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Age related changes in strigolactone control of adventitious root initiation

Final Report Summary - ADVROOTAGESTRIG2010 (Age related changes in strigolactone control of adventitious root initiation)

Strigolactones are a recently discovered group of small chemicals derived from the carotenoid pathway that are involved in the communication with organisms in the rhizosphere. For instance, the developmental pattern of myccorrhizae growing in the vicinity plants secreting strigolactones is modulated to improve physical interaction with roots. In addition to functions in the rhizosphere, strigolacones play a role in plant development regulating shoot branching and seed germination. In the lab of Prof. Christine Beveridge at the Department of Biological Science, University of Queensland, Australia, it was discovered that the synthetic strigolactone GR24 inhibits adventitious rooting in pea as well as in Arabidopsis thaliana hypocotyls. Pea and Arabidopsis mutants affected in strigolactone synthesis or signaling produce a higher number of adventitious roots, suggesting that controls the establishment of root architecture. Adventitious roots differ from other root systems in terms of physiological and structural features, and hence may perform functions that are quite different from that of the primary and lateral roots. Although that the distinction between different root systems has not been well-established, adventitious roots unlike any other root system emerge from non-root tissue. As strigolactone inhibits adventitious root formation but has only a minor effect on lateral root formation, it appears that strigolactone signalling may mediate the functional specification of adventitious roots.

To gain insight into the physiological and molecular mechanisms of adventitious rooting, a set of tools was developed that facilitates the study of strigolactone function. In planta synthesis of strigolactones takes place primarily in the root and is initiated via the hydrolysis of carotene by means of two carotene-deoxygenases (ccd7 and 8). A third gene, max1, which encodes a cytochrome P450 acts downstream. Some of the produced strigolactone seeps out of the root system into the rhizosphere and some will is transported to the shoot where it controls shoot branching. Because strigolactone have an effect in the nanomolar range and occur at very low concentrations in the plant, it is very difficult to detect it and study spatial distributions. To address this problem, we synthesised a strigolactone analog with fluorescent properties. The new strigolactone analog was shown to have strong biological activity in Orobanche seed germination, adventitious root inhibition and Arabidopsis inflorescence branching. The stability of CISA-1 was higher than GR24 and may explain the higher biological activity of CISA-1. Attempt to detect CISA-1 fluorescence in planta was not successful because of high autofluorescence in the 400 nm wavelength range that corresponds to the optimum fluorescence emission spectrum of CISA-1.

Adventitious rooting is controlled by environmental cues such as wounding and flooding, which trigger for instance ethylene and jasmonic acid signaling. How and if the associated signalling processes are linked is not known, yet represents an important question as the integration of external cues likely determines if rooting capacity. Ethylene has been shown to inhibit adventitious rooting in Arabidopsis and promote it in tomato. To determine a possible interaction between ethylene and strigolactone signalling or a possible epistatic regulation, adventitious rooting was determined in signalling mutants in the presence of one or the other hormone. The results are in accordance with separate signalling by strigolactone and ethylene and suggest that these may be driven by independent cues or mechanisms. Further support that ethylene and strigolactone operate independently is concluded from the spatial distribution of adventitious roots in the presence of ACC in comparison with the presence of GR24.

As well as there is evidence for environmental regulation of adventitious rooting, also life cycle processes influence the ability of non root tissue to form roots. In general, physiologically older plant material is known to show increased recalcitrance to regenerate. To test the effect of aging on rooting capacity, pea stem fragments of different nodal positions were used for rooting tests. In addition, nodal fragments from similar positions in the plant were taken from plants swon at different time intervals. The adventitious rooting measurements showed that in pea there was no significant difference in rooting, suggesting that there is no age-related control of root induction. However, at a late stage in the life cycle when the pea plants began flowering, a drop in adventitious rooting was noticed independent from strigolactone. The results seem to suggest that strigolactone is not involved in the control of organogenesis potential as function of plant developmental stage.

Strigolactone synthetic analogue CISA-1 can be obtained from Thomas Heugebaert ( via e-mail). For other requests, please refer to Danny Geelen Danny ( via e-mail).