75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

211 Subtilisin-like serine proteases are involved in a broad range of developmental and
physiological processes in Arabidopsis thaliana
Carsten Rautengarten, Dirk Buessis, Thomas Altmann
University of Potsdam
The Arabidopsis thaliana genome contains 56 genes encoding subtilisin-like serine proteases (AtSBTs). To gain
insight into the roles of the Arabidopsis subtilases we started a systematic functional genomics analysis program. It covers
detailed sequence and expression analyses on transcript and protein level, the collection and evaluation of knockout
mutants for the entire subtilase gene family, generation of multiple knockouts / knockdowns as well as overexpression
of selected subtilase genes. Whereas the mutation of almost any single member of the family did not result in any
informative phenotype under standard cultivation conditions, different approaches uncovered important key players in
plant developmental and physiological processes. Phenotypic analyses identified an Arabidopsis subtilase to be involved
in mucilage extrusion and hpRNA mediated silencing of closely related family members uncovered AtSBTs that act
redundantly in senescence associated processes. Ectopic expression identified an AtSBT to be involved in lateral root
initiation a hitherto poorly understood process. Proteomic analyses identified another AtSBT probably involved in the
initiation of seed storage protein degradation. Our current results emphasize the importance of subtilisin-like serine
proteases in plant growth and development.
212 The spd Mutants of Arabidopsis Disrupt Plastid Development in Embryo-Derived Cells
Nicholas Ruppel, Roger Hangarter
Indiana University
In plants, plastid development in seedling tissues can be critical for survival to adulthood. During germination, plastids
will develop to perform critical functions such as photosynthesis (chloroplasts) and gravity perception (amyloplasts). We
have identified a novel class of mutants in Arabidopsis that affect plastid development upon germination, specifically in
the embryo-derived tissues of the seedling. The mutants exhibit cotyledon and hypocotyl albinism upon germination due
to improper chloroplast development, whereas tissues derived from the shoot apical meristem are green and appear to
develop normal chloroplasts. Since the phenotype of these mutants is seen specifically in seedling tissues derived from the
embryo, they have been termed seedling plastid defective (spd) mutants. In this report, we describe the characterization of
spd2. When spd2 embryos are permitted to fully mature, less than 20% of seedlings survive to adulthood in the absence
of a supplemental carbon source. The SPD2 gene has been cloned and shown to encode for a plastid-targeted elongation
factor-G (EF-G). In prokaryotes, the EF-G is a critical component of the translation apparatus. SPD2 may therefore serve
a similarly important role in chloroplast translation. We have found that expression of SPD2 in a temperature-sensitive
E. coli EF-G mutant (fusA101) can rescue the wild-type phenotype, thus showing that SPD2 serves as a functional EF-
G. The EMS mutation that causes the spd2 phenotype is the result of a G to A base change, which converts a conserved
glycine residue to an arginine between the P-loop and Switch I regions of the GTP-binding domain found in elongation
factors. The amino acid change in spd2 may therefore disrupt the GTPase activity of the protein. Based on its mutant
phenotype and the role of EF-Gs in prokaryotes, the biological activity of SPD2 is most likely required for plastid protein
translation during late stages of embryogenesis and/or during germination. In summary, the spd2 mutant represents a
unique resource for studying critical, but poorly understood, aspects of plastid development that are an integral part of
embryo maturation and seedling establishment.