The mode of abscisic acid (ABA) action, and its relations to drought adaptive responses in particular, has been a captivating area of plant hormone research for much over a decade. The hormone triggers stomatal closure to limit water loss through transpiration, as well as mobilizes a battery of genes that presumably serve to protect the cells from the ensuing oxidative damage in prolonged stress. The signaling network orchestrating these various responses is, however, highly complex. This review summarizes several significant advances made within the last few years. The biosynthetic pathway of the hormone is now almost completely elucidated, with the latest identification of the ABA4 gene encoding a neoxanthin synthase, which seems essential for de novo ABA biosynthesis during water stress. This leads to the interesting question on how ABA is then delivered to perception sites. In this respect, regulated transport has attracted renewed focus by the unexpected finding of a shoot-to-root translocation of ABA during drought response, and at the cellular level, by the identification of a beta-galactosidase that releases biologically active ABA from inactive ABA-glucose ester. Surprising candidate ABA receptors were also identified in the form of the Flowering Time Control Protein A (FCA) and the Chloroplastic Magnesium Protoporphyrin-IX Chelatase H subunit (CHLH) in chloroplast-nucleus communication, both of which have been shown to bind ABA in vitro. On the other hand, the protein(s) corresponding to the physiologically detectable cell-surface ABA receptor(s) is (are) still not known with certainty. Genetic and physiological studies based on the guard cell have reinforced the central importance of reversible phosphorylation in modulating rapid ABA responses. Sucrose Non-Fermenting Related Kinases (SnRK), Calcium-Dependent Protein Kinases (CDPK), Protein Phosphatases (PP) of the 2C and 2A classes figure as prominent regulators in this single-cell model. Identifying their direct in vivo targets of regulation, which may include H(+)-ATPases, ion channels, 14-3-3 proteins and transcription factors, will logically be the next major challenge. Emerging evidence also implicates ABA as a repressor of innate immune response, as hinted by the highly similar roster of genes elicited by certain pathogens and ABA. Undoubtedly, the most astonishing revelation is that ABA is not restricted to plants and mosses, but overwhelming evidence now indicates that it also exists in metazoans ranging from the most primitive to the most advance on the evolution scale (sponges to humans). In metazoans, ABA has healing properties, and plays protective roles against both environmental and pathogen related injuries. These cross-kingdom comparisons have shed light on the surprising ancient origin of ABA and its attendant mechanisms of signal transduction.
SummaryThe late steps of both 16S and 5S ribosomal RNA maturation in the Gram-positive bacterium Bacillus subtilis have been shown to be catalysed by ribonucleases that are not present in the Gram-negative paradigm, Escherichia coli. Here we present evidence that final maturation of the 5Ј and 3Ј extremities of B. subtilis 23S rRNA is also performed by an enzyme that is absent from the Proteobacteria. Mini-III contains an RNase III-like catalytic domain, but curiously lacks the double-stranded RNA binding domain typical of RNase III itself, Dicer, Drosha and other well-known members of this family of enzymes. Cells lacking Mini-III accumulate precursors and alternatively matured forms of 23S rRNA. We show that Mini-III functions much more efficiently on precursor 50S ribosomal subunits than naked pre-23S rRNA in vitro, suggesting that maturation occurs primarily on assembled subunits in vivo. Lastly, we provide a model for how Mini-III recognizes and cleaves doublestranded RNA, despite lacking three of the four RNA binding motifs of RNase III.
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