[Show abstract][Hide abstract] ABSTRACT: Plant infections by the soil bacterium Agrobacterium rhizogenes result in neoplastic disease with the formation of hairy roots at the site of infection. Expression of a set of oncogenes residing on the stably integrated T-DNA is responsible for the disease symptoms. Besides the rol (root locus) genes, which are essential for the formation of hairy roots, the open reading frame orf13 mediates cytokinin-like effects, suggesting an interaction with hormone signaling pathways. Here we show that ORF13 induced ectopic expression of KNOX (KNOTTED1-like homeobox) class transcription factors, as well as of several genes involved in cell cycle control in tomato (Lycopersicon esculentum). ORF13 has a retinoblastoma (RB)-binding motif and interacted with maize (Zea mays) RB in vitro, whereas ORF13, bearing a point mutation in the RB-binding motif (ORF13*), did not. Increased cell divisions in the vegetative shoot apical meristem and accelerated formation of leaf primordia were observed in plants expressing orf13, whereas the expression of orf13* had no influence on cell division rates in the shoot apical meristem, suggesting a role of RB in the regulation of the cell cycle in meristematic tissues. On the other hand, ectopic expression of LeT6 was not dependent on a functional RB-binding motif. Hormone homeostasis was only altered in explants of leaves, whereas in the root no effects were observed. We suggest that ORF13 confers meristematic competence to cells infected by A. rhizogenes by inducing the expression of KNOX genes and promotes the transition of infected cells from the G1 to the S phase by binding to RB.
[Show abstract][Hide abstract] ABSTRACT: The regular arrangement of leaves around a plant's stem, called phyllotaxis, has for centuries attracted the attention of philosophers, mathematicians and natural scientists; however, to date, studies of phyllotaxis have been largely theoretical. Leaves and flowers are formed from the shoot apical meristem, triggered by the plant hormone auxin. Auxin is transported through plant tissues by specific cellular influx and efflux carrier proteins. Here we show that proteins involved in auxin transport regulate phyllotaxis. Our data indicate that auxin is transported upwards into the meristem through the epidermis and the outermost meristem cell layer. Existing leaf primordia act as sinks, redistributing auxin and creating its heterogeneous distribution in the meristem. Auxin accumulation occurs only at certain minimal distances from existing primordia, defining the position of future primordia. This model for phyllotaxis accounts for its reiterative nature, as well as its regularity and stability.
[Show abstract][Hide abstract] ABSTRACT: Auxin is of vital importance in virtually every aspect of plant growth and development, yet, even after almost a century of intense study, major gaps in our knowledge of its synthesis, distribution, perception, and signal transduction remain. One unique property of auxin is its polar transport, which in many well-documented cases is a critical part of its mode of action. Auxin is actively transported through the action of both influx and efflux carriers. Inhibition of polar transport by the efflux inhibitor N-1-naphthylphthalamic acid (NPA) causes a complete cessation of leaf initiation, a defect that can be reversed by local application of the auxin, indole-3-acetic acid (IAA), to the responsive zone of the shoot apical meristem. In this study, we address the role of the auxin influx carrier in the positioning and outgrowth of leaf primordia at the shoot apical meristem of tomato. By using a combination of transport inhibitors and synthetic auxins, we demonstrate that interference with auxin influx has little effect on organ formation as such, but prevents proper localization of leaf primordia. These results suggest the existence of functional auxin concentration gradients in the shoot apical meristem that are actively set up and maintained by the action of efflux and influx carriers. We propose a model in which efflux carriers control auxin delivery to the shoot apical meristem, whereas influx and efflux carriers regulate auxin distribution within the meristem.
