The Pellicle–Another Strategy of the Root Apex Protection against Mechanical Stress?

In grasses, the apical part of the root is covered by a two-layered deposit of extracellular material, the pellicle, which together with the outer periclinal wall of protodermal cells forms the three-layered epidermal surface. In this study, the effect of mechanical stress on the pellicle was examined. An experiment was performed, in which maize roots were grown in narrow diameter plastic tubes with conical endings for 24 h. Two groups of experimental roots were included in the analysis: stressed (S) roots, whose tips did not grow out of the tubes, and recovering (R) roots, whose apices grew out of the tube. Control (C) roots grew freely between the layers of moist filter paper. Scanning electron microscopy and confocal microscopy analysis revealed microdamage in all the layers of the epidermal surface of S roots, however, protodermal cells in the meristematic zone remained viable. The outermost pellicle layer was twice as thick as in C roots. In R roots, large areas of dead cells were observed between the meristematic zone and the transition zone. The pellicle was defective with a discontinuous and irregular outermost layer. In the meristematic zone the pellicle was undamaged and the protodermal cells were intact. The results lead to the conclusion that the pellicle may prevent damage to protodermal cells, thus protecting the root apical meristem from the negative effects of mechano-stress.

Differential biosynthesis and cellular permeability explain longitudinal gibberellin gradients in growing roots

Control over cell growth by mobile regulators underlies much of eukaryotic morphogenesis. In plant roots, cell division and elongation are separated into distinct longitudinal zones and both division and elongation are influenced by the growth regulatory hormone gibberellin (GA). Previously, a multicellular mathematical model predicted a GA maximum at the border of the meristematic and elongation zones. However, GA in roots was recently measured using a genetically encoded fluorescent biosensor, nlsGPS1, and found to be low in the meristematic zone grading to a maximum at the end of the elongation zone. Furthermore, the accumulation rate of exogenous GA was also found to be higher in the elongation zone. It was still unknown which biochemical activities were responsible for these mobile small molecule gradients and whether the spatiotemporal correlation between GA levels and cell length is important for root cell division and elongation patterns. Using a mathematical modeling approach in combination with high-resolution GA measurements in vivo, we now show how differentials in several biosynthetic enzyme steps contribute to the endogenous GA gradient and how differential cellular permeability contributes to an accumulation gradient of exogenous GA. We also analyzed the effects of altered GA distribution in roots and did not find significant phenotypes resulting from increased GA levels or signaling. We did find a substantial temporal delay between complementation of GA distribution and cell division and elongation phenotypes in a GA deficient mutant. Together, our results provide models of how GA gradients are directed and in turn direct root growth.

How do three cytosolic glutamine synthetase isozymes of wheat perform N assimilation and translocation?

To understand how the three cytosolic glutamine synthetase (GS1) isozymes of wheat (Triticum aestivum L., TaGS1) perform nitrogen assimilation and translocation, we studied the kinetic properties of TaGS1 isozymes, the effects of nitrogen on the expression and localization of TaGS1 isozymes with specific antibodies, and the nitrogen metabolism. The results showed TaGS1;1, the dominant TaGS1 isozyme, had a high affinity for substrates, and was widely localized in the mesophyll cells, root pericycle and root tip meristematic zone, suggesting it was the primary isozyme for N assimilation. TaGS1;2, with a high affinity for Glu, was activated by Gln, and was mainly localized in the around vascular tissues, indicating that TaGS1;2 catalyzed Gln synthesis in low Glu concentration, then the Gln returned to activate TaGS1;2, which may lead to the rapid accumulation of Gln around the vascular tissues. TaGS1;3 had low affinity for substrates but the highest Vmax among TaGS1, was mainly localized in the root tip meristematic zone; exogenous NH4+ could promote TaGS1;3 expressing, indicating that TaGS1;3 could rapidly assimilate NH4+ to relieve NH4+ toxicity. In conclusion, TaGS1;1, TaGS1;2 and TaGS1;3 have different role in N assimilation, Gln translocation and relieving ammonium toxicity, respectively, and synergistically perform nitrogen assimilation and translocation.HighlightThree cytosolic glutamine synthase isozymes of wheat have different role and synergistically perform nitrogen assimilation and translocation.

Light dynamically regulates growth rate and cellular organisation of the Arabidopsis root meristem

1AbstractLarge-scale methods and robust algorithms are needed for a quantitative analysis of cells status/geometry in situ. It allows the understanding the cellular mechanisms that direct organ growth in response to internal and environmental cues. Using advanced whole-stack imaging in combination with pattern analysis, we have developed a new approach to investigate root zonation under different dark/light conditions. This method is based on the determination of 3 different parameters: cell length, cell volume and cell proliferation on the cell-layer level. This method allowed to build a precise quantitative 3D cell atlas of the Arabidopsis root tip. Using this approach we showed that the meristematic (proliferation) zone length differs between cell layers. Considering only the rapid increase of cortex cell length to determine the meristematic zone overestimates of the proliferation zone for epidermis/cortex and underestimates it for pericycle. The use of cell volume instead of cell length to define the meristematic zone correlates better with cell proliferation zone.

Effects of water soluble oncostatic fraction from Rheum officinale Baill. rhizomes on Allium cepa root meristem. II. Meristem length and mitotic activity distribution

Incubation in 5 and 12.5 per cent extract from <i>Rheum officinale</i> rhizomes causes disturbance of the dynamic equilibrium between the number of dividing cells and the number of those passing to the elongation zone. The zone of meristematic cells is shortened to 2/3 and the zone of mitoses to 1/2 after 24-h incubation in 5 per cent extract. 12-h incubation in 12.5 per cent extract does not reduce the zone of meristematic cells, although it shortens the mitosis zone to 1/5. This suggests that a high concentration of the inhibitor arrests elongation. growth. Mitotic activation of the meristem in the beginning of postincubation period occurs on a wide area since the last mitotic cycle runs in the cells of the basal part of the meristem. During further postincubation (48 and 72 h after 5% and 72 h after 12.5% extract) the meristematic zone is greatly shortened and the zone of highest mitosis frequency shifts in apical direction. The mitotic activity in the apical sector much higher than in the control suggests, that the quiscent centre takes part in the reconstruction of the meristem.

Evolution of spur-length diversity in Aquilegia petals is achieved solely through cell-shape anisotropy

The role of petal spurs and specialized pollinator interactions has been studied since Darwin. Aquilegia petal spurs exhibit striking size and shape diversity, correlated with specialized pollinators ranging from bees to hawkmoths in a textbook example of adaptive radiation. Despite the evolutionary significance of spur length, remarkably little is known about Aquilegia spur morphogenesis and its evolution. Using experimental measurements, both at tissue and cellular levels, combined with numerical modelling, we have investigated the relative roles of cell divisions and cell shape in determining the morphology of the Aquilegia petal spur. Contrary to decades-old hypotheses implicating a discrete meristematic zone as the driver of spur growth, we find that Aquilegia petal spurs develop via anisotropic cell expansion. Furthermore, changes in cell anisotropy account for 99 per cent of the spur-length variation in the genus, suggesting that the true evolutionary innovation underlying the rapid radiation of Aquilegia was the mechanism of tuning cell shape.