pulse of soluble exudate just behind the root tip and that the shape of this pulse was ... __ ______ _. I. Date --"n.._P. ... Olsson, S., Soderstrom, B. and Nordbring-Hertz, B. (1984). A fast and ... the rhizosphere (â¢ectorhizosphere) and the general soil is called ..... chosen to fit the bacterial growth curve obtained in paper. IV.
Effects of root exudation on growth of bacteria and fungal pathogens in the rhizosphere
Stefan Olsson
19 8 7
Lund
University
EFFECTS OF ROOT EXUDATION ON GROWTH OF BACTERIA AND FUNGAL PATHOGENS IN THE RHIZOSPHERE
by
Stefan Olsson FK. Bl
Akademisk avhandling, som for avlaggande av filosofie doktorsexamen vid matematisk-naturvetenskaplig fakulteten vid Lunds universitet korruner att offentligen forsvaras a Forelasningssalen, Ekologihuset, Lund, fredagen den 5 februari 1988, kl 10.00.
Docwnent name OOCTORAL DISSERTATION
Organization LUND UNIVERSITY
Department of Microbiology Solvegatan 21, S-223 62 Lund
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Authotcreascd germination percentages with culture age over a 4 11-weck period. Microseh:rotial germination pem:ntages were always higher in mineral salts solution plus sucrose than in mineral salts solution alone or waler. In a sand culture system with thi: intact rape plane. micrnsclerotial germin.Hion percentages were high close to the root and decreased in a s1eep gradient to background le,·cls within 5 mm from the root.
2. INTRODUCTION The effect or plant roots on the germination of fungal propagules has attracred much interest, as it is the first event in thi: infection cycle: of a 0168·6496/ SS/ SOJ.30
soil-borne plan1 pathogen. Root exudate. in a broad sense. is the trigger of fungal propugule gemunauon and acllvi: subsranccs in the exudate have been charac1erized Ill- The roo1 exudates diffuse inlo the soil and an e:rndaie gradient dt:\elops around the root This gradient is innuenced by factors such as s01l hum1d11y and uptake by microorg.m1sms (2) Fungal propagules stimulated to germinate by root cxudJtes may form a 'germina· lion gradient' around the root that may influence the relation berween the disease index and the ino~ulum denSll) for a given host - pathogen- soil situJ11on (3.4) V. dalrlwt• 1s a soil-borne pathogen that sun 1ves in soil by means or microsclerotia. The fungus infects and causes disease m a widi: range of dicotyledonous hosts (51 but does not sei:m to cause disease m monocotyledons. even when inrccring the: root cortex (6 8). Se\eral investigations have dealt w11h the factors affecting V. dahlwe microsclcrollal gcrmination (9. IOJ and. in some cases. tht> effccrs of root exudatcs on the germination havi: been ci.umated [11 J. Wuhin an ongoing study of the biological and chemical inleractrons in the rhizosphcrc of the rape plane ( D napus). the ecology and inkction biology of I'. dahlwe has been included. In this work. microsdcrotial germination. as orfected by
t985 Federation of EuropcJn Mrcrohiolo;k:il Sodciics
29J
the culture age and size of microsdero11a .. "as determined in different solutions. In addition, the germination of microscleroti:i in the rhizosphere of the host plant was assessed. In both c:ises a germination assa) using a Ouon:scent probe. nuorescein diacclatc:. w:is used.
3. MATERIALS AND l\IETHODS
J. I.
/so/au~
Three isolates of V. dalrliae Kleb. were used (S. P2 and Vd 71 36). All three were isolated from rape plants ( B. naptts L.) from different localtues in southern Swedcn. Thc choice of the 1sol.11cs ".is bused on the d1fferencc:s in their microsclerouaand co111d1a-fo rmmg patterns. Isolate S forms mi · crosclero tm in abundance and conidia m rel.ill\ el) large numbers. whereas P2 produces fe\\Cr microst.lerotia but conidia m abundance. \'d 711 36 forms lower numbers or bo th mic.:rosch:rc-ua and conidia than the other two isolates. J.2. PrnJ11H1m1 of microsdaotia Stock cultures of I'. dulr/iae were kept on
Czapd.-Do' Agar (Czapck-Dox sol.. Q;'(o1d; l.5'C Bal.lo agar. Dif
4
%
ISOLATE S
7
11 weeks
NET PORE SIZE
40um
100
E
5£
'
%
ISOLATE S
7 NET PORE SlZE
11 weeks 80um
100
F
I
SE
I
• so
•
50
•
• 4
7
u
31 weeks
Fig. 2. lnOuencc of culture age and ~ite of m1cro-clem1tJ nn m:, ro...:lero 11al gcrr.unauon percentage> nf three V Jahhu~ 1sola1es VJ71 / 36. P2 anJ S 1n three $Cllu11ons • . 00-"'~ter. "'· mineral "'11ls solutinn. • · mineral sails w lut1on '"nh 1.S'\ >ucrm e
297
the microsclerotia. since the old and presumably de3d hyphae were not FDA-active.
....% IO
4.2. The effects of culture age and miaosclemtiul si=e on n11crosclerotial germ111a11on The large microsclerotia (trapped in the pores or the 80-µm nets) had a mean germination or 51%. and the smaller microsclerotia (trapped in the pores or the 40-µm nets) had a mean germina· lion or 27%. The microsclero11al germination per· cenlages decreased with the age or the culture for 2 or the isolates (Vd 71/ 36 and P2. Fig. 2A-C). Isolate P2 seemed to be less aHected by the culture age. since the germination percentage on the 80-µm nets, in Pfcfrt:rs solution with sucro~e. was constant over the 4- 11-week period (Fig 20). The microscleroual germination percentage.; or isolale S were little .irrected by the age or the culture. and m1crosclero11a from 31-weel..·old cultures showc:d no decrease in germination pen:entages compared 10 microscleroua from 4-\1.eel..-old cullures (Fig.
2E. F). The germinalion pcrccnlagc~ in the solutions tested wen! alwa)S lower in DD-.... ater o r mineral sahs solullon 1han in min.:ral salts solution with 1.5'< sucrose. DD-\\aler gave the lowest germination percen1.igcs. except in one casi: (Fig. 20). The 40-l'm nets trapped about 10 times as many mi\: roscleroua as the 80-l'm neK from the same: m1crosclero11al suspension Therc:furc:. although large m1croscleroti.i germm.1ted better than sm.ill.:r ones. the: 40-µm nets wi:rc; used in all subsequent expenmc:nts. ./.J. ,\/1ao.ulero11ul germi11ario11 in rlu rhi:mpht•n•
The: microsclcro1i;1 germ1nati:d m dc\:reasmg numbers with increasing distance from the root :uds (Fig. 3). Th.: gc:rminalion gradient "'a~ rather steep. and dccrased 10 background lc\c:I (ahout 30%) within 5 mm of the: root .im. Wlul.: still within the: reach of the: root hairs (.ihout 2 5 mm). the: germination percentages were< 45'C. I.I!.. r.ir below hair thi: diHercncc between the h.ickl?round lc\'el and the li:\'el obtained close to the ro~t The n:lativc:ly high levels or microsclerotial gcrmm,1tion obtained for away from the root \\C:rc: probahly an effi:ct of the limi1.:d fung1stas1s in thc s.ind systi:m. since these g.:rmination percentages weri:
IO
••
•0•1taM• from ••feol 1 u1
"
Fig. 3 PcrecntJgc or gcrmmJlmn or I' Juh/tu~ m1~ro...lcmt1a on n) Ion nels pJ.,,11eJ aga1ns1 Jo.lance (mm root aus Bars m u c
