1988. Competitive inhibition of the auxin-induced elongation by a-D-oligogalacturonides in pea stem segments. -Physiol. Plant. 72: 499-504.a-D-galacturonide oligomers (OG) were prepared by partial hydrolysis of sodium polypectate with an homogeneous Aspergillus niger endopolygalacturonase (EC 3.2.1.15). OG, obtained after digestion for 10, 20, 30, 60, 120 min and 24 h, were assayed for their ability to interfere with the IAA-induced elongation of pea (Pisum sativum L. cv. Alaska) stems. Maximum inhibiting activity was exhibited by oligomers with an approximate degree of polymerization higher than 8. Inhibition by longer O G was much lower, and the products of the 24 h digestion and the unhydrolysed polypectate were ineffective. The addition of O G to pea stems caused a parallel shift to the right of the IAA dose-effect curve. The shift depended on the amount of OG used, showing that oligogalacturonides behave as competitive antagonists of IAA. The presence of OG caused the disappearance of the second maximum of the elongation rate and reduced the first maximum. O G were also tested for their ability to inhibit IAA-induced ethylene evolution of pea stem segments. Maximal inhibition was obtained with OG of the same size as those that interfered with IAA-induced elongation. Inhibition of the auxin action seemed to be specific as O G did not interfere with the activity of gibberellic acid (GA,) or kinetin. It was concluded that oligogalacturonides strongly interfere with the activity of IAA, although they are by themselves incapable to influence the elongation of pea stem segments directly.
Leaf explants of hairy root tobacco (Nicotiana tabacum) regenerants characteristically differentiate roots from the wound margins on hormonefree medium. The same response can be elicited on normal tobacco by culturing the explants in the presence of auxin. We show here that the spontaneous rooting of transformed plants is neither due to the activity of right T-DNA-borne auxin genes nor to a substantially altered balance of endogenous hormones. Rather, an increased sensitivity to auxin is conferred to transformed cells by the left T-DNA (TL-DNA). Analysis of the morphogenetic behavior of transgenic tobacco plants obtained by transferring segments of TL-DNA cloned in a binary vector system allowed us to pinpoint TL-DNA genes responsible for this increased auxin sensitivity of hairy root tissues. Three genes (open reading frames 10,11,12) are responsible for the spontaneous rooting of leaf explants and confer to transgenic plants an exaggerated response to auxin.Agrobacterium rhizogenes is the causative agent of the hairy root syndrome of dicotyledonous plants, a neoplastic growth of adventitious roots at the site of bacterial infection (10). Hairy root cells are transformed by a fragment (T-DNA) of large bacterial plasmids (Ri plasmids) which controls plant cell growth and differentiation (7,18,21,23 transformed tobacco slowly necrotize and die with no sign of root differentiation. Thus, Ri plasmid T-DNA confers to transformed plant cells a permanent rhizogenic potential. Agropine-type Ri plasmids harbor two distinct T-regions, denoted TL (left T-DNA) and TR (right T-DNA) (9, 22); genes encoding for the synthesis of the opine agropine and for the plant hormone IAA account for most, if not all, of the coding capability of the TR-DNA (4, 9, 22). Auxin synthetic genes recently have been shown (5) to play a rather accessory role in hairy root induction in that they provide the auxin necessary to trigger differentiation of cells transformed bv the TL-DNA whenever endogenous auxin levels are not sufficient. It is the as yet functionally uncharacterized TL-DNA that confers to otherwise unresponsive cells the competence to respond to auxin by differentiating roots (5). In other words, TL-DNA-transformed cells appear to be far more sensitive to auxin than their untransformed counterparts.In the work presented here, we give further substance to this view; seeking the rationale for the morphogenetic behavior of hairy root plants we in fact rule out a possible role of TR-auxin synthetic genes and show that leaves from hairy root plants are not substantially altered in their hormonal balance as compared to normal tobacco. We furthermore show that transgenic tobacco obtained by introducing only a small complement (genes 10, 11, and 12) of Ri TL-DNA is morphogenetically equivalent to plants containing the whole of the TL-DNA and are much more sensitive to auxin than is normal tobacco.
MATERIALS AND METHODS
Tuber tissue of Helianthus tuberosus L. (cv. OB1) contains a low amount of polyamines during dormancy but they are rapidly synthesized when tuber cells are activated in a growth medium and enter a new cell cycle. It was assumed that one of the reasons for this synthesis is that polyamines are necessary for the active conformation and correct functioning of nucleic acids. Complexes were found between spermine, spermidine and putrescine and rRNA, tRNA and an RNA fraction which contains poly(A) RNA and proteins. The amount of RNA‐bound polyamines in the parenchyma cells of dormant tubers is dependent on the stage of dormancy and clearly increases (especially putrescine) when cells are activated. There are both tightly‐bound and non‐tightlybound polyamines. The significance of these bound polyamines is discussed in relation to their stabilizing role on nucleic acids.
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