A plausible mechanism, based upon Short-Root movement, for regulating the number of cortex cell layers in roots.

Formation of specialized cells and tissues at defined times and in specific positions is essential for the development of multicellular organisms. Often this developmental precision is achieved through intercellular signaling networks, which establish patterns of differential gene expression and ultimately the specification of distinct cell fates. Here we address the question of how the Short-root (SHR) proteins from Arabidopsis thaliana (AtSHR), Brachypodium distachyon (BdSHR), and Oryza sativa (OsSHR1 and OsSHR2) function in patterning the root ground tissue. We find that all of the SHR proteins function as mobile signals in A. thaliana and all of the SHR homologs physically interact with the AtSHR binding protein, Scarecow (SCR). Unlike AtSHR, movement of the SHR homologs was not limited to the endodermis. Instead, the SHR proteins moved multiple cell layers and determined the number of cortex, not endodermal, cell layers formed in the root. Our results in A. thaliana are consistent with a mechanism by which the regulated movement of the SHR transcription factor determines the number of cortex cell layers produced in the roots of B. distachyon and O. sativa. These data also provide a new model for ground tissue patterning in A. thaliana in which the ability to form a functional endodermis is spatially limited independently of SHR.

The TrpA protein of Trichodesmium erythraeum IMS101 is a non-fibril-forming collagen and a component of the outer sheath.

Collagen molecules are structural in nature and primarily found in eukaryotic, multicellular organisms. Recently, a collagen-like protein, TrpA, was identified and characterized in the marine cyanobacterium Trichodesmium erythraeum IMS 101, and it was shown to be involved in maintaining the structural integrity of the trichomes. The TrpA protein contains one glycine interruption in the otherwise perfectly uninterrupted collagenous domain. In this study, we used phylogenetic analysis to determine that the TrpA protein sequence is most closely associated with non-fibril-forming collagen proteins. Structural modelling and circular dichroism data suggest that the glycine insertion decreases the stability of TrpA compared to uninterrupted collagen sequences. Additionally, scanning electron microscopy revealed that TrpA is expressed entirely on the surface of the trichomes, with no specific pattern of localization. These data indicate that the TrpA protein is part of the outer sheath of this organism. As such, this protein may function to promote adhesion between individual T. erythraeum trichomes, and between this organism and heterotrophic bacteria found in the same environment.

Characterization of a novel collagen-like protein TrpA in the cyanobacterium Trichodesmium erythraeum IMS101.

The collagen protein family is diverse and its membership is continually expanding as new collagen-like molecules are identified. Identification of collagen in unicellular eukaryotes and prokaryotes has opened discussion on the function of these collagens and their role in the emergence of multicellularity. The previous identification of a collagen gene in Trichodesmium erythraeum raises the question of function of this structural protein in a prokaryote. In this study, we show that this gene is expressed during all phases of growth, indicating that it may be required for all phases of growth. Using immunofluorescence techniques, we demonstrate that the collagen-like protein is localized in a specific manner between adjacent cells along the trichome of T. erythraeum. Trichomes treated with the enzyme collagenase exhibited fragmentation, supporting our immunofluorescence localization data that this collagen-like protein is found between adjacent cells. Our data strongly suggest that the collagen-like protein found in T. erythraeum functions to maintain the structural integrity of the trichome through the adhesion of adjacent cells.