Impact of Agrobacterium tumefaciens-induced stem tumors on NO3- uptake in Ricinus communis

Developing tumors induced by Agrobacterium tumefaciens, strain C58, on stem s of Ricinus communis L. var. gibsonii cv. Carmencita were shown to be stro ng metabolic sinks for sucrose and amino acids, thus causing higher nutrien t demand in the host plant. However, NO3- uptake and, to a lesser extent, a lso H2PO4- uptake were strongly inhibited. Correspondingly, NO3- concentrat ion was lower in tumorised than in the control plants. NO(3)(-)reductase ac tivity was the same in both plant types, but it was completely suppressed i n the tumors. The electrical membrane potential difference of root cells wa s unaffected in tumorised plants when soil-grown, but significantly lowered when grown hydroponically. Consistent with the low NO3- uptake rate, NO3-- dependent membrane depolarisation at the onset of NO3-/2H(+)-cotransport wa s nearly zero. In the phloem sap, sucrose and amino acid concentrations wer e considerably lower in tumorised than in control plants, and lower below t han above the tumor. The qualitative pattern of amino acids of the phloem s ap of stems was almost the same in tumorised and control plants. It is conc luded that neither the overall amino acid concentration nor special amino a cids nor ammonium in the transport phloem suppress NO3- uptake in the roots . Aminocyclopropane-carboxylate, the precursor of ethylene, which is produc ed in the tumors in high amounts, was low in the stems and the same in both plant types. Thus, ACC and ethylene were ruled out as directly interfering with nutrient uptake in the roots. Root morphology was strongly affected d uring tumor development. Root fresh weight decreased to 50% of the controls and lateral root development was almost completely prevented. This suggest s that the high tumor ethylene production, together with an increasing conc entration of phenolic compounds, severely inhibits the basipetal auxin flow to the roots. Auxin accumulation and retention was confirmed by specifical ly enhanced expression of the auxin-responsive promoter of the soybean gene GH3:GUS in tumors induced in transgenic Trifolium repens L. Hence, root de velopment is poorer and anion uptake inhibited in tumorised plants. This ma y be aggravated by abscisic acid accumulation in the tumor and its basipeta l export into the roots. Moreover, sucrose depletion of the sieve tubes lea ds to energy shortage at the root level for maintaining energy-dependent an ion uptake.