Arabidopsis BNT1, an atypical TIR-NBS-LRR gene, acting as a regulator of the hormonal response to stress

During their life cycle, plants have to cope with fluctuating environmental conditions. The perception of the stressful environmental conditions induces a specific stress hormone signature specifying a proper response with an efficient fitness. By reverse genetics, we isolated and characterized a novel mutation in Arabidopsis, associated with environmental stress responses, that affects the At5g11250/BURNOUT1 (BNT1) gene which encode a Toll/Interleukin1 receptor-nucleotide binding site leucine-rich repeat (TIR-NBS-LRR) protein. The knock-out bnt1 mutants displayed, in the absence of stress conditions, a multitude of growth and development defects, suchas severe dwarfism, early senescence and flower sterility, similar to those observed in vitro in wild type plants upon different biotic and/or abiotic stresses. The disruption of BNT1 causes also a drastic increase of the jasmonic, salicylic and abscisic acids as well as ethylene levels. Which was consistent with the expression pattern observed in bnt1 showing an over representation of genes involved in the hormonal response to stress? Therefore, a defect in BNT1 forced the plant to engage in an exhausting general stress response, which produced frail, weakened and poorly adapted plants expressing "burnout" syndromes. Furthermore, by in vitro phenocopying experiments, physiological, chemical and molecular analyses, we propose that BNT1 could represent a molecular link between stress perception and specific hormonal signature.

De novo shoot organogenesis: from art to science

In vitro shoot organogenesis and plant regeneration are crucial for both plant biotechnology and the fundamental study of plant biology. Although the importance of auxin and cytokinin has been known for more than six decades, the underlying molecular mechanisms of their function have only been revealed recently. Advances in identifying new Arabidopsis genes, implementing live-imaging tools and understanding cellular and molecular networks regulating de novo shoot organogenesis have helped to redefine the empirical models of shoot organogenesis and plant regeneration. Here, we review the functions and interactions of genes that control key steps in two distinct developmental processes: de novo shoot organogenesis and lateral root formation.

Role of vitronectin-like protein in Agrobacterium attachment and transformation of Arabidopsis cells

The role of plant vitronectin-like protein (Vn) in Agrobacterium-host plant interactions and receptor-specific bacterial attachment is unclear and still open to debate. Using a well-established Agrobacterium-mediated Arabidopsis transformation system, the marker gene beta-glucuronidase (GUS) of Escherichia coli, and biochemical and cytological methods, such as ELISA tests, immunoblots, immunolocalization, and functional in vitro binding assays, we have reassessed the role of Vn in receptor-specific bacterial attachment and transformation. We provide evidence that Vn is present in the host plant cells and anti-human vitronectin antibody cross-reacts with a 65-kDa protein from Arabidopsis cells. The specificity of the immunological cross-reactivity of anti-vitronectin antibodies was further demonstrated by ELISA competition experiments. Immunogold labeling showed that Vn is localized in the plant cell wall, and its level increased considerably after phytohormone treatment of the petiole explants. However, Agrobacterium attachment was unaffected, and no inhibition of petiole cell transformation was detected in the presence of human vitronectin and anti-vitronectin antibodies in the media. Additionally, no correlation between the occurrence of Vn, attachment of bacteria to the cells, and susceptibility to Agrobacterium-mediated transformation was observed. Taken together, our data do not support a functional role of plant Vn as the receptor for site-specific Agrobacterium attachment leading to the transformation of Arabidopsis cells.

Occurrence of circadian rhythms in hairy root cultures grown under controlled conditions

Hairy roots obtained by transformation via Agrobacterium rhizogenes provide an artificial plant material devoid of aerial parts with high growth on hormone-free media. Fundamental knowledge of hairy root physiology is essential to develop and control its culture. In contrast to shake-flask cultures, a bioreactor set-up combined with on-line data logging provides an efficient tool to study rapid physiological variations in hairy root cultures. Datura innoxia hairy roots were grown in a bioreactor equipped with on-line data analyses of pH, dissolved oxygen (pO2), conductivity, oxygen, and carbon dioxide. The experiments were done at a constant temperature and in the absence of light cues. The results obtained showed that the carbon dioxide evolution rate (CER) presented regular oscillations during the culture. Similar oscillations were also observed for the oxygen uptake rate (OUR). These signals were treated mathematically to look for the existence of a rhythm. An autocorrelation function was used to detect any periodic components. The results demonstrate that hairy root respiration exhibited peaks of 1 day. These oscillations, having a period of about 24 h, were also observed in pH and conductivity signals, although not for the pO2 signal. The data acquired in the absence of hairy roots showed that the observed periodic behavior was not an artifact. No effect on rhythms was observed by the imposition of an external "day/night" cycle. The fact that oscillations persisted in the absence of external stimuli, with a free-running period of 24 h, suggests that a circadian rhythm exists in hairy roots of D. innoxia.

