No abstract

No abstract

Dehydrodiconiferyl alcohol glucosides (DCGs) Candidates for such a non-adenyl metabolite are the cell division-promoting substances found in fractions isolated from hormone-autonomous Catharanthus roseus crown gall tumors by . These tumors, now also known to synthesize zeatin and ZR (18), were recently reinvestigated ( 16). The fractions isolated by Wood et al.(29) were shown to contain DCG4 isomers capable of replacing cytokinin in the promotion of tobacco pith growth in culture. The individual DCG isomers differed in their ability to stimulate proliferation with isomers A and B (differing only in the enantiomeric nature of the phenylpropanoid dimer) being the most active (5). The relatively inactive isomers D and E differed from A and B only with respect to the location of the glucose moiety, at the C-13 hydroxyl and C'-4 hydroxyl, respectively (5). Moreover, the active DCGs were found in quiescent tobacco pith tissue at low levels (approximately 0.05 yM) but at 20-to 50-fold higher levels in actively growing cultured pith tissue (5). Such results indicated that these naturally occurring, phenolic derivatives could stimulate cell division. To demonstrate that the purified DCGs were, in fact, responsible for the biological activity observed, DCG isomers A and B (hereafter referred to as DCG A + B) have been synthesized chemically and their activities reexamined. The results presented here indicate that synthetic DCG A + B possess cell division-promoting activity but do not share several other biological activities with the adenine-derived cytokinins. In addition, we show that the accumulation of DCG A and DCG B in cultured tobacco pith explants is 4Abbreviations: DCG, dehydrodiconiferyl alcohol glucoside;

Agrobacterium tumefaciens initiates the expression of pathogenic genes (vir genes) in response to hostderived phenolic signals through a two-component regulatory system consisting of VirA and VirG. a-Bromoacetosyringone (ASBr) was developed as an inhibitor of this induction process and found to be a specific and irreversible inhibitor of vir gene induction in this pathogen. Formal replacement of one of the methoxy groups of ASBr with Iodine gave an equally effective inhibitor that could carry an 'mI label. We report here that the resulting radiolabeled inhibitor does not react with the sensory component of this system, VirA, either in vivo or in vitro. Rather, two small proteins, p1O and p2l, bind labeled inhibitor in vivo in a time period that is consistent with the exposure time required for the inhibition of vir gene expression. Labeling to these proteins was protected by preexposure to ASBr but not by a-bromo-3,5-dlmethoxyacetophenone, a compound of comparable chemical reactivity but previously shown not to inhibit vir gene expression. Our findings suggest that proteins that are not tumor-inducing plasmid-encoded mediate vir gene activation in a step prior to the VirA/VirG two-component regulatory system. Agrobacterium tumefaciens is the causative agent of crown gall tumors, a neoplastic disease of many dicotyledonous plants (1,2). A small oncogenic segment [transferred DNA (T-DNA)] of the tumor-inducing (Ti) plasmid harbored by these bacteria is transferred to the host cell, inducing the disease state. Many ofthe genes of the virulence regulon (vir) of the Ti plasmid required for this transfer are only expressed in the presence of host-derived phenolic compounds (3), one of which has been implicated in the control of division in the host cell (4-6).Sequence homology of two constitutively expressed genes of the vir regulon, virA and virG, with what now appears to be a large family of procaryotic two-component regulatory proteins (7) led to the proposal that these proteins served as the sensor/response elements controlling vir expression (8).This model predicts that VirA would interact with the phenolic inducer as the initiation step. Autophosphorylation in the C-terminal cytoplasmic domain of VirA (9, 10) and subsequent phosphate transfer to the transcriptional regulator, VirG (11), have been demonstrated in vitro and together constitute a phenol-activated signaling pathway. At least two other environmental stimuli, acidic pH and monosaccharides, act synergistically with the phenolic signals to activate vir gene expression (2).The dissection of the structures of these phenolic inducers has provided sufficient insight for the development of specific, irreversible inhibitors of vir gene expression (12). We now provide evidence that these inhibitors do not interact directly with VirA but rather with two other proteins, p10 and p21. We propose that these chromosomally encoded proteins modulate the signal transduction pathway prior to the autophosphorylation of VirA. MATERIALS AND METHODS Syntheses. a-Bro...

Striga asiatica (Scrophulariaceae), an obligate parasite of grasses including many of the world's major grain crops, switches from vegetative to parasitic development by the differentiation of the root meristem into the host attachment organ, the haustorium. This change was induced in culture by the exposure to a single, low molecular weight signal molecule, 2,6-dimethoxy-p-benzoquinone. A concentration of 10-6 molar quinone and an exposure time of >6 hours were required before the developmental process could be completed. With shorter exposure times, haustorial development was prematurely aborted and meristematic elongation was reestablished. The new meristem was capable of developing a second haustorium if reexposed to the signal molecule. These results are discussed in terms of the transition to the parasitic phase and the general control of plant cellular development.The initial gene expression events involved in the induction of both parasitic (7, 24) and symbiotic (14,19,20) relation-ships have been shown to be controlled in many cases by specific, structurally simple, signal molecules. These compounds must convey critical information required for host selection since, in most cases, the shift from a vegetative to a parasitic mode is both sudden and dramatic, requiring a significant recommitment of resources. The magnitude of this change is particularly striking in the parasitic angiosperms that are strictly dependent on the host for continued survival ( 15). In general, these plant holoparasites are dormant as seeds and apparently break dormancy only when exposed to a specific host-derived signal (10, 13). The change from the vegetative to a parasitic mode involved formation ofa primary haustorium, an organ specialized for host plant attachment.Several theories have been proposed for the mechanism of haustorial induction in parasitic plants, each generally involving haustorial inducing signals (2, 3, 8, 9, 12,17,26 suggests an active screening process on the part ofthe parasite and ensures intimate host-parasite association before the induction of haustoria.During primary haustorial induction, cellular expansion at the root meristem is redirected from longitudinal to radial dimensions followed closely by the formation of specialized haustorial hairs just distal to the swollen tip. The process is one of the most rapid differentiation processes known. Agalinis purpurea (Scrophulariaceae) has been shown to attach within 24 h following induction (22), and Striga attachment occurs at least as rapidly (8). Therefore, haustorial development is dependent to a large extent on cellular resources already present in the meristem. Nevertheless, the cost of this meristematic recommitment cannot be minimized. The redirection of meristematic growth results in a cessation of elongation. Therefore, host contact must be established for successful attachment (8, 9), and premature commitment to haustorial formation would certainly restrict the possibility of such contact. The importance of this differentiation event ...