Molecular cloning and characterisation of a putative pectin methylesterase cDNA in Arabidopsis thaliana (L.)

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Abstract
Pectin methylesterase (PME) is a cell wall enzyme that catalyses the de-esterification of pectins leading to fundamental changes which confer new properties to the micro-environment of each cell. In order to elucidate the meaning of PME-mediated changes of pectin in the time course of cell differentiation, we attempted to study the regulation of PME genes in Arabidopsis thaliana. In this report, the first full cDNA sequence showing sequence similarities with other PME genes characterised so far in other plant species has been isolated from an Arabidopsis shoot cDNA library. This ATPME1 cDNA is 1,970 bp long and contains an open reading frame encoding a protein of 64,1 kDa and a basic pI of 8.7 as predicted from the nucleotide sequence. Northern blot analyses denoted changes in the expression level of the ATPME1 mRNA according to plant organs. High mRNA levels were found in young developing organs such as cauline leaves while they were significantly lower in rosette leaves, stems and inflorescences, and almost undetectable in roots. Beside this molecular approach, isoelectrofocusing analyses revealed the occurrence of three PME isoforms in Arabidopsis. Two PME isoforms with pI values of 4.9 and 9.1 were found throughout the plant, but at a higher level in the root, while an other PME isoform with a pI of 5.7 was essentially detected in the inflorescence. The relationship between our observations and the data reported for other plant species is discussed.