The Ins and Outs of Homeodomain-Leucine Zipper/Hormone Networks in the Regulation of Plant Development.

The generation of complex plant architectures depends on the interactions among different molecular regulatory networks that control the growth of cells within tissues, ultimately shaping the final morphological features of each structure. The regulatory networks underlying tissue growth and overall plant shapes are composed of intricate webs of transcriptional regulators which synergize or compete to regulate the expression of downstream targets. Transcriptional regulation is intimately linked to phytohormone networks as transcription factors (TFs) might act as effectors or regulators of hormone signaling pathways, further enhancing the capacity and flexibility of molecular networks in shaping plant architectures.

HD-Zip II transcription factors control distal stem cell fate in Arabidopsis roots by linking auxin signaling to the FEZ/SOMBRERO pathway.

In multicellular organisms, specialized tissues are generated by specific populations of stem cells through cycles of asymmetric cell divisions, where one daughter undergoes differentiation and the other maintains proliferative properties. In Arabidopsis thaliana roots, the columella - a gravity-sensing tissue that protects and defines the position of the stem cell niche - represents a typical example of a tissue whose organization is exclusively determined by the balance between proliferation and differentiation. The columella derives from a single layer of stem cells through a binary cell fate switch that is precisely controlled by multiple, independent regulatory inputs.

Transcriptional induction of cell wall remodelling genes is coupled to microtubule-driven growth isotropy at the shoot apex in .

The shoot apical meristem of higher plants continuously generates new tissues and organs through complex changes in growth rates and directions of its individual cells. Cell growth, which is driven by turgor pressure, largely depends on the cell walls, which allow cell expansion through synthesis and structural changes. A previous study revealed a major contribution of wall isotropy in organ emergence, through the disorganization of cortical microtubules.

Flower development: from morphodynamics to morphomechanics.

The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. We will discuss here flower formation at the SAM, which involves a complex network of regulatory genes and signalling molecules. A major downstream target of this network is the extracellular matrix or cell wall, which is a local determinant for both growth rates and growth directions.

Mechanical stress contributes to the expression of the STM homeobox gene in Arabidopsis shoot meristems.

The role of mechanical signals in cell identity determination remains poorly explored in tissues. Furthermore, because mechanical stress is widespread, mechanical signals are difficult to uncouple from biochemical-based transduction pathways. Here we focus on the homeobox gene SHOOT MERISTEMLESS (STM), a master regulator and marker of meristematic identity in Arabidopsis.

When biochemistry meets mechanics: a systems view of growth control in plants.

The emergence of complex shapes during the development of plants is under the control of genetically determined molecular networks. Such regulatory networks, comprising hormones and transcription factors, regulate the collective behavior of cell growth within a tissue. Because all the cells within a tissue are linked together by the cell wall, their collective growth generates a good amount of mechanical stress.

New insights in shoot apical meristem morphogenesis: Isotropy comes into play.

The great complexity and plasticity of aerial plant shapes largely results from the activity of the shoot apical meristem (SAM), a group of undifferentiated cells which produces all the aboveground organs of the plant. Organogenesis at the SAM is regulated by the hormone auxin, which, through an integration of active transport, signalling and transcriptional regulation, determines the positional and temporal information dictating where, when, and how a new organ will be formed. At the cellular level, the information stemming from the regulatory molecular networks influences the growth of the cells within the tissue to give rise to the final organ shape.

The Arabidopsis RNA-binding protein AtRGGA regulates tolerance to salt and drought stress.

Salt and drought stress severely reduce plant growth and crop productivity worldwide. The identification of genes underlying stress response and tolerance is the subject of intense research in plant biology. Through microarray analyses, we previously identified in potato (Solanum tuberosum) StRGGA, coding for an Arginine Glycine Glycine (RGG) box-containing RNA-binding protein, whose expression was specifically induced in potato cell cultures gradually exposed to osmotic stress.

An auxin-mediated shift toward growth isotropy promotes organ formation at the shoot meristem in Arabidopsis.

To control morphogenesis, molecular regulatory networks have to interfere with the mechanical properties of the individual cells of developing organs and tissues, but how this is achieved is not well known. We study this issue here in the shoot meristem of higher plants, a group of undifferentiated cells where complex changes in growth rates and directions lead to the continuous formation of new organs. Here, we show that the plant hormone auxin plays an important role in this process via a dual, local effect on the extracellular matrix, the cell wall, which determines cell shape.

