The following phylogenetic study includes P1B-ATPases from 22 photosynthetic organisms whose full genomes have been published on the Phytozome v.6.0 website (http://www.phytozome.net/). Then, the recently published HMA sequences from the hyperaccumulator species A. halleri and T. caerulescens were added and the two sequences found in the yeast Saccharomyces cerevisiae (http:// www.yeastgenome.org/). Finally, 205 sequences coding HMA transporters were identified and analyzed (Fig. 3). All photosynthetic land plants (moss, lycophytes, monocots and eudicots) present about the same number of HMA transporters, an average of 9 sequences, with the exception of two species, Manihot esculenta (genome size ~760 Mbp) with only 5 HMAs, and at the opposite end, Glycine max which presents the highest number of HMAs coding genes with 18 members. Unicellular organisms, photosynthetic or not, Chlamydomonas reinhardtii, Volvox carteri and S. cerevisiae possess a reduced number of sequences (four for the microalgae and two for the yeast). All HMAs analyzed in the present study respected the classification from Baxter into six clusters, although those from the unicellular organisms appear rather apart. Four of the six clusters (clusters II, IV, V and VI) present an equivalent distribution of HMAs, each being subdivided into three subclusters with generally one containing eudicot proteins, a second one the Poaceae or monocot proteins, and the last one including moss and lycophyte transporters. Cluster I presents a higher degree of divergence (P1B-1), while cluster III is the largest including 54 proteins and contains 2 subclusters segregating the eudicot or Poaceae members. Sequences in each of the first subclusters showed 75-90% identities, whereas sequences belonging to the second subcluster presented only 50% identities.
Plants are the only eukaryotic organisms possessing P1B_2 Zn2+-ATPases with the exception of PCA1 from S. cerevisiae which is a particular case. The P1B_2-ATPase cluster II contains 48 Zn2+/Cd2+/Co2+/Pb2+ divalent cation transporters, in reference to the metal specificity found for those in A. thaliana. In this species, the three transporters of this subcluster played major roles (AtHMA4 and AtHMA2 in the Zn2+ vascular vessel loading, but also in Cd2+ translocation, and AtHMA3 in the vacuolar storage of these two metals). The Poaceae species present in general the same number of transporters, while a larger diversity was found in eudicots, exhibiting from one to four members. Thus, due to this diversity, it is not possible to directly spread the physiological roles of HMA2-4 in Arabidopsis to other plant species.
The main divergences between Arabidopsis and other Brassicaceae occurred at around 45-20 Mya (Beilstein et al. 2010; Ueno et al. 2011). Among the Brassicaceae family, only A. halleri and the genus Thlaspi unequivocally contain characterized Zn2+ and Cd2+ hyperaccumulator species. Moreover, the occurrence of a constitutive metal tolerance phenotype in metallicolous and non-metallicolous A. halleri populations strongly suggests that these traits had only arisen during two recent evolutionary events (Roosens et al. 2008). Unfortunately the full genome of
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