Pectin methylesterases (PMEs) modify homogalacturonan's chemistry and thereby play a key role in regulating primary cell wall mechanical properties. How PME activity can fine-tune pectin structure in the growing plant has remained elusive, in part due to the lack of available biochemically-characterized enzymes to empirically test functional properties. Here we report on AtPME2, which we found to be highly expressed during lateral root emergence as well as root and hypocotyl elongation. Production of mature active enzyme in Pichia pastoris allowed its biochemical characterization. We show that AtPME2 can switch from full processivity (at pH 8), creating large blocks of unmethylated galacturonic acid, to low processivity (at pH 5) and relate these observations to the differences in electrostatic potential of the protein. We also produced a generic plant PME antiserum suitable for detecting recombinant and native enzyme independent of species source. In the context of acidified apoplast, we showed using reverse genetics that low-processive demethylesterification by AtPME2 can loosen the cell wall, with consequent increase in cell elongation and etiolated hypocotyl length. Our study brings insights into how the pH-dependent regulation by PME activity could affect pectin structure and associated cell wall mechanical properties in expansion.
We used secondary ion mass spectrometry to image cellular targets of nitrogen oxides (widespread air pollutants) in pollen grains of birch (Betula verrucosa Ehrh.) and cockfoot (Dactylis glomerata L.). The pollen samples were exposed to air supplemented with high doses of 15NO. The pollen grains were then fixed, dehydrated using a newly developed 'vapour phase' preparation method and embedded in LRW resin. Semithin sections were then analysed. Imaging was performed in scanning mode. As usual, the two isotopes 14N and 15N were imaged as 12C14N- and 12C15N-, respectively. The isotopic percentages of 15N were quantitatively determined either by image processing or by direct analysis. We show that the preferential areas of NO fixation in the pollen cell are the sporoderm and discrete intracytoplasmic structures that we tentatively describe as globoid-like structures similar to those encountered in seeds.
This paper considers the control of a system composed of several interconnected subsystems. Each subsystem i s described by a canonical nonl i n e a r s t a t e model, w i t h t h e r e s t r i c t i o n t h a t i nt e r a c t i o n between d i f f e r e n t subsystems i s l i n e a r . Under v e r y m i l d a d d i t i o n a l c o n d i t i o n s a decentrali z e d d i t h e r feedback c o n t r o l i s c o n s t r u c t e d w h i c h w i l l drive the system state to zero. The d i t h e r c o n t r o l method i s shown t o s t a b i l i z e t h i s system f o r a l l i n i t i a l c o n d i t i o n s .
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