Plant development: from biochemistry to biophysics and back.

Since plant cells cannot move relative to each other, plant organogenesis mainly depends on the strict coordination of cell growth and proliferation. Recent work suggests that this implies a subtle combination of biochemical and physical interactions between neighboring cells.

Dynamics of the shade-avoidance response in Arabidopsis.

Shade-intolerant plants perceive the reduction in the ratio of red light (R) to far-red light (FR) as a warning of competition with neighboring vegetation and display a suite of developmental responses known as shade avoidance. In recent years, major progress has been made in understanding the molecular mechanisms underlying shade avoidance. Despite this, little is known about the dynamics of this response and the cascade of molecular events leading to plant adaptation to a low-R/FR environment.

Auxin and self-organization at the shoot apical meristem.

Plants continuously generate new tissues and organs throughout their life cycle, due to the activity of populations of specialized tissues containing stem cells called meristems. The shoot apical meristem (SAM) generates all the aboveground organs of the plant, including leaves and flowers, and plays a key role in plant survival and reproduction. Organ production at the SAM occurs following precise spatio-temporal patterns known as phyllotaxis.

Arabidopsis HD-Zip II transcription factors control apical embryo development and meristem function.

The Arabidopsis genome encodes ten Homeodomain-Leucine zipper (HD-Zip) II proteins. ARABIDOPSIS THALIANA HOMEOBOX 2 (ATHB2), HOMEOBOX ARABIDOPSIS THALIANA 1 (HAT1), HAT2, HAT3 and ATHB4 are regulated by changes in the red/far red light ratio that induce shade avoidance in most of the angiosperms. Here, we show that progressive loss of HAT3, ATHB4 and ATHB2 activity causes developmental defects from embryogenesis onwards in white light.

Shedding light on auxin movement: light-regulation of polar auxin transport in the photocontrol of plant development.

By being sessile, plants have evolved a remarkable capacity to perceive and respond to changes in environmental conditions throughout their life cycle. Light represents probably the most important environmental factor that impinge on plant development because, other than supplying the energy source for photosynthesis, it also provides seasonal and positional information that are essential for the plant survival and fitness. Changes in the light environment can dramatically alter plant morphogenesis, especially during the early phases of plant life, and a compelling amount of evidence indicates that light-mediated changes in auxin homeostasis are central in these processes.

COP1 mediates the coordination of root and shoot growth by light through modulation of PIN1- and PIN2-dependent auxin transport in Arabidopsis.

When a plant germinates in the soil, elongation of stem-like organs is enhanced whereas leaf and root growth is inhibited. How these differential growth responses are orchestrated by light and integrated at the organismal level to shape the plant remains to be elucidated. Here, we show that light signals through the master photomorphogenesis repressor COP1 to coordinate root and shoot growth in Arabidopsis.

A novel regulatory circuit underlying plant response to canopy shade.

A plant growing in the field has the unique ability to sense the presence of other plants growing near by and adjust its growth rate accordingly. This ability to detect neighbors, which is referred to as shade avoidance response, is mediated by members of the phytochrome family which detect light in the red (R) and far-red (FR) region of the spectrum. Work done by several laboratories has shown that low R/FR provides the signal for shade avoidance response during which the elongation of stem-like organs occurs at the expense of leaf development.

Canopy shade causes a rapid and transient arrest in leaf development through auxin-induced cytokinin oxidase activity.

A plant grown under canopies perceives the reduction in the ratio of red (R) to far-red (FR) light as a warning of competition, and enhances elongation growth in an attempt to overgrow its neighbors. Here, we report that the same low R/FR signal that induces hypocotyl elongation also triggers a rapid arrest of leaf primordium growth, ensuring that plant resources are redirected into extension growth. The growth arrest induced by low R/FR depends on auxin-induced cytokinin breakdown in incipient vein cells of developing primordia, thus demonstrating the existence of a previously unrecognized regulatory circuit underlying plant response to canopy shade.

A dynamic balance between gene activation and repression regulates the shade avoidance response in Arabidopsis.

Plants grown under dense canopies perceive through the phytochrome system a reduction in the ratio of red to far-red light as a warning of competition, and this triggers a series of morphological changes to avoid shade. Several phytochrome signaling intermediates acting as positive regulators of accelerated elongation growth and induction of flowering in shade avoidance have been identified. Here we report that a negative regulatory mechanism ensures that in the presence of far-red-rich light an exaggerated plant response does not occur.