Plant and animal stem cells: conceptually similar, molecularly distinct?

Plant stem cells carve their own niche

Stem cells are the precursors of differentiated cells and are, thus, indispensable for growth and development in plants and animals. Stem cells from both types of organisms share the fundamental features of a capacity for self-renewal and an ability to generate differentiated cells. The maintenance of stem cells in both systems is dependent upon reciprocal signalling between stem cells and the specialized tissue microenvironment known as the niche, which provides intercellular signals for stem cell regulation. One significantly underexplored facet of plant stem cells is the nature of their intrinsic transcriptional programme. A potentially rich avenue for addressing this deficiency is to combine laser-assisted microdissection and genome-wide transcriptional profiling to unravel the molecular road map controlling plant stem cells and their niches.

Molecular control of stem cell maintenance in shoot apical meristem

Sustained post-embryonic organ initiation and development in plants depends on coordinating the formation and differentiation of pluripotent stem cells in apical meristems. Transcriptional regulation and intercellular signalling appear to play key roles in this coordination process. Here we discuss the current knowledge about the molecular regulation of stem cell maintenance in the shoot apical meristem and recent attempts to delineate the molecular signatures of "stemness" in flowering plants. We also outline contemporary molecular approaches for deciphering the process of stem cell renewal in the shoot apical meristem.

REGULATION OF ROOT APICAL MERISTEM DEVELOPMENT

The establishment of the Angiosperm root apical meristem is dependent on the specification of a stem cell niche and the subsequent development of the quiescent center at the presumptive root pole. Distribution of auxin and the establishment of auxin maxima are early formative steps in niche specification that depend on the expression and distribution of auxin carriers. Auxin specifies stem cell niche formation by directly and indirectly affecting gene activities. Part of the indirect regulation by auxin may involve changes in redox, favoring local, oxidized microenvironments. Formation of a QC is required for root meristem development and elaboration. Many signals likely pass between the QC and the adjacent root meristem tissues. Disappearance of the QC is associated with roots becoming determinate. Given the many auxin feedback loops, we hypothesize that roots evolved as part of an auxin homeostasis mechanism.

Signals that regulate stem cell activity during plant development

Plant stem cells are used continuously to generate new structures during the entire life-span of the organism. In the adult plant, stem cells are found in specialized structures called meristems. The meristems contain the stem cell niche together with rapidly dividing daughter cells that will ultimately differentiate into specific cell types. Some of the master genes that orchestrate the establishment and maintenance of the stem cell niche have now been identified in both the root and the shoot. Recent results show that these genes also determine the fate of the stem cells and that feedback signals from differentiated cells are involved in stem cell specification. These advances have provided a framework to understand how short-range and long-range signals are integrated to specify and position the stem cell niche in the meristems, and how the differentiation potential of plant stem cells is controlled.

Intercellular signalling in the transition from stem cells to organogenesis in meristems

Meristems continuously produce new cells to sustain plant growth. Stem cells are maintained in the centre of the meristem and provide the precursor cells for the initiation of new organs and tissues in the periphery. The structure of the meristem is maintained while cells are constantly displaced by new divisions. Recent advances have been made in understanding the intercellular signals that maintain meristem structure by adjusting gene expression according to cell position. In addition to refinements in our understanding of how the position and size of the stem-cell population is regulated, there have been advances in understanding how the location of new organ primordia is controlled and how the meristem influences organ polarity.