T-DNA of Agrobacterium tumefaciens encodes an enzyme of cytokinin biosynthesis.

Abstract

Phytohormone overproduction in crown gall tumors is due to the expression of several T-DNA genes. The Infection of a wound site by Agrobacterium tumefaciens strains containing Ti (tumor-inducing) plasmids results in the neoplastic plant disease known as crown gall (1-5). In the plant genome, the integrated region (T-DNA) of the Ti plasmid expresses two phenotypic traits characteristic of transformed cells: phytohormone autonomy and opine synthesis. The T-DNA in tumors incited by octopine-type Ti plasmids encodes eight transcripts (6). Five of these map within three loci (tmr, tms, tml), which affect tumor morphology (7-11). Transposon inactivation of the tms locus (transcripts 1 and 2) or the tmr locus (transcript 4) results in tumors that proliferate shoots or roots, respectively. Inactivation of the tml locus (transcripts 6a and 6b) results in larger tumors on certain plant species. The region of T-DNA containing these loci is shown to be highly conserved in octopine and nopaline plasmids (12)(13)(14). The nucleotide sequences of the tmr (15,16) and tms (15,(17)(18)(19) genes have been recently determined and have been shown to be highly conserved. The synthesis of several proteins from the T-DNA region has been demonstrated in Escherichia coli minicells and in coupled transcription/translation systems of E. coli and A. tumefaciens (20). Three of these proteins had molecular weights identical to the predicted sizes of the tmr and tms gene products, as determined from the nucleotide sequence.Previous studies have also shown that the tms and tmr loci are involved in auxin and cytokinin metabolism, respectively (9-11, 21, 22). Tumors incited by tms mutants have decreased endogenous auxin levels (21) and are auxin dependent in culture (23,24

Cytokinins - recent news and views of evolutionally old molecules

“…The first identification of a gene encoding a CK de novo biosynthetic enzyme was carried out in the gall-forming bacteria Agrobacterium tumefaciens (Akiyoshi et al 1984;Barry et al 1984). This gene, designated Tmr (tumour morphology root), is located in the T-DNA of the tumour-inducing (Ti) plasmid and encodes an IPT that is primarly associated with synthesis of iPRMP from DMAPP and AMP (Blackwell and Horgan 1991), but with the same K m value that also recognises 4-hydroxy-3-methyl-2-(E)-butenyl diphosphate (HMBPP) as the side chain substrate .…”

Roles of Arabidopsis ATP/ADP isopentenyltransferases and tRNA isopentenyltransferases in cytokinin biosynthesis

“…This reaction is catalyzed by adenylate isopentenyltransferases (AMP IPTs) (19)(20)(21)(22). In plants, AtIPT4 isopentenylate ATP and ADP, forming isopentenyl ATP and isopentenyl ADP, using dimethylallyldiphosphate as the sidechain donor (23).…”

An alternative cytokinin biosynthesis pathway

“…This finding, and studies on the metabolism of isopentenyl-type cytokinins (7,8), led to the proposal that iPMP is also the primary cytokinin intermediate in plants, and zeatin cytokinins are formed by hydroxylation of iPMP and its derivatives (9, 10) ( Fig. 1).Later, the product of the T-DNA gene 4 (ipt) of the crown gall-forming bacterium, Agrobacterium tumefaciens, also was described as a DMAPP:AMP isopentenyltransferase (ipt) (11,12). AMP was found to be the preferred adenylic substrate for the Agrobacterium ipt enzyme but searches for alternative side-chain donors have been limited.…”