The Lateral Root Cap Acts as an Auxin Sink that Controls Meristem Size.

Plant developmental plasticity relies on the activities of meristems, regions where stem cells continuously produce new cells [1]. The lateral root cap (LRC) is the outermost tissue of the root meristem [1], and it is known to play an important role during root development [2-6]. In particular, it has been shown that mechanical or genetic ablation of LRC cells affect meristem size [7, 8]; however, the molecular mechanisms involved are unknown.

RETINOBLASTOMA RELATED directly regulates DNA damage responses through functions beyond cell cycle control.

The rapidly proliferating cells in plant meristems must be protected from genome damage. Here, we show that the regulatory role of the RETINOBLASTOMA RELATED (RBR) in cell proliferation can be separated from a novel function in safeguarding genome integrity. Upon DNA damage, RBR and its binding partner E2FA are recruited to heterochromatic γH2AX-labelled DNA damage foci in an ATM- and ATR-dependent manner.

RETINOBLASTOMA-RELATED protein stimulates cell differentiation in the Arabidopsis root meristem by interacting with cytokinin signaling.

Maintenance of mitotic cell clusters such as meristematic cells depends on their capacity to maintain the balance between cell division and cell differentiation necessary to control organ growth. In the Arabidopsis thaliana root meristem, the antagonistic interaction of two hormones, auxin and cytokinin, regulates this balance by positioning the transition zone, where mitotically active cells lose their capacity to divide and initiate their differentiation programs. In animals, a major regulator of both cell division and cell differentiation is the tumor suppressor protein RETINOBLASTOMA.

Spatial coordination between stem cell activity and cell differentiation in the root meristem.

A critical issue in development is the coordination of the activity of stem cell niches with differentiation of their progeny to ensure coherent organ growth. In the plant root, these processes take place at opposite ends of the meristem and must be coordinated with each other at a distance. Here, we show that in Arabidopsis, the gene SCR presides over this spatial coordination.

Growth and development of the root apical meristem.

A key question in plant developmental biology is how cell division and cell differentiation are balanced to modulate organ growth and shape organ size. In recent years, several advances have been made in understanding how this balance is achieved during root development. In the Arabidopsis root meristem, stem cells in the apical region of the meristem self-renew and produce daughter cells that differentiate in the distal meristem transition zone.

Analysis of root meristem size development.

Plant post-embryonic development takes place in the meristems. In the root of the model plant Arabidopsis thaliana, stem cells organized in a stem-cell niche in the apex of the root meristem generate transit-amplifying cells, which undergo additional division in the proximal meristem and differentiate in the elongation/differentiation zone. For meristem maintenance, and therefore continuous root growth, the rate of cell differentiation must equal the rate of generation of new cells: how this balance is achieved is a central question in plant development.

The rate of cell differentiation controls the Arabidopsis root meristem growth phase.

Upon seed germination, apical meristems grow as cell division prevails over differentiation and reach their final size when division and differentiation reach a balance. In the Arabidopsis root meristem, this balance results from the interaction between cytokinin (promoting differentiation) and auxin (promoting division) through a regulatory circuit whereby the ARR1 cytokinin-responsive transcription factor activates the gene SHY2, which negatively regulates the PIN genes encoding auxin transport facilitators. However, it remains unknown how the final meristem size is set, i.

The molecular basis of cytokinin function.

Cytokinins are a class of phytohormones that regulate a wide variety of physiological and developmental processes such as shoot and root growth. Cytokinin signaling relies on a phosphorelay mechanism similar to the prokaryotic two-component system. Although the principal components mediating this cascade have been identified, only recently have we begun to understand the molecular basis of cytokinin action.

A genetic framework for the control of cell division and differentiation in the root meristem.

Plant growth and development are sustained by meristems. Meristem activity is controlled by auxin and cytokinin, two hormones whose interactions in determining a specific developmental output are still poorly understood. By means of a comprehensive genetic and molecular analysis in Arabidopsis, we show that a primary cytokinin-response transcription factor, ARR1, activates the gene SHY2/IAA3 (SHY2), a repressor of auxin signaling that negatively regulates the PIN auxin transport facilitator genes: thereby, cytokinin causes auxin redistribution, prompting cell differentiation.