Intracellular redox status is a critical parameter determining plant development in response to biotic and abiotic stress. Thioredoxin (TRX) and glutathione are key regulators of redox homeostasis, and the TRX and glutathione pathways are essential for postembryonic meristematic activities. Here, we show by associating TRX reductases (ntra ntrb) and glutathione biosynthesis (cad2) mutations that these two thiol reduction pathways interfere with developmental processes through modulation of auxin signaling. The triple ntra ntrb cad2 mutant develops normally at the rosette stage, undergoes the floral transition, but produces almost naked stems, reminiscent of the phenotype of several mutants affected in auxin transport or biosynthesis. In addition, the ntra ntrb cad2 mutant shows a loss of apical dominance, vasculature defects, and reduced secondary root production, several phenotypes tightly regulated by auxin. We further show that auxin transport capacities and auxin levels are perturbed in the mutant, suggesting that the NTR-glutathione pathways alter both auxin transport and metabolism. Analysis of ntr and glutathione biosynthesis mutants suggests that glutathione homeostasis plays a major role in auxin transport as both NTR and glutathione pathways are involved in auxin homeostasis.

SUMMARYFew regulators of phenylpropanoids have been identified in monocots having potential as biofuel crops. Here we demonstrate the role of the maize (Zea mays) R2R3-MYB factor ZmMYB31 in the control of the phenylpropanoid pathway. We determined its in vitro consensus DNA-binding sequence as ACC T / A ACC, and chromatin immunoprecipitation (ChIP) established that it interacts with two lignin gene promoters in vivo. To explore the potential of ZmMYB31 as a regulator of phenylpropanoids in other plants, its role in the regulation of the phenylpropanoid pathway was further investigated in Arabidopsis thaliana. ZmMYB31 downregulates several genes involved in the synthesis of monolignols and transgenic plants are dwarf and show a significantly reduced lignin content with unaltered polymer composition. We demonstrate that these changes increase cell wall degradability of the transgenic plants. In addition, ZmMYB31 represses the synthesis of sinapoylmalate, resulting in plants that are more sensitive to UV irradiation, and induces several stress-related proteins. Our results suggest that, as an indirect effect of repression of lignin biosynthesis, transgenic plants redirect carbon flux towards the biosynthesis of anthocyanins. Thus, ZmMYB31 can be considered a good candidate for the manipulation of lignin biosynthesis in biotechnological applications.

In eukaryotes the primary cleavage of the precursor rRNA (pre-rRNA) occurs in the 5 external transcribed spacer (5ETS). In Saccharomyces cerevisiae and animals this cleavage depends on a conserved U3 small nucleolar ribonucleoprotein particle (snoRNP), including fibrillarin, and on other transiently associated proteins such as nucleolin. This large complex can be visualized by electron microscopy bound to the nascent pre-rRNA soon after initiation of transcription. Our group previously described a radish rRNA gene binding activity, NF D, that specifically binds to a cluster of conserved motifs preceding the primary cleavage site in the 5ETS of crucifer plants including radish, cauliflower, and Arabidopsis thaliana (D. Caparros-Ruiz, S. Lahmy, S. Piersanti, and M. Echeverria, Eur. J. Biochem. 247:981-989, 1997). Here we report the purification and functional characterization of NF D from cauliflower inflorescences. Remarkably NF D also binds to 5ETS RNA and accurately cleaves it at the primary cleavage site mapped in vivo. NF D is a multiprotein factor of 600 kDa that dissociates into smaller complexes. Two polypeptides of NF D identified by microsequencing are homologues of nucleolin and fibrillarin. The conserved U3 and U14 snoRNAs associated with fibrillarin and required for early pre-rRNA cleavages are also found in NF D. Based on this it is proposed that NF D is a processing complex that assembles on the rDNA prior to its interaction with the nascent pre-rRNA.The rRNAs 18S, 5.8S, and 25S are encoded by tandemly repeated single transcriptional units. Transcription of these units by RNA polymerase I (RNA Pol I) produces a primary transcript (pre-rRNA) containing the rRNA flanked by external spacer sequences (ETS) and internal spacer sequences. The pre-rRNA is then subjected to a complex maturation process that involves the accurate removal of the spacers and the modification of numerous rRNA residues (49). In vivo, all these events are accomplished by large small nucleolar ribonucleoprotein particle (snoRNP) complexes that transiently interact with the pre-rRNA in the nucleolus (47).One of the earliest processing events on the pre-rRNA is an endonucleolytic cut in the 5ЈETS upstream from the 18S rRNA. This primary pre-RNA cleavage is conserved in all eukaryotes, but its position within the 5ЈETS is distinct in each species. In Saccharomyces cerevisiae it occurs at site A0, located 90 nucleotides upstream from the 5Ј end of the 18S rRNA (49). Genetic studies have shown that A0 cleavage depends on the U3 snoRNP, a large complex containing the C/D snoRNA U3 and associated nucleolar proteins (16). This implicates a basepair interaction of U3 with the 5ЈETS that may stabilize or promote a pre-RNA structure required for processing and subsequent production of 18S rRNA (5, 6). The U3 snoRNP proteins, including Nop1p (fibrillarin in vertebrates), and three other core proteins associated with all C/D snoRNAs are essential for this cleavage. In addition, there are proteins which are specific to U3 snoRNP and are not fou...