A perspective on cell proliferation kinetics in the root apical meristem.

Organogenesis in plants is primarily postembryonic and relies on a strict balance between cell division and cell expansion. The root is a particularly well-suited model to study cell proliferation in detail since the two processes are spatially and temporally separated for all the different tissues. In addition, the root is amenable to detailed microscopic analysis to identify cells progressing through the cell cycle. While it is clear that cell proliferation activity is restricted to the root apical meristem (RAM), understanding cell proliferation kinetics and identifying its parameters have required much effort over many years. Here, we review the main concepts, experimental settings, and findings aimed at obtaining a detailed knowledge of how cells proliferate within the RAM. The combination of novel tools, experimental strategies, and mathematical models has contributed to our current view of cell proliferation in the RAM. We also discuss several lines of research that need to be explored in the future.

Tools for Assessing Cell-Cycle Progression in Plants.

Estimation of cell-cycle parameters is crucial for understanding the developmental programs established during the formation of an organism. A number of complementary approaches have been developed and adapted to plants to assess the cell-cycle status in different proliferative tissues. The most classical methods relying on metabolic labeling are still very much employed and give valuable information on cell-cycle progression in fixed tissues. However, the growing knowledge of plant cell-cycle regulators with defined expression pattern together with the development of fluorescent proteins technology enabled the generation of fusion proteins that function individually or in conjunction as cell-cycle reporters. Together with the improvement of imaging techniques, in vivo live imaging to monitor plant cell-cycle progression in normal growth conditions or in response to different stimuli has been possible. Here, we review these tools and their specific outputs for plant cell-cycle analysis.

Defence sugarcane glycoproteins disorganize microtubules and prevent nuclear polarization and germination of Sporisorium scitamineum teliospores.

Microtubules (MTs) are involved in the germination of Sporisorium scitamineum teliospores. Resistant varieties of sugar cane plants produce defence glycoproteins that prevent the infection of the plants by the filamentous fungi Sporisorium scitamineum. Here, we show that a fraction of these glycoproteins prevents the correct arrangement of MTs and causes nuclear fragmentation defects. As a result, nuclei cannot correctly migrate through the growing hyphae, causing germinative failure. Arginase activity contained in defence glycoproteins is already described for preventing fungal germination. Now, its enzymatically active form is presented as a link between the defensive capacity of glycoproteins and the MT disorganization in fungal cells. Active arginase is produced in healthy and resistant plants; conversely, it is not detected in the juice from susceptible varieties, which explains why MT depolarization, nuclear disorganization as well as germination of teliospores are not significantly affected by glycoproteins from non-resistant plants. Our results also suggest that susceptible plants try to increase their levels of arginase after detecting the presence of the pathogen. However, this signal comes "too late" and such defensive mechanism fails.