BRK1 and apical growth

From Purdue Genomics Database Facility

Victor A. Albert

This article discusses results obtained by others (functional data reported February, 2008) that together may be of phylogenetic/functional significance regarding apical growth in land plants.

Perroud and Quatrano (e-pub; see below) have just reported functional implications of the BRK1 homolog of Physcomitrella. The BRK1 protein is part of the Wave/SCAR complex that participates in regulation of the assembly of actin networks. These authors show that in Physcomitrella, BRK1 participates in the elongation and orientation of protonemal cell division, specifically by expressing in the apical cell. The knockout mutant severely affects protonemal cell dision and filament growth, whereas it does not affect growth of the leafy shoots.

Under the article Cytoskeletons (these pages), Murata et al. have annotated the single Selaginella BRK1 homolog locus, and in their external link [1] they have provided an alignment and neighbor joining and maximum likelihood trees including Selaginella, Physcomitrella, seed plants (in this case, angiosperms; I have also found spruce and cycad homologs), as well animal outgroups (see bolded point below Re: phylogenetic reconstruction concerns!).

Of potential functional importance is that Murata et al.'s trees show that Selaginella and Physcomitrella BRK1 homologs group together as a monophyletic lineage sister to the BRK1 homologs of angiosperms. That is, there is not the simple (Physcomitrella(Selaginella(angiosperm))) phylogenetic progression as would be expected for a single copy gene inherited only following organismal phylogeny.

As such, it could be that the BRK1 homologs of Selaginella and Physcomitrella are both specialized for apical growth based on a single-celled "meristematic" format (at least for some organs, maybe; see Harrison et al., 2007, re: 2 apical initials in Selaginella leafy shoots). The Arabidopsis BRK1 mutant does not appear to show apical meristem defects, and among other sites of single-celled tip growth (e.g., trichomes, root hairs, pollen tubes) only trichomes show a phenotype.

The phylogenetic evidence reported above suggests that a BRK1 duplication could have occurred on the stem lineage of land plants. Indeed, there appear to be interesting phylogenetic dynamics associated with this protein among eukaryotes. Importantly, BRK1 homologs do NOT appear to be present in single-celled green plants; I have not been able to find them in either Chlamydomonas or Ostreococcus. However, they are not only present in animals, but also Dictyostelium and Ectocarpus (a brown alga)! Losses can be hypothesized to have occurred, but their precise phylogenetic positions will require a broader sampling of green-plant BRK1 homologs. If BRK1 did duplicate in the land plant stem lineage then it may be that copies were differentially deleted in the seed-plant vs. other lineages. Or perhaps the tree is misleading because of taxonomic sampling effects (long branch attraction from outgroups?). Or perhaps convergent evolution of protein sequence (and function) occurred in Selaginella and mosses? Or, perhaps the BRK1 protein of the seed plant ancestor diverged substantially enough in sequence so as to obscure organismal phylogenetic history? Sampling of pteridophyte BRK1 homologs could be helpful for choosing among these alternatives.

Note that the unexpected Selaginella/Physcomitrella sister-group relationship also appears in some of Murata et al.'s trees of members of the actin-related protein2/3 (Arp2/3) complex (see, e.g., [2], [3], [4], [5], [6]), which are also related to single-celled tip growth (see Harries et al., 2005, below).

References:

• Perroud PF, Quatrano RS. BRICK1 Is Required for Apical Cell Growth in Filaments of the Moss Physcomitrella patens but Not for Gametophore Morphology. Plant Cell. 2008 Feb 8; [Epub ahead of print]
• Harries PA, Pan A, Quatrano RS. Actin-related protein2/3 complex component ARPC1 is required for proper cell morphogenesis and polarized cell growth in Physcomitrella patens. Plant Cell. 2005 Aug;17(8):2327-39.
• Harrison CJ, Rezvani M, Langdale JA. Growth from two transient apical initials in the meristem of Selaginella kraussiana. Development. 2007 Mar;134(5):881-9.