Aerobic fermentation during tobacco pollen development

In vegetative organs of plants, the metabolic switch from respiration to fermentation is dictated by oxygen availability. The two genes dedicated to ethanolic fermentation, pyruvate decarboxylase and alcohol dehydrogenase, are induced by oxygen deprivation and the gene products are active under oxygen stress. In pollen, these two genes are expressed in a stage-specific manner and transcripts accumulate to high levels, irrespective of oxygen availability. We have examined the expression pattern of pyruvate decarboxylase and alcohol dehydrogenase at the protein level in developing pollen and show that the active proteins are localized to the gametophytic tissue and begin to accumulate at microspore mitosis. A flux through the ethanolic fermentation pathway could already be detected very early in pollen development, occurring in all stages from premeiotic buds to mature pollen. This flux was primarily controlled not by oxygen availability, but rather by sugar supply. At a high rate of sugar metabolism, respiration and fermentation took place concurrently in developing and germinating pollen. We propose that aerobic fermentation provides a shunt from pyruvate to acetyl-CoA to accommodate the increased demand for energy and biosynthetic intermediates during pollen development and germination. A possible undesirable side-effect is the potential accumulation of toxic acetaldehyde. Our results support a model for cms-T-type male sterility in maize, in which degeneration of the tapetum is caused by the toxic effects of acetaldehyde on mitochondria weakened by the presence of the URF13 protei

Tnt1 retrotransposon tagging of STF in Medicago truncatula reveals tight coordination of metabolic, hormonal and developmental signals during leaf morphogenesis

Tnt1 (transposable element if Nicotiana tabaccum cell type 1) is one of the very few active LTR retrotransposons used for gene tagging in plants. In the model legume Medicago truncatula, Tnt1 has been effectively used as a gene knock-out tool to generate several very useful mutants. stenofolia (stf) is such a mutant identified by Tnt1 insertion in a WUSCHEL-like homeobox transcription factor. STF is required for blade outgrowth, leaf vascular patterning and female reproductive organ development in barrel medic and woodland tobacco. Using transcript profiling and metabolite analysis, we uncovered that mutant leaves are compromised in steady-state levels of multiple phytohormones, sugar metabolites and derivatives including flavonoids and polyamines. In the lam1 mutant (caused by deletion of the STF ortholog in Nicotiana sylvestris), while glucose, fructose, mannose, galactose, myo-inositol and aromatic aminoacids are dramatically reduced, sucrose is comparable to wild-type levels, and glutamine, proline, putrescine, nicotine and sorbitol are highly increased. We demonstrated that both stf and lam1 mutants accumulate reduced levels of free auxin and ABA in their leaves, and ectopic expression of STF in tobacco leads to auxin and cytokinin overproduction phenotypes including formation of tumors on the roots and crown. These data suggest that STF mediated integration of metabolic and hormonal signals are required for lateral organ morphogenesis and elaboration

<em>NODULE ROOT</em> and <em>COCHLEATA</em> Maintain Nodule Development and Are Legume Orthologs of Arabidopsis <em>BLADE-ON-PETIOLE</em> Genes

During their symbiotic interaction with rhizobia, legume plants develop symbiosis-specific organs on their roots, called nodules, that house nitrogen-fixing bacteria. The molecular mechanisms governing the identity and maintenance of these organs are unknown. Using Medicago truncatula nodule root (noot) mutants and pea (Pisum sativum) cochleata (coch) mutants, which are characterized by the abnormal development of roots from the nodule, we identified the NOOT and COCH genes as being necessary for the robust maintenance of nodule identity throughout the nodule developmental program. NOOT and COCH are Arabidopsis thaliana BLADE-ON-PETIOLE orthologs, and we have shown that their functions in leaf and flower development are conserved in M. truncatula and pea. The identification of these two genes defines a clade in the BTB/POZ-ankyrin domain proteins that shares conserved functions in eudicot organ development and suggests that NOOT and COCH were recruited to repress root identity in the legume symbiotic organ

A WD40 Repeat Protein from Medicago truncatula

WD40 repeat proteins regulate biosynthesis of anthocyanins, proanthocyanidins (PAs), and mucilage in the seed and the development of trichomes and root hairs. We have cloned and characterized a WD40 repeat protein gene from Medicago truncatula (MtWD40-1) via a retrotransposon-tagging approach. Deficiency of MtWD40-1 expression blocks accumulation of mucilage and a range of phenolic compounds, including PAs, epicatechin, other flavonoids, and benzoic acids, in the seed, reduces epicatechin levels without corresponding effects on other flavonoids in flowers, reduces isoflavone levels in roots, but does not impair trichome or root hair development. MtWD40-1 is expressed constitutively, with highest expression in the seed coat, where its transcript profile temporally parallels those of PA biosynthetic genes. Transcript profile analysis revealed that many genes of flavonoid biosynthesis were down-regulated in a tissue-specific manner in M. truncatula lines harboring retrotransposon insertions in the MtWD40-1 gene. MtWD40-1 complemented the anthocyanin, PA, and trichome phenotypes of the Arabidopsis (Arabidopsis thaliana) transparent testa glabrous1 mutant. We discuss the function of MtWD40-1 in natural product formation in M. truncatula and the potential use of the gene for engineering PAs in the forage legume alfalfa (Medicago sativa)