1754 Plant Cell Rep (2009) 28:1747–1758 Fig. 7 Phylogenetic tree of ABP1 proteins among 13 land plants. The ABP1 sequences for each organism are indicated by a ﬁve letter code, as follows: Arath: Arabidopsis thaliana, Avesa: Avena sativa, Cerpu: Ceratodon purpureus, Cerri: Ceratopteris richardii, Glyma: Glycine max, Medtr: Medicago truncatula, Orysa: Oryza sativa, Phypa: Physcomitrella patens, Poptr: Populus tremula, Selmo: Selaginella programmes [i.e. TargetP (Emanuelsson et al. 2000), SherLoc (Shatkay et al. 2007), MultiLoc (Hoglund et al. 2006), WoLF PSORT (Horton et al. 2007)] to localise this moss ABP1 homologous protein suggested that it is not localised to the ER (data not shown). It therefore seems possible that a true homologue of ABP1 with a classical KDEL ER retention sequence may not be present in mos- ses. Attempts were also made to investigate if the putative ABP1 homologues use HDEL as a signal sequence for its retention in the ER. The HDEL retention sequence is suf- ﬁcient for secretory plant proteins to be retained in the ER while promoting vascular targeting of the proteins that escapes ER (Gomord et al. 1997). None of the putative ABPs were detected with the respective antibody, although as a positive control a 80 kDa protein (most likely to be a Bip, binding protein, a member of HSP70 family and a major chaperone of ER) was detected in corn extracts (data not shown). There is a report from animal systems describing signal sequences such as DKEL, RDEL, KNEL to be used as the ER retention signal (Andres et al. 1990). 123 moellendorfﬁi, Sorbi: Sorghum bicolor, Vitvi: Vitis vinifera, Zeama: Zea mays. The phylogenetic tree was constructed using Neighbour Joining as implemented in quicktree (Howe et al. 2002) using the ScoreDist distance matrix (Sonnhammer and Hollich 2005), 1,000 bootstrap replicates and rooted at the longest internal branch However, neither the P. patens nor the C. purpureus ABP1 nucleotide sequences contain such alternative retention signals. Moreover, there is a remarkable conservation between regulatory as well as signal sequences between moss and mammals (Schaaf et al. 2005; Gitzinger et al. 2009), making it unlikely that mosses contain hitherto unknown ER retention signals. Therefore, it is tempting to suggest that the ER retention sequence of ABP1 in higher plants could have evolved independently and at a later stage along with the needs for a specialised auxin signalling during the evolution of higher land plants, as was recently described for the evo- lution of nuclear auxin signalling (Paponov et al. 2009). It is also possible that these proteins are functional homo- logues of higher plant ABP1 and control essential non- nuclear processes such as cell elongation, polarity and control of ion channels, fundamental to plant development. Not only in the moss Funaria, but also in the extracts of corn and sorghum (data not shown) higher molecular mass putative ABPs were detected in the current study by