Kinetic study of littorine rearrangement in Datura innoxia hairy roots by (13)C NMR spectroscopy

The kinetics of tropane alkaloid biosynthesis, particularly the isomerization of littorine into hyoscyamine, were studied by analyzing the kinetics of carbon-13 ((13)C) in metabolites of Datura innoxia hairy root cultures fed with labeled tropoyl moiety precursors. Both littorine and hyoscyamine were the major alkaloids accumulated, while scopolamine was never detected. Feeding root cultures with (RS)-phenyl[1,3-(13)C(2)]lactic acid led to (13)C spin-spin coupling detected on C-1' and C-2' of the hyoscyamine skeleton, which validated the intramolecular rearrangement of littorine into hyoscyamine. Label from phenyl[1-(13)C]alanine or (RS)-phenyl[1,3-(13)C(2)]lactic acid was incorporated at higher levels in littorine than in hyoscyamine. Initially, the apparent hyoscyamine biosynthesized rate (v(app)()hyo = 0.9 micromol (13)C.flask(-1).d(-1)) was lower than littorine formation (v(app)()litto = 1.8 micromol (13)C.flask(-1).d(-1)), suggesting that the isomerization reaction could be rate limiting. The results obtained for the kinetics of littorine biosynthesis were in agreement with the role of this compound as a direct precursor of hyoscyamine biosynthesis.

Monocotyledonous C4 NADP(+)-malate dehydrogenase is efficiently synthesized, targeted to chloroplasts and processed to an active form in transgenic plants of the C3 dicotyledon tobacco

Chloroplastic NADP(+)-malate dehydrogenase (cpMDH, EC 1.1.1.82) is a key enzyme in the carbon-fixation pathway of some C4 plants such as the monocotyledons maize or Sorghum. We have expressed cpMDH from Sorghum vulgare Pers. in transgenic tobacco (Nicotiana tabacum L.) (a dicotyledonous C3 plant) by using a gene composed of the Sorghum cpMDH cDNA under the control of cauliflower mosaic virus 35S promoter. High steady-state levels of cpMDH mRNA were observed in isogenic dihaploid transgenic tobacco lines. Sorghum cpMDH protein was detected in transgenic leaf extracts, where a threefold higher cpMDH activity could be measured, compared with control tobacco leaves. The recombinant protein was identical in molecular mass and in N-terminal sequence to Sorghum cpMDH. The tobacco cpMDH protein which has a distinct N-terminal sequence, could not be detected in transgenic plants. Immunocytochemical analyses showed that Sorghum cpMDH was specifically localized in transgenic tobacco chloroplasts. These data indicate that Sorghum cpMDH preprotein was efficiently synthesized, transported into and processed in tobacco chloroplasts. Thus, C3-C4 photosynthesis specialization or monocotyledon-dicotyledon evolution did not affect the chloroplastic protein-import machinery. The higher levels of cpMDH in transgenic leaves resulted in an increase of L-malate content, suggesting that carbon metabolism was altered by the expression of the Sorghum enzyme.

Characterization of competent cells and early events of Agrobacterium-mediated genetic transformation in Arabidopsis thaliana

The insertion of foreign DNA in plants occurs through a complex interaction between Agrobacteria and host plant cells. The marker gene β-glucuronidase of Escherichia coli and cytological methods were used to characterize competent cells for Agrobacterium-mediated transformation, to study early cellular events of transformation, and to identify the potential host-cell barriers that limit transformation in Arabidopsis thaliana L. Heynh. In cotyledon and leaf explants, competent cells were mesophyll cells that were dedifferentiating, a process induced by wounding and-or phytohormones. The cells were located either at the cut surface or within the explant after phytohormone pretreatment. In root explants, competent cells were present in dedifferentiating pericycle, and were produced only after phytohormone pretreatment. Irrespective of their origin, the competent cells were small, isodiametric with thin primary cell walls, small and multiple vacuoles, prominent nuclei and dense cytoplasm. In both cotyledon and root explants, histological enumeration and β-glucuronidase assays showed that the number of putatively competent cells was increased by preculture treatment, indicating that cell activation and cell division following wounding were insufficient for transformation without phytohormone treatment. Exposure of explants for 48 h to A. tumefaciens produced no characteristic stress response nor any gradual loss of viability nor cell death. However, in the competent cell, association between the polysaccharide of the host cell wall and that of the bacterial filament was frequently observed, indicating that transformation required polysaccharide-to-polysaccharide contact. Flow cytofluorometry and histological analysis showed that abundant transformation required not only cell activation (an early state exhibiting an increase in nuclear protein) but also cell proliferation (which in cotyledon tissue occurred at many ploidy levels). Noncompetent cells could be made competent with the appropriate phytohormone treatments before bacterial infection: this should aid analysis of critical steps in transformation procedures and should facilitate developing new strategies to transform recalcitrant plants.