Induction of localized auxin response during spontaneous nodule development in Lotus japonicus

In leguminous plants, rhizobial infection of the epidermis triggers proliferation of cortical cells to form a nodule primordium. Recent studies have demonstrated that two classic phytohormones, cytokinin and auxin, have important functions in nodulation. The identification of these functions in Lotus japonicus was facilitated by use of the spontaneous nodule formation 2 (snf2) mutation of the putative cytokinin receptor LOTUS HISTIDINE KINASE 1 (LHK1). Analyses using snf2 demonstrated that constitutive activation of cytokinin signaling causes formation of spontaneous nodule-like structures in the absence of rhizobia and that auxin responses are induced in proliferating cortical cells during such spontaneous nodule development. Thus, cytokinin signaling positively regulates the auxin response. In the present study, we further investigated the induction of the auxin response using a gain-of-function mutation of Ca²⁺/calmodulin-dependent protein kinase (CCaMK) that causes spontaneous nodule formation. We demonstrate that CCaMK^T265D-mediated spontaneous nodule development is accompanied by a localized auxin response. Thus, a localized auxin response at the site of an incipient nodule primordium is essential for nodule organogenesis.     

The effect of auxin on cytokinin levels and metabolism in transgenic tobacco tissue expressing an ipt gene

The ipt gene from the T-DNA of Agrobacterium tumefaciens was transferred to tobacco (Nicotiana tabacum L.) in order to study the control which auxin appears to exert over levels of cytokinin generated by expression of this gene. The transgenic tissues contained elevated levels of cytokinins, exhibited cytokinin and auxin autonomy and grew as shooty calli on hormone-free media. Addition of 1-naphthylacetic acid to this culture medium reduced the total level of cytokinins by 84% while 6-benzylaminopurine elevated the cytokinin level when added to media containing auxin. The cytokinins in the transgenic tissue were labelled with ³H and auxin was found to promote conversion of zeatin-type cytokinins to ³H-labelled adenine derivatives. When the very rapid metabolism of exogenous [³H]zeatin riboside was suppressed by a phenylurea derivative, a noncompetitive inhibitor of cytokinin oxidase, auxin promoted metabolism to adenine-type compounds. Since these results indicated that auxin promoted cytokinin oxidase activity in the transformed tissue, this enzyme was purified from the tobacco tissue cultures. Auxin did not increase the level of the enzyme per unit tissue protein, but did enhance the activity of the enzyme in vitro and promoted the activity of both glycosylated and non-glycosylated forms. This enhancement could contribute to the decrease in cytokinin level induced by auxin. Studies of cytokinin biosynthesis in the transgenic tissues indicated that trans-hydroxylation of isopentenyladenine-type cytokinins to yield zeatin-type cytokinins occurred principally at the nucleotide level.Abbreviations Ade adenine - Ados adenosine - BA 6-benzylaminopurine - C control - Con A concanavallin A - CP cellulose phosphate - IPT isopentenyl transferase - NAA 1-naphthylacetic acid - NP normal phase - NPPU N-(3-nitrophenyl)-N-phenylurea - RIA radioimmunoassay - RP reversed phase We wish to thank Dr. J. Zwar for supplying phenylurea derivitives.     

Auxin levels and auxin binding protein availability inrolB transformedBeta vulgaris cells

The final biological effect of auxin depends both on free auxin levels and on auxin perception capacity.RolB transformedBeta vulgaris L. hairy roots provide a system for studying both factors. Highly purified plasma membrane fractions were prepared with aqueous two-phase partitioning. Individual hairy root clones were assessed for the binding activities of plasma membrane-bound auxin binding proteins and for their free intracellular indole-3-acetic acid levels. The presence of a high affinity auxin binding protein with a dissociation constant of 9.07 x 10^?7 M was detected in the plasma membrane fractions isolated from non-transformed seedling roots and the six clones ofrolB transformed hairy roots. However, the levels of specific IAA binding considerably varied among different hairy root clones and between transformed and non-transformed roots. The levels of the detectable polypeptide in immunoblotting with an antibody against maize 22-kD auxin binding protein subunit were in good agreement to the levels that were detected in auxin binding assays. Differences in the indole-3-acetic acid levels were found between transformed and non-transformed roots and also between different transformed hairy root clones. A negative correlation was observed between free intracellular IAA levels and its specific binding to the plasma membrane-bound auxin binding proteins. A latency study indicated that the binding site for auxin may be located on the exterior face of the plasma membrane     

Phosphorylation of membrane-located proteins of soybean in vitro and response to auxin

