Description

The turgor-driven enlargement of cells, which is controlled by phytohormones like auxin, has been linked to the growth of hypocotyls and epicotyls. However, auxin-mediated segment elongation ceases after one day, whereas hypocotyl growth continues in the intact system, according to the experiments presented here and elsewhere using developing sunflower (Helianthus annuus L.) seedlings raised either in darkness (skotomorphogenesis) or in white light (photomorphogenesis). We propose that hypocotyl growth is comprised of three interrelated processes on the basis of these findings and data from the literature: 1) division of cells in the meristematic apical regions; 2) cell elongation along the stem caused by turgor; and thirdly, the maturation of cells in the organ's basal region. We show that the location where cell division takes place is the closed apical hook-or the corresponding region after opening in WL-and that the epidermis and outer cortex play a significant role in a "pacemaker system" for cell division. The hypothesis that pectin metabolism is involved in wall-loosening and-stiffening in the expansion-limiting epidermal cell walls is supported by the literature. In H. annuus, there is no hydrostatic pressure-regulated growth because turgor pressure is largely maintained during hypocotyl growth in darkness and WL. The "loss of stability theory" of cell expansion is not supported by these data. In conclusion, we demonstrate that sucrose catabolism via vacuolar acid invertases, which results in the production of hexoses, is responsible for maintaining turgor during organ elongation.

Cell Division Activity

Turgor-driven cell elongation has been attributed to the growth of axial organs like hypocotyls. Hypocotyl growth is correlated with cell enlargement and cell division activity in sunflower seedlings. Contrary to the "loss of stability theory," sucrose catabolism is responsible for maintaining turgor. Wall loosening involves pectin metabolism in the epidermal wall(s). An integrative model of light and dark stem elongation is presented. The cells that plants need to make new leaves are made by stem cells at the apex of their shoots. Multiple tissue layers are arranged along the dorso-ventral axis of adult leaves.

REV simultaneously establishes a negative feedback loop due to the fact that ZPR-type micro-proteins are known to decrease HD-ZIPIII protein activity. Plant polarity configuration and stem cell activity may be regulated, according to our hypothesis, by the interaction of these microRNA/micro-protein feedback loops. The precise control of transcription factor activities guides the differentiation processes necessary for tissue formation in eukaryotic organisms during development. Cells can progress from a non-differentiated state to a highly specialized one by altering the transcriptional program. Stem cells are cells that have not been differentiated and can take on a wide range of cell fates. The Shoot Apical Meristem (SAM) is a population of stem cells at the plants shoot tip that is necessary for growth and development. Multiple factors involved in the organization and maintenance of the meristem have been identified through forward and reverse genetic methods.

Plant Ago Proteins

Several additional protein factors are required for the microRNA-mediated suppression of post-transcriptional genes. The Argonaute proteins, which bind the mature microRNA and direct the riboprotein complex to their target mRNAs, and the DICER-like proteins, which participate in the processing of longer precursor RNAs, stand out. In both plants and animals, microRNA (miRNA) function relies heavily on AGOs. Based on their biochemical properties, there are five distinct clades of plant AGO proteins. By controlling positive and negative feedback loops, we propose that HD-ZIPIII transcription factors can directly influence their activity state, which is crucial for the regulation of biological processes like the maintenance of meristems or the establishment of polarity in leaves. The biological significance of these feedback loops is emphasized by the fact that mutation or transgenic overexpression approaches significantly alter the developmental processes controlled by HD-ZIPIIIs. As a result, REV directly initiates two distinct feedback mechanisms that reflect on its own activity. MicroRNA inhibition leads to positive regulation, while micro-protein action leads to negative regulation.

Understanding the function of HD-ZIPIII proteins in stem cell maintenance and development in general will be improved by further characterizing how these feedback loops connect in the wild type plant. A novel strategy for altering the amount and composition of lignin in poplar has been demonstrated by our findings that miRNA overexpression of the MIR156 class has significant effects on plant architecture. The potential function of MIR156 in regulating poplar developmental processes could be investigated using these transgenic plants as a tool. The 35S: Cg1 poplars may also be useful in the paper-making industry and as a cellulosic feedstock for the production of biofuels. Stem cell pluripotency is maintained by the transcription factor WUSCHEL's potentiation of signaling from beneath the cells by this repression. A mechanistic framework for the localization of stem cells at the tip of the meristem is provided by the interaction of two opposing signaling centers. In the self-organizing meristem, the surface layer serves as a stable reference point despite the changing constituent cells. Despite a rapidly shifting cellular context, the spatial organization of stem cell niches must be maintained. A population of pluripotent stem cells at the tip of the plant shoot apical meristem gives rise to all of the plant's aerial organs throughout its life. Instead of asymmetric division, the shoot meristem stem cell niche operates in a population mode in which positional information determines the fate of each stem cell daughter. Because every cell divides in the shoot meristem, the spatial pattern cannot be oriented to a fixed point of reference but instead maintains itself in a self-organizing manner.

The mechanisms by which the shoot meristem maintains its spatial organization have largely remained elusive despite the discovery of these regulatory pathways. L2 and L3 cells showed altered cell division orientation and terminal differentiation when the L1 was surgically removed from the apices of tomato shoots. This suggests that the L1 is responsible for regulating the cell layers below it. It is possible to imagine both mechanical forces and molecular signals as the underlying processes. Ectopic WUS activity only induces CLV3 expression in the outermost cell layers of the shoot apex, as previously discovered, indicating the presence of unknown factors that give these cells stem cell competence. The single-layered protoderm serves as a signaling source for the underlying cells during shoot meristem formation, establishing a link between stem cell competence and the shoot meristem tip in this study.