Strigolactones Suppress Adventitious Rooting in Arabidopsis and Pea      

Abstract

Adventitious root formation is essential for the propagation of many commercially important plant species and involves the formation of roots from nonroot tissues such as stems or leaves. Here, we demonstrate that the plant hormone strigolactone suppresses adventitious root formation in Arabidopsis (Arabidopsis thaliana) and pea (Pisum sativum). Strigolactone-deficient and response mutants of both species have enhanced adventitious rooting. CYCLIN B1 expression, an early marker for the initiation of adventitious root primordia in Arabidopsis, is enhanced in more axillary growth2 (max2), a strigolactone response mutant, suggesting that strigolactones restrain the number of adventitious roots by inhibiting the very first formative divisions of the founder cells. Strigolactones and cytokinins appear to act independently to suppress adventitious rooting, as cytokinin mutants are strigolactone responsive and strigolactone mutants are cytokinin responsive. In contrast, the interaction between the strigolactone and auxin signaling pathways in regulating adventitious rooting appears to be more complex. Strigolactone can at least partially revert the stimulatory effect of auxin on adventitious rooting, and auxin can further increase the number of adventitious roots in max mutants. We present a model depicting the interaction of strigolactones, cytokinins, and auxin in regulating adventitious root formation.

Strigolactones fine-tune the root system

“…As cytokinin is known to be a potent inhibitor of adventitious root formation (Bollmark and Eliasson 1986;De Klerk et al 2001), cytokinin levels and signaling have been analyzed in Arabidopsis. Strigolactone synthesis and signaling mutants all responded to exogenous cytokinin, and vice versa; both the triple ipt1,5,7 cytokinin synthesis and the double ahk3,4 cytokinin signaling mutants responded to exogenous strigolactones (Rasmussen et al 2012). Together these findings demonstrate that the increased adventitious rooting in strigolactone mutants is a result of both low strigolactone and low cytokinin levels, independently controlling adventitious rooting (Bollmark and Eliasson 1986;De Klerk et al 2001;Rasmussen et al 2012).…”

“…However, auxin-induced strigolactone signaling is not the key process regulating adventitious rooting, because both auxin and strigolactone mutants respond to strigolactone and auxin treatments, respectively (Rasmussen et al 2012). The levels of auxin were found to be higher at the base of tomato stem cuttings ) and auxin and strigolactones applied simultaneously to reduce rooting when compared to auxin treatments alone Rasmussen et al 2012). Together these results could support several hypotheses.…”

“…One of the most striking effects of strigolactones on the general root structure is the inhibition of adventitious root initiation (Rasmussen et al 2012). This phenotype has been observed in stem cuttings of pea (Fig.…”

“…2) (Rasmussen et al 2012) and tomato ) and etiolated hypocotyls of Arabidopsis (Rasmussen et al 2012) and as well as in lower stems of intact petunia lacking endogenous strigolactones (Napoli 1996). Exogenous strigolactone treatments inhibited adventitious rooting in a MAX2/RMS4-dependent manner Rasmussen et al 2012) and biosynthesis mutants from pea (rms5) but not from Arabidopsis (max4) were more sensitive to exogenous GR24 application than the wild type (Rasmussen et al 2012). In stem cuttings, root primordia typically emerge from cambial cell division activity, a stem cell-like cell layer that is required for vascular bundle formation and in addition, secondary growth.…”

“…Auxin application was also shown to promote the expression of strigolactone synthesis genes in plant stems (Foo et al 2005;Hayward et al 2009), suggesting that auxin should reduce adventitious rooting. However, auxin-induced strigolactone signaling is not the key process regulating adventitious rooting, because both auxin and strigolactone mutants respond to strigolactone and auxin treatments, respectively (Rasmussen et al 2012). The levels of auxin were found to be higher at the base of tomato stem cuttings ) and auxin and strigolactones applied simultaneously to reduce rooting when compared to auxin treatments alone Rasmussen et al 2012).…”