Influence of N-hydroxyurea on the growth of seedlings and the process of crown-gall tumour formation on sunflower plants

Hydroxyurea (HU) strongly inhibits formation of tumours induced with <i>Agrobacterium tumefaciens</i> in sunflower stems. This effect may partly be ascribed to the direct action of this substance on the bacterium. The course of the HU activity curve in the transformation process leads, however, to the supposition that it acts mainly on the host cells at the time corresponding to the induction phase. Maximal plant cell susceptibility to HU coincides with the wave of DNA synthesis induced by injury to the plant. Under the experimental conditions the time of HU activity in the tissues of the control test plant was limited, and the effects receded during its growth and development.

Changes in auxin level in the course of growth of a sunflower crown-gall suspension culture

The auxin level in the cell mass and culture medium was determined by means of the <em>Avena</em> straight caleoptile test in various periods of the suspension culture cycle of the sunflower crown-gall tumour. The investigations were performed in the course of the zero passage (PO) and first one (Pl), differing in their time of duration of maximum growth and its intensity. In both passages the intra- and extra-cellular auxin levels reach values of the same order. At the beginning of the maximal growth phase the activity corresponding to IAA in the cells prevails over that of the other auxin-like compounds. This disproportion diminishes with further development of the culture, and with the beginning of the stationary phase the cellular IAA level is lower than that of the remaining auxin-like compounds. The short phase of maximal growth (PO) occurs with an auxin level decreasing in the cell mass and increasing in the medium, and towards the end of the cycle these levels become equal. During the long phase of maximal growth (Pl) the total amount of auxins in the cells increases and is 2-3 times higher than in the medium, whereas IAA in the cells remains at a constant level. These results suggest that the participation of IAA in the intracellular pool of auxin-like substances is decisive for the mitotic activity of the cells and maintenance of growth in the culture.