Changes in Cytokinin Concentrations in Xylem Extrudate following Infection of Eucalyptus marginata Donn ex Sm with Phytophthora cinnamomi Rands

Abstract

ABSTRACIThe concentrations of zeatin-type and isopentenyladenine-type cytokinins were reduced in the xylem extrudate collected from seedlings of Eucalyptus species following infection by Phytophthora cinnamomi Rands. The use of an enzyme-linked immunosorbent assay (ELISA) allowed the detection of these cytokinins over the range of 03 to 7 picomoles for the isopentenyladenine-type and 1 to 1000 picomoles for the zeatin-type. Isopentenyladenine-type cytokinins occurred in concentrations less than 10% of the zeatin-type, but they could be readily detected and measured. This is the first report of their presence in xylem. The sensitivity of the assay allowed a short collection period (30 minutes) reducing any confusion with trauma-induced changes. Infection of the susceptible species Eucalyptus marginata Donn. ex Sm. resulted in significant reduction of zeatin-type cytokinins within 3 days of infection, and at 14 days postinfection the concentration of both cytokinin types was reduced to 26% of uninoculated controls. No reduction in cytokinins occurred with the field resistant Eucalyptus calophylla R. Br. It is suggested that failure of cytokinin transport from the root system may be responsible for the failure in water transport and symptoms of P. cinnamomi infection observed in infected susceptible eucalypts.The fungus Phytophthora cinnamomi Rands is a pathogen with a broad host range, and the diseases it causes are ofeconomic importance throughout the temperate and tropical zone (36). While the primary symptom of infection by P. cinnamomi in a susceptible species is root rot, the organism also induces secondary symptoms in the shoots of many perennial host plants which resemble droughting. These include wilting, chlorosis, microphylly, and shoot and bud death (dieback) (36).Studies of seedling eucalypts grown under controlled conditions showed that susceptible species died when as little as 8% of the root system was infected (9), indicating that loss ofroot tissue alone is unlikely to be the cause ofeither the secondary symptoms or of host death. Moreover, the same study demonstrated a significant reduction in hydraulic conductivity ofthe root system of a susceptible species 2 to 4 d after inoculation. This decline in root conductivity preceded reductions in leaf xylem water poten- tial, leaf transpiration rate, and wilting in these plants. These changes were not observed in seedlings of field resistant species.Histological examination failed to show xylem blockage or extensive damage to the conducting system under these conditions (9) and there is no evidence of toxin production sufficient to account for the secondary symptoms (7).However, reductions in the concentration of cytokinin-like compounds in the tracheal fluid of crop plants infected with wilt by other fungal root pathogens, Verticillium spp., have been reported (16,20,24) with symptoms which also resemble drought. In this paper, we report that infection of susceptible eucalypts by P. cinnamomi also induces a major reduction in the concentration of cyt...

Root Hypoxia Reduces Leaf Growth

“…Earlier reports on the role of cytokinins in the regulation of leaf responses to root environments have often relied on the premise that changes in the levels of cytokinins exported from the roots would cause a reduction in the levels of cytokinins in the leaves (5). In this study, we found that the levels of ZR cytokinins were lower in xylem sap expressed from hypoxic roots than in sap from aerated roots (Fig.…”

“…The effects of ABA and ZR2 on in vitro leaf growth and the effects of root hypoxia on their endogenous levels in xylem sap and leaves were also measured. (5 discs/dish), containing 10 mL of 10 mm sucrose with 10 mM KCI. Discs from aerated plants grew at similar rates to leaves on the intact plant.…”

Phytophthora cinnamomi and oak decline in southern Europe. Environmental constraints including climate change

“…These are motile by virtue of their two flagellae and require free water and warm soil conditions for their dispersal (Crandall et al, 1945;Zentmyer, 1980;Shearer and Tippett, 1989 (Shearer and Tippett, 1989). In addition to the root necrosis, there is some evidence that the pathogen may affect host stomatal control by reducing its cytokinin production, leading to crown symptoms resembling those of drought (Cahill et al, 1985(Cahill et al, , 1986. The fungus survives dry periods either as thick-walled chlamydospores embedded in soil or roots, or deep in the soil profile (to 3 m) around infected sinker roots (Shearer and Tippett, 1989).…”

“…One relevant factor is that Quercus spp are probably in general more resistant to P cinnamomi than Castanea spp (Crandall et al, 1945 The imposition of drought stress on oaks already exhibiting chronically low levels of root infection might critically reduce their tolerance of the fungus (cf, Griffin et al, 1993). Moreover, drought stress is already known to enhance the development of P cinnamomi within the infected host (Weste and Rupin, 1975;Cahill et al, 1985Cahill et al, , 1986Marçais et al, 1993).…”

“…An additional possibility that the pathogen may directly influence host stomatal regulation, so mimicking the effects of drought (Cahill et al, 1985(Cahill et al, , 1986 (Montoya, personal communication).…”