The Plant Journal 12/2002; 32(4):509-17. · 6.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chloroplasts from 17-d-old pea leaves ( Pisum sativum L.) were isolated to elucidate the requirements for the light-induced degradation of stromal proteins. The influence of electron transport through the thylakoids and the influence of ATP on protein degradation were investigated. When chloroplasts were incubated in the light (45 μmol m−2s−1), glutamine synthetase, the large subunit of ribulose-1,5-bisphosphate carboxylase and glutamate synthase were degraded, whereas phosphoribulokinase, ferredoxin-NADP+ reductase and the 33 kDa protein of photosystem II remained more stable. Major protein degradation was not observed over 240 mm in darkness. The electron transport inhibitor dichlorophenyldimethylurea reduced protein degradation in the light over several hours, whereas dibromothymoquinone was less effective. Inhibiting the production of ATP with tentoxin or by destroying the δpH with the ionophores valinomycin and nigericin had no effect or even a stimulating influence on protein degradation when chloroplasts were exposed to light. Furthermore, adding ATP to chloroplasts incubated in the dark had no effect on proteolysis. From these results it is concluded that the transport of electrons through the thylakoids or photooxidative processes associated with it (especially in presence of DTT), rather than the availability of ATP caused the acceleration of stromal protein degradation by light in isolated pea chloroplasts.
[Show abstract][Hide abstract] ABSTRACT: The influence of waterlogging on leaf senescence and protein remobilisation was investigated in winter wheat (Triticum aestivum L., cv. Arina), which was grown in large pots embedded in the field in spring. The soil of intact pots was flooded permanently from anthesis to maturity, while in perforated control pots the soil was well aerated throughout the maturation period. No major effects of waterlogging were observed in the third leaf from the top, which was already senescing when the treatment was started. The degradation of proteins and chlorophyll was accelerated slightly in the second leaf from the top and considerably in the flag leaf. The contents of free amino acids in the uppermost leaf blades were lower on waterlogged soil than on control pots. Furthermore, amino acids in the blades did not accumulate during the earlier protein remobilisation observed on flooded soil, indicating that the export of amino acids via the phloem was fully functional. The loss of leaf proteins and the changes in the pattern of peptide hydrolases during senescence were similar in treated and control plants. Endopeptidase activities increased in an early phase of senescence at pH 5 and later also at pH 9, while aminopeptidase activities decreased. Therefore, nitrogen remobilisation was accelerated on flooded soil, but the senescence-related changes were very similar to those observed in control plants.
Plant and Soil 09/1994; 166(2):173-179. · 3.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Wheat plants (Triticum aestivum L., cv. Arina) growing in large pots (perforated at the bottom for controls, intact for flooding) were embedded in the field in spring. Waterlogging was initiated at anthesis and was maintained throughout the maturation period. Grain yield as well as potassium, phosphorus and magnesium contents in the shoot were decreased on flooded soil, while manganese and iron contents increased considerably. Total calcium and zinc contents per shoot remained comparable to those in controls. The reduction of potassium, phosphorus and magnesium contents by waterlogging was greatest in the grains, while manganese and iron accumulated mostly in the vegetative parts and the glumes. Zinc contents were also lowered in the grains during waterlogging due to an inhibited redistribution from the vegetative parts to the grains. Our results indicate that flooding caused not only an accumulation of manganese and iron in the shoot, but also affected the redistribution of macro- and micronutrients to the maturing gains.
Plant and Soil 01/1994; 160(1):87-95. · 3.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chloroplasts from pea leaves (Pisum sativum L.) were isolated to study the influence of activated oxygen species on stromal protein catabolism. Either lysed or intact chloroplasts were incubated in the light in the presence or absence of oxygen radical-generating systems. In lysed chloroplasts, phosphoglycolate phosphatase (EC 188.8.131.52), glutamine synthetase (EC 184.108.40.206) and the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 220.127.116.11) were cleaved when Fe, H2O2 and ascorbate were present in the incubation medium. In intact chloroplasts, stromal proteins were degraded when chloroplasts were exposed to light (45 μmol m−2s−1) at 25°C. High light (2700 μmol m−2s−1) or the addition of methyl viologen to the incubation medium accelerated the degradation of glutamine synthetase, phosphoglycolate phosphatase and the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase and increased the content of carbonyl groups in stromal proteins. These results suggest that the degradation of some stromal proteins is accelerated after exposure to free radicals of oxygen.