Abstract
The newly defined phytohormones strigolactones (SLs) were recently shown to act as regulators of root development. Their positive effect on root-hair (RH) elongation enabled examination of their cross talk with auxin and ethylene. Analysis of wild-type plants and hormone-signaling mutants combined with hormonal treatments suggested that SLs and ethylene regulate RH elongation via a common regulatory pathway, in which ethylene is epistatic to SLs. The SL and auxin hormonal pathways were suggested to converge for regulation of RH elongation; this convergence was suggested to be mediated via the ethylene pathway, and to include regulation of auxin transport.
Key words: strigolactone, auxin, ethylene, root, root hair, lateral root
Strigolactones (SLs) are newly identified phytohormones that act as long-distance shoot-branching inhibitors (reviewed in ref. 1). In Arabidopsis, SLs have been shown to be regulators of root development and architecture, by modulating primary root elongation and lateral root formation.2,3 In addition, they were shown to have a positive effect on root-hair (RH) elongation.2 All of these effects are mediated via the MAX2 F-box.2,3
In addition to SLs, two other plant hormones, auxin and ethylene, have been shown to affect root development, including lateral root formation and RH elongation.4–6 Since all three phytohormones (SLs, auxin and ethylene) were shown to have a positive effect on RH elongation, we examined the epistatic relations between them by examining RH length.7 Our results led to the conclusion that SLs and ethylene are in the same pathway regulating RH elongation, where ethylene may be epistatic to SLs.7 Moreover, auxin signaling was shown to be needed to some extent for the RH response to SLs: the auxin-insensitive mutant tir1-1,8 was less sensitive to SLs than the wild type under low SL concentrations.7
On the one hand, ethylene has been shown to induce the auxin response,9–12 auxin synthesis in the root apex,11,12 and acropetal and basipetal auxin transport in the root.4,13 On the other, ethylene has been shown to be epistatic to SLs in the SL-induced RH-elongation response.7 Therefore, it might be that at least for RH elongation, SLs are in direct cross talk with ethylene, whereas the cross talk between SL and auxin pathways may converge through that of ethylene.7 The reduced response to SLs in tir1-1 may be derived from its reduced ethylene sensitivity;7,14 this is in line with the notion of the ethylene pathway being a mediator in the cross talk between the SL and auxin pathways.
The suggested ethylene-mediated convergence of auxin and SLs may be extended also to lateral root formation, and may involve regulation of auxin transport. In the root, SLs have been suggested to affect auxin efflux,3,15 whereas ethylene has been shown to have a positive effect on auxin transport.4,13 Hence, it might be that in the root, the SLs' effect on auxin flux is mediated, at least in part, via the ethylene pathway. Ethylene's ability to increase auxin transport in roots was associated with its negative effect on lateral root formation: ethylene was suggested to enhance polar IAA transport, leading to alterations in the quantity of auxin that unloads into the tissues to drive lateral root formation.4 Under conditions of sufficient phosphate, SL's effect was similar to that of ethylene: SLs reduced the appearance of lateral roots; this was explained by their ability to change auxin flux.3 Taken together, one possibility is that the SLs' ability to affect auxin flux and thereby lateral root formation in the roots is mediated by induction of ethylene synthesis.
To conclude, root development may be regulated by a network of auxin, SL and ethylene cross talk.7 The possibility that similar networks exist elsewhere in the SLs' regulation of plant development, including shoot architecture, cannot be excluded.
References
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