An Ethylene-Mediated Increase in Sensitivity to Auxin Induces Adventitious Root Formation in Flooded Rumex palustris Sm

Abstract

The hormonal regulation of adventitious root formation induced by flooding of the root system was investigated in the wetland species Rumex palusfris Sm. Adventitious root development at the base of the shoot is an important adaptation to flooded conditions and takes place soon afler the onset of flooding. Decreases in either endogenous auxin or ethylene concentrations induced by application of inhibitors of either auxin transport or ethylene biosynthesis reduced the number of adventitious roots formed by flooded plants, suggesting an involvement of these hormones in the rooting process. The rooting response during flooding was preceded by increased endogenous ethylene concentrations in the root system. The endogenous auxin concentration did not change during flooding-induced rooting, but a continuous basipetal transport of auxin from the shoot to the rooting zone appeared to be essential in maintaining stable auxin concentrations. These results suggest that the higher ethylene concentration in soil-flooded plants increases the sensitivity of the root-forming tissues to endogenous indoleacetic acid, thus initiating the formation of adventitious roots.Flooding causes many changes in the hormone physiology of plants. For instance, transport of auxin from shoots to roots may be inhibited by soil flooding, resulting in accumulation of auxin at the base of the shoot (Phillips, 1964;Wample and Reid, 1979). Other hormones such as ethylene in hypoxic roots are produced in larger amounts during flooding (Voesenek et al., 1990; Brailsford et al., 1993). In wetland plants, the change in hormonal status of the flooded plant is followed by a number of responses that alleviate the negative effects of flooding on plant growth. The mechanisms that underlie these adaptations to flooding have been explained in terms of changes in hormone concentrations or sensitivity to a hormone (see reviews by Reid and Bradford, 1984;Jackson, 1990;Voesenek et al., 1992; Blom et al., 1994). However, for one major adaptation to flooding, i.e. adventitious root formation, persuasive evidence for a hormone-mediated regulation is still lacking.

A Co-Opted Hormonal Cascade Activates Dormant Adventitious Root Primordia upon Flooding in Solanum dulcamara

“…While no change in the endogenous free auxin concentration was detected upon flooding, blocking polar auxin transport with N-1-naphtylphtalamic acid (NPA) decreased the number of ARs after flooding (Visser et al, 1996c). Reconciling these findings, it was found that accumulation of ethylene leads to AR emergence by increasing the sensitivity to auxin in the rooting zone (Visser et al, 1996c).…”

“…In R. palustris, ethylene accumulation was observed in roots following 24 h of submergence, and inhibition of ethylene production decreased the number of ARs induced by flooding (Visser et al, 1996b). While no change in the endogenous free auxin concentration was detected upon flooding, blocking polar auxin transport with N-1-naphtylphtalamic acid (NPA) decreased the number of ARs after flooding (Visser et al, 1996c). Reconciling these findings, it was found that accumulation of ethylene leads to AR emergence by increasing the sensitivity to auxin in the rooting zone (Visser et al, 1996c).…”

“…In both species, ethylene is a key regulator of the process (Visser et al, 1996b(Visser et al, , 1996cMergemann and Sauter, 2000). In rice, the ethylene that accumulates upon submergence induces formation of the reactive oxygen species (ROS) hydrogen peroxide, which in turn leads to cell death in the epidermal cell layer that covers the AR primordia at the nodes, thereby facilitating their emergence (Mergemann and Sauter, 2000).…”

Flooding tolerance of Carex species in relation to field distribution and aerenchyma formation

“…Ramets of each species were taken from the plants in the glasshouse and placed in rafts floating in containers of well aerated nutrient solution as described in Visser et al (1996c). After the development of a new root system, five to eight plants per species were transferred to a stagnant, oxygen-deficient nutrient solution containing 0n1% agar (Visser et al, 1996c).…”

Rooting and establishment of Limoniastrum monopetalum (L.) Boiss stem-tip cuttings

“…It is well established that exogenous application of auxin accelerates the rates of rooting, increases final rooting percentage and the number of produced roots in leafy cuttings (Leakey, 1990;Larson, 1992;De Klerk et al, 1999), which could be attributed to the translocation of carbohydrates and other nutrients to the rooting zone (Middleton et al, 1980;Leakey et al, 1982). However, exogenous application of auxin may be promotive, ineffective or even inhibitory for the rooting of cuttings, depending on the endogenous level of growth-regulating substances (Haissig, 1979) or the tissue sensitivity (Visser et al, 1996). Relatively high concentrations of auxins have been reported to be inhibitory to rooting, indicating that in many species, optimal concentrations for rooting have to be defined (Leakey et al, 1982).…”