The role of auxin in adventitious root formation of grevillea

Abstract

Grevillea (Proteaceae) is one of the woody Australian native species that have a high commercial value in the floriculture market. Grevillea spp. varies greatly in their ease of propagation from cuttings, and some Grevillea cultivars are considered to be difficult-to- root. Efforts to improve propagation success are essential if this species is to be developed for commercial production. The objectives of this study were to observe the response of Grevillea cuttings to exogenous auxin, and to determine whether or not rooting differences between the difficult and easy-to-root cultivars could be explained by differences in endogenous auxin levels, auxin uptake, transport and/or metabolism.GC-MS analysis was used to measure the levels of auxin in the plants. The level of auxin in the endogenous pool was measured by adding 13C-labelled IAA (I3C6-IAA) and 13C1- labelled IBA (13C1-IBA) as internal standards for quantitative mass-spectral analysis. The capacity for auxin transport in cuttings of the two Grevillea cultivars was estimated by applying radio labelled auxin with high specific activity (3H-IBA, Bioscientific Pty) to the cuttings. The distribution of radioactivity was determined in the cutting base, leaves and the rest of the stem harvested at various time points. Confirmation of the quantity of radiolabel maintained as [3H]-IBA during this period was monitored by HPLC-radio detection.Apical auxin application to the difficult-to-root cultivar Coastal Dawn resulted in a higher or comparable percentage of rooting compared to basal application in autumn and might potentially reduce the amount of auxin required to induce rooting.Rooting differences between the difficult and easy-to-root Grevillea cultivars are unlikely to be related to their endogenous auxin levels or their ability to uptake applied auxin. This conclusion was based on the findings that (1) there were no significant differences in the endogenous IAA and IBA levels between the easy and difficult-to-root cultivars and (2) both cultivars demonstrated an increase in endogenous IAA and IBA levels following IBA application; the increase in the difficult-to-root Coastal Dawn being more pronounced than in the easy-to-root Royal Mantle.A different distribution pattern of the applied [3H]-IBA was noted between Coastal Dawn and Royal Mantle. The applied IBA in Coastal Dawn moved to the leaf rather than the stem base, whereas in Royal Mantle it accumulated in the stem base.Both Greviilea cultivars demonstrated the ability to convert the IBA to IAA. However, the rate of IBA metabolism appears to vary between cultivars and between tissues within the same cultivar. Between cultivars, the stem base of Royal Mantle cuttings metabolised IBA more than Coastal Dawn. Since roots formed in the tissues which showed more IBA metabolism, i.e. in the stem base of Royal Mantle and in the upper stem of Coastal Dawn, it is possible that root formation in Grevillea correlates with the cultivar's ability to metabolise IBA. These findings led to the conclusion that auxin distribution and metabolism might contribute to the different rooting abilities of Grevillea cultivars.