Abstract
Plants have evolved a range of cellular responses to maintain developmental homeostasis and to survive over a range of temperatures. Here, we describe the in vivo and in vitro functions of BOBBER1 (BOB1), a NudC domain containing Arabidopsis (Arabidopsis thaliana) small heat shock protein. BOB1 is an essential gene required for the normal partitioning and patterning of the apical domain of the Arabidopsis embryo. Because BOB1 loss-of-function mutants are embryo lethal, we used a partial loss-of-function allele (bob1-3) to demonstrate that BOB1 is required for organismal thermotolerance and postembryonic development. Recombinant BOB1 protein functions as a molecular chaperone and prevents the aggregation of a model protein substrate in vitro. In plants, BOB1 is cytoplasmic at basal temperatures, but forms heat shock granules containing canonical small heat shock proteins at high temperatures. In addition to thermotolerance defects, bob1-3 exhibits pleiotropic development defects during all phases of development. bob1-3 phenotypes include decreased rates of shoot and root growth as well as patterning defects in leaves, flowers, and inflorescence meristems. Most eukaryotic chaperones play important roles in protein folding either during protein synthesis or during cellular responses to denaturing stress. Our results provide, to our knowledge, the first evidence of a plant small heat shock protein that has both developmental and thermotolerance functions and may play a role in both of these folding networks.
Highlights
Plants have evolved a range of cellular responses to maintain developmental homeostasis and to survive over a range of temperatures
We previously demonstrated that BOB1 is required for partitioning the apical domain of Arabidopsis embryos during embryogenesis
In order to gain insights into the mechanistic basis of BOB1-mediated patterning, we took advantage of a partial loss-of-function allele as well as a BOB1:GFP fusion and an in vitro chaperone assay to establish that BOB1 functions as a sHSP
Summary
BOB1 is required for the establishment of auxin gradients and for patterning the apical domain of the embryo (Jurkuta et al, 2009). Seven out of 20 ethyl methanesulfonate (EMS) alleles identified were predicted to disrupt BOB1 protein function by SIFT (Ng and Henikoff, 2001) One of these alleles, bob, is a substitution of a Glu for a Gly at the highly conserved amino acid position 141 (G141E), which is just upstream of the NudC domain (Fig. 1). Mutant plants grow more slowly, flower later, and have smaller rosette diameters at flowering compared to wild-type siblings. All of these phenotypes are stronger in bob THs than in bob homozygotes. No ectopic expression of KNAT1, KNAT2, or STM was detected using either approach (Supplemental Fig. S1), demonstrating that the developmental defects observed during embryogenesis and leaf margin patterning are caused by different molecular mechanisms
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