A sensitive radioimmunoassay for isopentenyladenosine

Phytohormones in the formation of crown gall tumors

Crown gall tumors were initiated in a variety of plant species by infection with Agrobacterium tumefaciens strain B6 and the concomitant changes in the tissue levels of phytohormones, mainly indole-3-acetic acid (IAA) and cytokinins, were analyzed. A comparison was made of these hormones with those produced by virulent and avirulent strains of the bacterium in liquid culture and with those of bacteria-free crown gall callus cultures. Specific radioimmunoassays were employed for hormone determinations. An assay for the quantitation of femto-mol amounts of isopentenyladenosine and related cytokinins was newly developed and is described in detail. The results can be summarized as follows: Virulence in strain B 6 is associated with the ability to release trans-zeatin and increased amounts of IAA into the surrounding environment. In many, but not all plants analyzed, the development of crown gall tumors is also associated with a sharp rise in the levels of trans-zeatin-type zytokinins and IAA (e.g., Euphorbia lathyris, Catharanthus roseus). Crown gall calli growing on hormone-free media varied greatly in their cytokinin levels. In a culture of Nicotiana tabacum, both trans-zeatin and isopentenyladenine or related cytokinins were not detected. Thus, tumor growth cannot be explained on the basis of elevated levels of IAA and/or cytokinins alone.

Specificity of deoxycytidylate antibodies

Antibodies to deoxycytidylate (dpC) were elicited in rabbits using a thyroglobulin (Tg) conjugate of dpC. The specificity of the antibodies was determined by measuring the inhibition of the binding of [3H]dpC to the antibodies by various non-radioactive nucleotides or derivatives. The antibodies were found to distinguish dpC from pC and DNA from RNA, probably due to their specificity for the conformation of the deoxycytidylate.

Radioimmunoassays for trans-zeatin and related cytokinins

Radioimmunoassays for the quantitation of trans-zeatin and related cytokinins have been developed. Antisera produced against bovine serum albumin conjugates of trans-zeatinriboside have a high affinity (Ka=2.4.10(-11) M) for zeatinriboside and for zeatin, but show a negligible cross reaction to isopentenyladenosine (0.1%) and cis-zeatinriboside (0.4%), only a slight cross reaction to dihydrozeatin (1.7%), and no cross reaction at all to other purines, such as adenosine and related, compounds, was observed. The assays are sensitive and measuring ranges extend from 0.06-30 pmol (0.02-10 ng) of zeatinriboside. This has been achieved by employing as tracers immunoreactive zeatin derivatives with high-specific activity, (tritiated zeatinriboside-dialcohol: 8.37.10(11) Bq mmol(-1) and zeatinribosyl-[(125)I]tyramine: ca. 1.9.10(13) Bq mmol(-1). The detection limit is 40 fmol (15pg) for the assay employing the tritiated tracer, and assay reproducibility is high (variation coefficients of triplicates less than 5%). Several hundred assays can be completed in one day, and, due to the high specificity of this assay, crude extracts may be used for analysis. The course of zeatin levels in developing fruits of Lycopersicon esculentum cv. Moneymaker is given.