Question

1 -What effects do microtubule poisons usually have on the directionality of plant cell expansion, and why?

2- Possible modifications of the maturing cell wall that may contribute to cell wall rigidification.

Answer 1

Answer : 1

A spindle poison, also known as a spindle toxin or microtubule poision, is a poison that disrupts cell division by affecting the protein threads that connect the centromere regions of chromosomes, known as spindles. Spindle poisons effectively cease the production of new cells by interrupting the mitosis phase of cell division at the spindle assembly checkpoint (SAc) .

Many plant mutants are known that exhibit some degree of helical growth. This ‘twisted’ phenotype has arisen frequently in mutant screens of model organisms, but it is also found in cultivars of ornamental plants, including trees. The phenomenon, in many cases, is based on defects in cell expansion symmetry. Any complete model which explains the anisotropy of plant cell growth must ultimately explain how helical cell expansion comes into existence – and how it is normally avoided. While the mutations observed in model plants mainly point to the microtubule system, additional affected components involve cell wall functions, auxin transport and more. Evaluation of published data suggests a two‐way mechanism underlying the helical growth phenomenon: there is, apparently, a microtubular component that determines handedness, but there is also an influence arising in the cell wall that feeds back into the cytoplasm and affects cellular handedness. This idea is supported by recent reports demonstrating the involvement of the cell wall integrity pathway. In addition, there is mounting evidence that calcium is an important relayer of signals relating to the symmetry of cell expansion. These concepts suggest experimental approaches to untangle the phenomenon of helical cell expansion in plant mutants.

Answer : 2

Directional (anisotropic) cell wall expansion is an integral part of most plant developmental processes, where it facilitates the structural changes necessary for proper cell and organ morphogenesis. The initial expansive growth phase, which requires both addition of new extracellular polymers and remodeling of existing components in the primary cell walls, is often succeeded by cell wall thickening and rigidification processes to create secondary cell walls that enhance the structural integrity of the organ, but also curtail further wall extension. These sequential processes require a high degree of dynamic, context-specific coordination of cell wall building, reconstruction and fortification in order to harness the underlying driving force of turgor pressure in a spatially-defined manner .Consistent with such developmental complexity, at least one thousand genes in Arabidopsis have been shown to have some association with cell wall synthesis and remodeling .

The gene expression patterns associated with cell wall expansion and/or secondary cell wall formation in Arabidopsis have been analyzed in several studies in efforts to identify participating genes and understand the biological roles of their products . To specifically address cell expansion processes, for example, transcript profiles have been compared with protein accumulation profiles in Arabidopsis seedling hypocotyls that were undergoing rapid cell elongation without significant cell division . Transcript profiling has also been conducted with in vitro cultured Zinnia mesophyll cells and Arabidopsis subcultured cells that were induced to trans-differentiate into tracheary element-like cells, a process that is accompanied by deposition of distinctive secondary cell wall thickenings on top of the original primary cell walls. In another approach, large-scale correlation analysis of public microarray data enabled the in silico identification of genes whose expression is strongly aligned with expression of specific members of the Arabidopsis cellulose synthase (CesA) gene family that are believed to be predominantly involved in either primary or secondary cell wall biogenesis