Unraveling the function of bacterial-type phosphoenolpyruvate carboxylase in vascular plants

Two distinct phosphoenolpyruvate carboxylase (PEPC) isozymes occur in vascular plants and green algae: plant-type PEPC (PTPC) and bacterial-type PEPC (BTPC). PTPC polypeptides typically form a tightly regulated cytosolic Class-1 PEPC homotetramer. BTPCs, however, appear to be less widely expressed and to exist only as catalytic and regulatory subunits that physically interact with co-expressed PTPC subunits to form hetero-octameric Class-2 PEPC complexes that are highly desensitized to Class-1 PEPC allosteric effectors. Yeast two-hybrid studies indicated that castor plant BTPC (RcPPC4) interacts with all three Arabidopsis thaliana PTPC isozymes, and that it forms stronger interactions with AtPPC2 and AtPPC3, suggesting that specific PTPCs are preferred for Class-2 PEPC formation. In contrast, Arabidopsis BTPC (AtPPC4) appeared to interact very weakly with AtPPC2 and AtPPC3, suggesting that BTPCs from different species may have different physical properties, hypothesized to be due to sequence dissimilarities within their ~10 kDa intrinsically disordered region. Recent RNA-seq and microarray data were analyzed to obtain a better understanding of BTPC expression patterns in different tissues of various monocot and dicot species. High levels of BTPC transcripts, polypeptides and Class-2 PEPC complexes were originally discovered in developing castor seeds, but the analysis revealed a broad range of diverse tissues where abundant BTPC transcripts are also expressed, such as the developing fruits of cucumber, grape, and tomato. Marked BTPC expression correlated well with the presence of ~116 kDa immunoreactive BTPC polypeptides, as well as Class-2 PEPC complexes in the immature fruit of cucumbers and tomatoes. It is therefore hypothesized that in vascular plants BTPC and thus Class-2 PEPC complexes maintain anaplerotic PEP flux in tissues with elevated malate levels that would potently inhibit ‘housekeeping’ Class-1 PEPCs. Elevated levels of malate can be used by biosynthetically active sink tissues such as immature tomatoes and cucumbers for rapid cell expansion, drought or salt stressed roots for osmoregulation, and developing seeds and pollen as a precursor for storage lipid and protein biosynthesis.

The Composition of Holocene Aeolianites; Western Eyre Peninsula, Great Australian Bight, South Australia

The Australian southern continental margin is the world’s largest site of cool-water carbonate deposition, and the Great Australian Bight is its largest sector. The Eyre Peninsula is fringed by coastal beaches with aeolianites and marks the eastern edge of the Great Australian Bight. Five shoreline transects of varying lengths spanned a 150km longitudinal distance and at each the intertidal, beach, dune and secondary dune environments were sampled, for a total of 18 samples. Sediments are a mixture of modern, relict, and Cenozoic carbonates, and quartz grains. Carbonate aeolianites on the western Eyre Peninsula are mostly composed of modern carbonate grains: predominantly molluscs (23-33%) and benthic foraminifera (10-26%), locally abundant coralline algae (3-28%), echinoids (2-22%), and bryozoans (2-14%). Cenozoic grain abundance ranges from 1-6% whereas relict grain abundance ranges from 0-17%. A southward increase in bryozoan particles correlates with a nutrient element abundance and decrease in temperature due to a large seasonal coastal upwelling system that drives 2-3 major upwelling events per year, bringing cold, nutrient rich, Sub-Antarctic Surface Water (<12°C) onto the shelf. In southern, mostly wind protected locations, the beach and dune sediment compositions are similar, indicating that wind energy has successfully carried all sediment components of the beach into the adjacent dunes. In northern, exposed locations, the composition is not the same everywhere, and trends indicate that relative wind energy has the ability to impact grain composition through preferential wind transport. Aeolianite composition is therefore a function of both upwelling and the degree of coastal exposure.