Strigolactones and hormonal interaction in control of branching in Zantedeschia and other horticultural species : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Palmerston North, New Zealand

Abstract

Shoot branching that involves development of lateral buds into shoots is one of the important factors influencing crop productivity. Strigolactones have recently been found to be involved in the control of branching, but the actual bioactive compound/s that inhibits bud outgrowth is still unknown. A germination assay utilizing the seeds of a parasitic weed (Orobanche minor), detected strigolactones within the xylem exudates of different horticultural crop species; the strigolactone concentration negatively correlated with branching of cultivars or mutants. In Zantedeschia grown in vivo, the concentration of strigolactones was independent on the volume of guttation fluid (xylem exudates) suggesting the difference in concentration of strigolactones in high and low branched cultivars was due to the difference in potential of producing strigolactones between these cultivars and not due to differences in volume of guttation fluid. While identifying a bioactive compound using germination and branching assays in combination with liquid chromatography and mass spectrometry, compounds containing „N? were detected in the low branched wild-type Petunia, but not in the highly branched mutant, suggesting the possibility of such compounds being SL-conjugates which may be associated with bud outgrowth inhibition. In Zantedeshia (in vitro) and pea stems, strigolactone reduced the axillary shoot number stimulated by the cytokinin suggesting an antagonistic interaction between these two hormones on bud release. However, as cytokinin may stimulate subsequent growth of released buds by increasing the auxin transport out of the bud, strigolactone may have reduced subsequent growth by reducing auxin transport. Since GA3 enhanced subsequent growth of buds in pea stems, but not the release, an antagonistic interaction between strigolactone and gibberellins on subsequent growth is possible. Interestingly, strigolactone successfully reduced adventitious bud formation in Zantedeschia grown in vitro, adding a new role for strigolactones in plant development. Despite correlation between strigolactone and branching inhibition in different horticultural crops such as apple, kiwifruit, Zantedeschia and Acer, further studies relating to strigolactone and its interaction with other hormones on branching of these crops could be performed using in vitro techniques for a clear understanding of strigolactones? role on branching inhibition. More importantly, quantification of strigolactones using the germination assay may have significant implications in horticultural crop breeding for obtaining desired shoot branching.