Isolated membranes of soybean incorporate ³²P from γ-[³²P]ATP in vitro. The incorporation was rapid and did not require added calcium. When displayed on 10% sodium dodecyl sulfate-polyacrylamide gels, several protein bands were revealed. An apparent auxin (2,4-dichlorophenoxyacetic acid) stimulation of ³²P incorporation into material from membrane vesicles insoluble in trichloroacetic acid-perchloric acid may be reflected partly in enhanced incorporation into protein bands with apparent molecular weights of 45,000 and 50,000. Additionally, a low molecular weight component was sometimes observed where incorporation was stimulated 2- to 3-fold by auxin. However, protein-bound radioactivity represented only a small fraction of the total radioactivity of the acid-insoluble material. Other labeled constituents, not retained on the gels, may contribute to the apparent, rapid (10 s or less) auxin response of the isolated membranes. Stimulation of incorporation into the low molecular weight component was given by diglyceride plus calcium, constituents known to augment protein kinase activities in other systems.     

The Diageotropica Gene Differentially Affects Auxin and Cytokinin Responses throughout Development in Tomato

The interactions between the plant hormones auxin and cytokinin throughout plant development are complex, and genetic investigations of the interdependency of auxin and cytokinin signaling have been limited. We have characterized the cytokinin sensitivity of the auxin-resistant diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) in a range of auxin- and cytokinin-regulated responses. Intact, etiolated dgt seedlings showed cross-resistance to cytokinin with respect to root elongation, but cytokinin effects on hypocotyl growth and ethylene synthesis in these seedlings were not impaired by the dgt mutation. Seven-week-old, green wild-type and dgt plants were also equally sensitive to cytokinin with respect to shoot growth and hypocotyl and internode elongation. The effects of cytokinin and the dgt mutation on these processes appeared additive. In tissue culture organ regeneration from dgt hypocotyl explants showed reduced sensitivity to auxin but normal sensitivity to cytokinin, and the effects of cytokinin and the mutation were again additive. However, although callus induction from dgt hypocotyl explants required auxin and cytokinin, dgt calli did not show the typical concentration-dependent stimulation of growth by either auxin or cytokinin observed in wild-type calli. Cross-resistance of the dgt mutant to cytokinin thus was found to be limited to a small subset of auxin- and cytokinin-regulated growth processes affected by the dgt mutation, indicating that auxin and cytokinin regulate plant growth through both shared and separate signaling pathways.     

Purification and Characterization of a Low-Molecular-Weight Phospholipase A2 from Developing Seeds of Elm

Phospholipase A₂ (PLA₂) was purified about 180,000 times compared with the starting soluble-protein extract from developing elm (Ulmus glabra) seeds. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis the purified fraction showed a single protein band with a mobility that corresponded to 15 kD, from which activity could be recovered. When analyzed by matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry, the enzyme had a deduced mass of 13,900 D. A 53-amino acid-long N-terminal sequence was determined and aligned with other sequences, giving 62% identity to the deduced amino acid sequence of some rice (Oryza sativa) expressed sequence tag clones. The purified enzyme had an alkaline pH optimum and required Ca²⁺ for activity. It was unusually stable with regard to heat, acidity, and organic solvents but was sensitive to disulfide bond-reducing agents. The enzyme is a true PLA₂, neither hydrolyzing the sn-1 position of phosphatidylcholine nor having any activity toward lysophosphatidylcholine or diacylglycerol. The biochemical data and amino acid sequence alignments indicate that the enzyme is related to the well-characterized family of animal secretory PLA₂s and, to our knowledge, is the first plant enzyme of this type to be described.     

Cytokinin binding proteins from mammalian sera

A cytokinin-binding protein fraction was isolated from normal rabbit sera by affinity chromatography. The protein fraction bound tritium labelled N⁶- (δ2-risopentenyl) adenosine and the order of inhibition of this binding by competing non-radioactive compounds was, N⁶-(δ²-isopentenyl) adenosine < N⁶-benzyIadenosine < zeatin-riboside > N⁶-(δ²-isopentenyl) adenine < kinetin riboside > adenosine. The protein fraction showed broad specificity, the prefered cytokinin being N⁶-(δ²-isopentenyl) adenosine. This is the first report of the isolation of cytokinin binding proteins from mammalian sources.     

