Changes in the geometry of the apical meristem and concomitant changes in cell wall properties during photoperiodic induction of flowering in Chenopodium rubrum

Invited Review: Cryo-scanning electron microscopy (CSEM) in the advancement of functional plant biology. Morphological and anatomical applications

Cryo-scanning electron microscopy (CSEM) is reviewed by exploring how the images obtained have changed paradigms of plant functions and interactions with their environment. Its power to arrest and stabilise plant parts in milliseconds, and to preserve them at full hydration for examination at micrometre resolution has changed many views of plant function. For example, it provides the only feasible way of accurately measuring stomatal aperture during active transpiration, and volume and shape changes in guard cells, or examining the contents of laticifers. It has revealed that many xylem conduits contain gas, not liquid, during the day, and that they can be refilled with sap and resume water transport. It has elucidated the management of ice to prevent cell damage in frost tolerant plants and has revealed for the first time inherent biological and physical features of root/soil interactions in the field. CSEM is increasingly used to reveal complementary structural information in studies of metabolism, fungal infection and symbiosis, molecular and genetic analysis.

Flowering and apical meristem growth dynamics

The shoot apical meristem generates stem, leaves, and lateral shoot meristems during the entire shoot ontogeny. Vegetative leaves are generated by the meristem in the vegetative developmental phase, while in the reproductive phase either bracts subtending lateral flower primordia (or paraclades), or perianth and strictly reproductive organs are formed. Meristem growth is fully characterized by the principal growth rates, directions, volumetric, and areal growth rates. Growth modelling or sequential in vivo methods of meristem observation complemented by growth quantification allow the above growth variables to be estimated. Indirectly, growth is assessed by cell division rates and other cell cycle parameters. Temporal and spatial changes of growth and geometry take place at the meristem during the transition from the vegetative to the reproductive phase. During the vegetative phase, meristem growth is generally indeterminate. In the reproductive phase it is almost always determinate, but the extent of determinacy depends on the inflorescence architecture. In the vegetative phase the central meristem zone is the slowest growing region. The transition from the vegetative to the reproductive phase is accompanied by an increase in mitotic activity in this zone. The more determinate is the meristem growth, the stronger is this mitotic activation. However, regardless of the extent of the activation, in angiosperms the tunica/corpus structure of the meristem is preserved and therefore the mitotic activity of germ line cells remains relatively low. In the case of the thoroughly studied model angiosperm plant Arabidopsis thaliana, it is important to recognize that the flower primordium develops in the axil of a rudimentary bract. Another important feature of growth of the inflorescence shoot apical meristem is the heterogeneity of the peripheral zone. Finally, the role of mechanical factors in growth and functioning of the meristem needs further investigation.