The primary endosymbiotic origin of chloroplasts is now well established but the identification of the present cyanobacteria most closely related to the plastid ancestor remains debated. We analyse the evolutionary trajectory of a subset of highly conserved cyanobacterial proteins (core) along the plastid lineage, those which were not lost after the endosymbiosis. We concatenate the sequences of 33 cyanobacterial core proteins that share a congruent evolutionary history, with their eukaryotic counterparts to reconstruct their phylogeny using sophisticated evolutionary models. We perform an independent reconstruction using concatenated 16S and 23S rRNA sequences. These complementary approaches converge to a plastid origin occurring during the divergence of one of the major cyanobacterial lineages that include N 2 -fixing filamentous cyanobacteria and species able to differentiate heterocysts.

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The cDNA clone (CanBGal-3) encoding a cell wall pectin-degrading b-galactosidase (bIII-Gal) from Cicer arietinum L. cv. Castellana has been identified. The identification was carried out by comparing the deduced amino acid sequences of several isolated chickpea b-galactosidase clones with the purified bIII-Gal protein sequence. The expression pattern of the gene corresponding to CanBGal-3 was in concordance with the fluctuations of the enzyme bIII-Gal in different seedling organs, being specific to elongating organs such as epicotyls and roots. Transformation of Solanum tuberosum plants with the chickpea CanBGal-3 clone indicated that the b-galactosidase encoded by this clone is a pectin-degrading enzyme. The authors propose an important role for chickpea bIII-Gal in pectin degradation in cell walls of vegetative organs such as epicotyls and roots. The degradation of galactan carried out by this enzyme may determine structural changes and affect cell wall porosity. It is suggested that the increase in the size of cell wall pores could permit access of other cell wall-modifying enzymes to their substrate.