The influence of cytokinin-auxin cross-regulation on cell-fate determination in Arabidopsis thaliana root development

Root growth and development in Arabidopsis thaliana are sustained by a specialised zone termed the meristem, which contains a population of dividing and differentiating cells that are functionally analogous to a stem cell niche in animals. The hormones auxin and cytokinin control meristem size antagonistically. Local accumulation of auxin promotes cell division and the initiation of a lateral root primordium. By contrast, high cytokinin concentrations disrupt the regular pattern of divisions that characterises lateral root development, and promote differentiation. The way in which the hormones interact is controlled by a genetic regulatory network. In this paper, we propose a deterministic mathematical model to describe this network and present model simulations that reproduce the experimentally observed effects of cytokinin on the expression of auxin regulated genes. We show how auxin response genes and auxin efflux transporters may be affected by the presence of cytokinin. We also analyse and compare the responses of the hormones auxin and cytokinin to changes in their supply with the responses obtained by genetic mutations of SHY2, which encodes a protein that plays a key role in balancing cytokinin and auxin regulation of meristem size. We show that although shy2 mutations can qualitatively reproduce the effect of varying auxin and cytokinin supply on their response genes, some elements of the network respond differently to changes in hormonal supply and to genetic mutations, implying a different, general response of the network. We conclude that an analysis based on the ratio between these two hormones may be misleading and that a mathematical model can serve as a useful tool for stimulate further experimental work by predicting the response of the network to changes in hormone levels and to other genetic mutations.     

Auxin and cytokinin levels in selected and temperature-induced morphologically distinct tissue lines of tobacco crown gall tumors

Two morphologically distinct tissue culture lines were selected from a tobacco (Nicotiana tabacum L. ‘Wisconsin 38’) crown gall tumor: (1) a green teratomous line producing numerous abnormal shoots, designated leafy; and (2) a white, hard, spherical tissue, designated white. These were originally isolated at 27°C, but temperature changes were also found to control these morphologies, with 33°C favoring the development of the leafy form and 21°C favoring the white form. The higher temperature was also correlated with a decrease in the endogenous levels of both auxin and cytokinin, whereas the lower temperature was correlated with an increase in the levels of these growth regulators. There was no significant difference, however, in the auxin and cytokinin levels in the two forms maintained at 27°C, suggesting that the basis for morphological changes at this temperature resides either in very subtle differences in auxin and cytokinin or in factors other than these two growth regulators.     

Flavonoids and Auxin Transport Inhibitors Rescue Symbiotic Nodulation in the Medicago truncatula Cytokinin Perception Mutant cre1

Initiation of symbiotic nodules in legumes requires cytokinin signaling, but its mechanism of action is largely unknown. Here, we tested whether the failure to initiate nodules in the Medicago truncatula cytokinin perception mutant cre1 (cytokinin response1) is due to its altered ability to regulate auxin transport, auxin accumulation, and induction of flavonoids. We found that in the cre1 mutant, symbiotic rhizobia cannot locally alter acro- and basipetal auxin transport during nodule initiation and that these mutants show reduced auxin (indole-3-acetic acid) accumulation and auxin responses compared with the wild type. Quantification of flavonoids, which can act as endogenous auxin transport inhibitors, showed a deficiency in the induction of free naringenin, isoliquiritigenin, quercetin, and hesperetin in cre1 roots compared with wild-type roots 24 h after inoculation with rhizobia. Coinoculation of roots with rhizobia and the flavonoids naringenin, isoliquiritigenin, and kaempferol, or with the synthetic auxin transport inhibitor 2,3,5,-triiodobenzoic acid, rescued nodulation efficiency in cre1 mutants and allowed auxin transport control in response to rhizobia. Our results suggest that CRE1-dependent cytokinin signaling leads to nodule initiation through the regulation of flavonoid accumulation required for local alteration of polar auxin transport and subsequent auxin accumulation in cortical cells during the early stages of nodulation.     

Kinetics of Determination in the Differentiation of Isolated Mesophyll Cells of Zinnia elegans to Tracheary Elements

Mechanically isolated mesophyll cells of Zinnia elegans L. cv Envy differentiate to tracheary elements when cultured in inductive medium containing 0.5 micromolar α-naphthaleneacetic acid and 0.5 micromolar benzyladenine. The cells do not differentiate when cultured in medium in which the concentration of auxin and/or cytokinin has been reduced to 0.005 micromolar. Cells require an initial 24-hour exposure to inductive cytokinin and 56-hour exposure to inductive auxin for differentiation at 72 hours of culture. Freshly isolated Zinnia cells can be maintained in medium having low concentrations of both auxin and cytokinin for only 1 day without significant loss of potential to differentiate upon transfer to inductive medium. Initial culture for up to 2 days in medium having high auxin and low cytokinin, or low auxin and high cytokinin, allows full differentiation on the third day after transfer to inductive medium and potentiates the early differentiation of some cells.     

