphological defects occurring in lateral root primordia. The latter are observed in some mutants and occasionally in wild-type plants, but may also result from application of external factors.

SCOPE AND CONCLUSIONS

In this review various morphological aspects of lateral branching in roots are analysed. Morphological events occurring during the formation of a typical lateral root are described. This process involves dramatic changes in the geometry of the developing organ that at early stages are associated with oblique cell divisions, leading to breaking of the symmetry of the cell pattern. Several types of defects in the morphology of primordia are indicated and described. Computer simulations show that some of these defects may result from an unstable field of growth rates. Significant changes in both primary and lateral root morphology may also be a consequence of various mutations, some of which are auxin-related. Examples reported in the literature are considered. Finally, lateral root formation is discussed in terms of mechanics. In this approach the primordium is considered as a physical object undergoing deformation and is characterized by specific mechanical properties.

Mechanical control of morphogenesis at the shoot apex

Morphogenesis does not just require the correct expression of patterning genes; these genes must induce the precise mechanical changes necessary to produce a new form. Mechanical characterization of plant growth is not new; however, in recent years, new technologies and interdisciplinary collaborations have made it feasible in young tissues such as the shoot apex. Analysis of tissues where active growth and developmental patterning are taking place has revealed biologically significant variability in mechanical properties and has even suggested that mechanical changes in the tissue can feed back to direct morphogenesis. Here, an overview is given of the current understanding of the mechanical dynamics and its influence on cellular and developmental processes in the shoot apex. We are only starting to uncover the mechanical basis of morphogenesis, and many exciting questions remain to be answered.