Understanding the function of bacterial and eukaryotic thiolases II by integrating evolutionary and functional approaches.

Acetoacetyl-CoA thiolase (EC 2.3.1.9), commonly named thiolase II, condenses two molecules of acetyl-CoA to give acetoacetyl-CoA and CoA. This enzyme acts in anabolic processes as the first step in the biosynthesis of isoprenoids and polyhydroxybutyrate in eukaryotes and bacteria, respectively. We have recently reported the evolutionary and functional equivalence of these enzymes, suggesting that thiolase II could be the rate limiting enzyme in these pathways and presented evidence indicating that this enzyme modulates the availability of reducing equivalents during abiotic stress adaptation in bacteria and plants. However, these results are not sufficient to clarify why thiolase II was evolutionary selected as a critical enzyme in the production of antioxidant compounds. Regarding this intriguing topic, we propose that thiolase II could sense changes in the acetyl-CoA/CoA ratio induced by the inhibition of the tricarboxylic acid cycle under abiotic stress. Thus, the high level of evolutionary and functional constraint of thiolase II may be due to the connection of this enzyme with an ancient and conserved metabolic route.

Somatic embryogenesis and plant regeneration capacity in Argentinean maize (Zea mays L. ) inbred lines

Somatic embryogenesis, which is still the method of choice for tissue culture, regeneration and transformation of maize, is largely considered highly genotype-dependent. The Hi II, a highly embryogenic genotype, has been extensively used in transformation protocols. However, this is not an inbred line; instead, it has a proportion of the undesirable A-188 background, and the progeny segregates for phenotypic characteristics and shows poor agronomic performance. In an effort to identify genotypes that combine a high somatic embryogenic response and good agronomic performance, we evaluated 48 advanced inbred lines developed at INTA. Callus development and somatic embryogenesis capacity were measured in 200 immature embryos per line. Embryogenic capacity [EC (mature somatic embryos/callus evaluated) x 100], Regeneration Capacity (RC) and Fertile Plant Recovery in greenhouse (FPR, fertile plants/regenerated plants) were recorded. A total of 17 lines reached an EC > 50 percent, and 14 out of those 17 lines regenerated seedlings. The FPR ranged between 50 and 100 percent. Also, we selected three promising lines with high agronomic performance, as alternatives to Hi II, in order to be included in a maize transformation scheme via somatic embryogenesis. In addition, we report the usefulness of Single Sequences Repeat (SSRs) in the determination of genetic diversity among 14 divergent lines for somatic embryogenesis response. The seven lines displaying good in vitro behaviour can be crossed to obtain hybrids combining desirable alleles for somatic embryogenesis response and different genetic backgrounds.