Analysis of gametophytic development in the moss,Physcomitrella patens,using auxin and cytokinin resistant mutants

Mutants altered in their response to auxins and cytokinins have been isolated in the moss Physcomitrella patens either by screening clones from mutagenized spores for growth on high concentrations of cytokinin or auxin, in which case mutants showing altered sensitivities can be recognized 3–4 weeks later, or by non-selective isolation of morphologically abnormal mutants, some of which are found to have altered sensitivities. Most of the mutants obtained selectively are also morphologically abnormal. The mutants are heterogeneous in their responses to auxin and cytokinin, and the behaviour of some is consistent with their being unable to make auxin, while that of others may be due to their being unable to synthesize cytokinin. Physiological analysis of the mutants has shown that both endogenous auxin and cytokinin are likely to play important and interdependent roles in several steps of gametophytic development. Although their morphological abnormalities lead to sterility, genetic analysis of some of the mutants has been possible by polyethyleneglycol induced protoplast fusion.Abbreviations NTG N-methyl-N-nitro-N-nitrosoguanidine - NAA 1-naphthalene acetic acid - 2,4D 2,4-dichlorophenoxyacetic acid - BAP 6-benzylaminopurine - IAP 6-( ²isopentenyl) aminopurine - NAR NAA resistant mutants - BAR BAP resistant mutants     

A high affinity binding site for cytokinin to a particulate fraction in carrot suspension cells

Summary Carrot suspension cells contain one class of high affinity binding sites for cytokinin in an 80,000 × g particulate fraction. Binding of [8-¹⁴C]-benzylaminopurine (BA) to this fraction assayed by a sedimentation method was found to be optimal at pH 6.0 and thermolabile. Specific binding was proved in competition experiments in which labelled BA was displaced by increasing concentrations of unlabelled BA. Scatchard plots of these results displayed a dissociation constant (K_d) of 33 ± 6 nM. The number of binding sites found was 1,100 ± 120 fmol g^–1 fresh weight which is equivalent to a frequency of 23,000 binding sites per cell. The specificity of the binding sites to cytokinins and their analogues followed the sequence BA with highest affinity, kinetin, zeatin, iP and adenine. The cytokinin ribosides generally had a lower affinity than their cytokinin bases, and the affinity decreased in the order [9 R] BA, [9 R] iP, [9 R] Z, [9 R] A.     

DAR2 acts as an important node connecting cytokinin,auxin, SHY2 and PLT1/2 in root meristem size control

Cytokinin and auxin antagonistically affect cell proliferation and differentiation and thus regulate root meristem size by influencing the abundance of SHORT HYPOCOTYL2 (SHY2/IAA3). SHY2 affects auxin distribution in the root meristem by repressing the auxin-inducible expression of PIN-FORMED (PIN) auxin transport genes. The PLETHORA (PLT1/2) genes influence root meristem growth by promoting stem cells and transit-amplifying cells. However, the factors connecting cytokinin, auxin, SHY2 and PLT1/2 are largely unknown. In a recent study, we have shown that the DA1-related protein 2 (DAR2) acts downstream of cytokinin and SHY2 but upstream of PLT1/2 to affect root meristem size. Here, we discuss the possible molecular mechanisms by which Arabidopsis DAR2 controls root meristem size.     

Hormonal Control of Gene Expression During Reactivation of the Cell Cycle in Tobacco Mesophyll Protoplasts

The roles of auxin and cytokinin in cell cycle reactivation were studied during the first 48 h of culture of mesophyll protoplasts of Nicotiana tabacum. Using hormone delay and withdrawal studies we found that auxin was required by 0–4 h of culture, whereas cytokinin was not required until hour 10–12, which is 6–10 h before S phase. Cycloheximide blocks division, indicating that protein synthesis is required. In an effort to detect a molecular response to either hormone, we examined the expression of the cell cycle marker, cdc2. Cdc2 expression was detected by 12 h of culture, coincident with the timing of the cytokinin requirement and well before the entry into S. However, cdc2 was partially induced by either auxin or cytokinin alone, suggesting that cdc2 expression is not the primary target of either hormone. Our hormone delay experiments suggest that there are separate signal transduction pathways leading from auxin and from cytokinin to reactivation of the cell cycle and that these pathways converge before S. The underlying mechanisms for these distinct pathways remain to be elucidated. Received November 4, 1997; accepted October 7, 1998