Plant development: From cells to individuals

Abstract

As the development of research tools in molecular biology, plant developmental biology has been changed its focus from the descriptive analysis of plant morphology to the cellular and gene regulation level. Plants have distinct postembryonic development patterns compare to the animal, which give the plant a flexible developmental plasticity in response to different growth environments. In the long-term of evolution, plants are adapted to the environment changes through the continuous adjustment of their development strategy, which makes the plant world highly diverse. The growth and development of multicellular organisms depend on the maintenance and constant differentiation of stem cells. In plants, most of the organs originate from stem cells where resided in the shoot apical meristem, root apical meristem and cambium. The functional conservation of stem cells in those diverse plants is the basis for ontogenesis. However, from the perspective of evolutionary development, the diversity of key regulatory genes expression in different stem cell populations fulfills the continuous adjustments of development strategy to adapt to environmental changes. The plasticity in stem cell regulations determines the flexibility of plant development, which is conserved in plant kingdom from moss to angiosperms. Of course, the mechanism of stem cell maintenance and differentiation are even more complicated in angiosperms. In model plant of Arabidopsis thanian , the molecular mechanism of stem cell regulations has been extensively studied over the past decades. Multiple signaling molecules and transcription factors are found to tightly control the stem cell fate in Arabidopsis thaliana . The homeodomain transcription factor WUSCHEL ( WUS ) where expressed in the organizing center (OC) is a key regulator for plant stem cell fate determination. While, stem cells expressed secreted peptide CLV3, negatively regulates the expression of WUS . They form a negative feedback loop that tightly control plant stem cell fate. In addition, the classic phytohormones cytokinin and auxin also play essential roles in the maintenance of stem cells in shoot apical meristem, root apical meristem and cambium, and exhibit complex functional interactions. The molecular organization of the RAM is quite similar to that of the shoot. WOX5 ( WUSCHEL- RELATED HOMEOBOX 5 ), a homologue of WUS , is expressed in the QC, and induces root stem cell fate in the surrounding cells. The GRAS family transcription factor SHORT-ROOT ( SHR ) is expressed in the innermost tissue of the root, and SHR moves to the surrounding cell layer activating SCARECROW ( SCR ), together with PLT1 and PLT2 , defines the stem cells fate. In this review, we focused on the functions of plant stem cells in plant postembryonic development, and covered recent findings on plant hormonal regulation in stem cells. We also discussed the integration of endogenous genetic information and external environmental factors in plant development, and how they affected the development of organs, morphology and yield of crops.