Increased plant growth regulator efficiency via slow release formulations and induced tissue competence via introduction of the rolB gene

The action of plant growth regulators (PGR's) depend on (1) PGR activity and (2) tissue sensitivity or tissue competence (the capacity to react to PGR's). PGR activity is often hampered by suboptimal uptake or transport in the plant tissue or by metabolic inactivation of the PGR after uptake. New PGR conjugates can be designed to overcome these problems. These conjugates consist of a PGR that is coupled via an ester or amide bond to a carrier molecule, e.g. bovine serum albumin (BSA). To 1 BSA molecule 35 auxins can be coupled by a bond between the amide moiety (Nconjugates) or the carboxylic moiety (C-conjugates) of the auxin. These bond can be hydrolysed enzymatically in the tissue. As a model system to test PGR conjugated auxin induced root regeneration was chosen. For optimal root regeneration metabolic inactivation of auxin was the main cause for suboptimal rooting of shootlets (in vivo and in vitro). IBA or IAA conjugates were used to decrease the rate of auxin metabolism. As long as the PGR is coupled to the carrier it is not metabolized and a pool of potentially actitve PGR is built up in the tissue. Inside the tissue the PGR's are slowly released and a significantly increased active PGR concentration (compared to standard PGR's) is obtained in the tissue over an extended period of time. The rate of auxin release of conjugates was about three times higher than that of the C-conjugates. In vitro, in a test system consisting of thin stem disks of apple, the conjugates induced optimal rooting at 30- to 100- fold lower auxin concentrations that non-conjugated hormones did. Shootlets of oak, which rooted for only 2 per cent with standard hormones, rooted up to 100 per cent with the conjugate. The role of tissue sensitivitycompetence on rooting was examined in thin stem disks cut from rolB transformed tobacco plantlets (the roIB gene was under control of its natural promoter). In control tobacco plants, disks cut from the basal part of the stem did not form roots (irrespective of the auxin concentration tested), however, the same type of disks cut from roIB plantlets formed roots (more than 70 per cent). So the roIB gene introduces tissue competence. All disks cut from the apical part of both genotypes formed roots. However, the disks cut from roIB plants formed three times more roots than the control disks. The dose response curve of the roIB plants did not shift significantly to a lower auxin concentration in comparison to that of the control plants. This indicates that roIB does not increase tissue sensitivity in tobacco stem disks but introduces higher tissue competence.