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BACTERIA FOR REDUCING EFFECTS OF ROOT-KNOT NEMATODE
INFECTION ON TOMATO SEEDLINGS
N.L. Bell, G. Burch, T.C. Rohan and L.T. Aalders
AgResearch, Hamilton, New Zealand. nigel.bell@agresearch.co.nz
ABSTRACT. Thirty-two strains of bacteria were tested in three tomato seedling bioassays against the root-knot nematode
Meloidogyne hapla. The bacteria were characterised by their ability to produce the potentially beneficial compounds
aminocyclopropane carboxylic acid (ACC) deaminase, indole acetic acid (IAA) or iron-chelating siderophores. Six of the
seven bacterial strains that produced ACC deaminase reduced tomato root galling with the two best strains reducing galling
by 50% compared to untreated controls. There was no consistent effect of IAA or siderophore-producing strains on root
galling or plant yield. Further research is warranted to investigate the mechanism of gall reduction by ACC and whether this
has long-term benefits for tomato yield.
INTRODUCTION
Tomatoes are grown commercially in many temperate and
tropical parts of the world, including New Zealand and
Australia. While glasshouse production of tomatoes is
common there is also a substantial proportion of the crop
which is grown outdoors. Root-knot nematodes
(Meloidogyne sp.) are one of the yield limiting pests of
tomato in outdoor growing situations due to their disruption
of root function.
Almost all outdoor crops are established from
transplanted seedlings which are grown in glasshouses and
hardened off before planting into beds. This transplant stage
offers an opportunity to introduce beneficial organisms (1),
in this case to tomato seedlings, that can then be transferred
to the outdoor situation at planting.
A number of possible mechanisms have been described
to explain how beneficial bacteria improve plant growth.
Three of these are production of ACC deaminase which
reduces ethylene production in plants; IAA which is a plant
growth promoter; and siderophores which allow bacteria to
sequester iron in the rhizosphere, depriving pathogens of
this resource.
The current study reports bioassays of bacterial strains
characterised for IAA, ACC and siderpohore production, to
quantify any benefits they gave to tomato seedling yield or
root galling under challenge from root-knot nematodes.
MATERIALS AND METHODS
Bacteria were selected from the AgResearch Ruakura
culture collection as being beneficial to plant growth in
previous testing or were freshly isolated from tomato roots.
Bacteria were characterised for their ability to produce IAA,
ACC and siderophores using standard methods.
Three bioassays were conducted as described by (2).
Briefly, tomato seed were individually planted into root
trainers and inoculated with bacterial suspension (12, 11 and
9 strains for bioassays 1–3 respectively) or distilled water
(untreated control) at sowing. For all bacterial strains, one
week after sowing a 1 ml solution of Meloidogyne nr
hispanica nematode eggs or distilled water were inoculated
into the substrate at 20 mm depth around each seedling.
Seedlings were maintained in a 20°C controlled
environment room with regular overhead watering. There
were eight replicate plants for each bacterial strain ×
nematode combination.
Approximately 3000 nematode eggs were inoculated per
plant for all bioassays. Four weeks after nematode
inoculation plant roots were washed free of substrate and
shoot and root fractions separated. The percentage of the
root system which was galled by the nematodes was scored
visually.
RESULTS
Fifteen of the bacterial strains produced only one of the
compounds measured (mostly siderophores) while a single
strain produced all three and eight produced no detectable
levels of any of the compounds.
Six of the seven bacterial strains that produced ACC
reduced tomato root galling with the two best strains
reducing galling by 50% compared to control (Table 1). The
strain that did not reduce galling was also the strongest
producer of siderophores of this group. The reduction in
galling caused by this group of bacteria did not, however,
result in any substantial increase in shoot growth for any of
the strains.
There was no consistent effect of IAA or siderophoreproducing
strains on root galling or plant yield with half the
strains of each increasing and half reducing these variables
compared to control. Similarly, those strains which did not
produce detectable levels of IAA, ACC or siderophores had
no consistent effect on galling or plant growth.
Table 1: Mean percent change (range in parentheses) in
galling, shoot dry weight (DW) and root DW in tomato
plants inoculated with Meloidogyne nematodes, compared to
untreated control.
Galling Shoot DW Root DW
ACC (n=7) -20
(11 to -50)
-10
(7 to -27)
-16
(18 to -35)
IAA (n=6) -11
2
10
Siderophore
(n=20)
(44 to -50)
8
(94 to -61)
(24 to -15)
-2
(24 to -27)
(52 to -21)
0
(50 to -35)
DISCUSSION
The reduction in galling caused by bacteria producing ACC
seems promising. Paradoxically, the reduction in root weight
is also promising in this case because root galls caused by
Meloidogyyne nematodes can add considerably to root
weight, so a reduction in galling may well lead to a
reduction in root weight. Further investigation would be
needed to determine if the reduction in galling was due to a
reduction in nematode invasion or merely an increase in root
growth due to the reduction in root elongation-limiting
ethylene in the plant. If a reduction in nematode invasion
was found, longer-term experiments would be needed to
determine any subsequent effects on tomato yield.
ACKNOWLEDGEMENTS
We thank Maria Tourna for her assistance with these
experiments. The work was funded from FRST contract
C10X0706.
REFERENCES
1. Kokalis-Burelle, N., Kloepper, J.W. and Reddy, M.S.
(2006). Appled Soil Ecology 31: 91-100.
2. Aalders, L.T., Minchin, R., Hill, R.A., Braithwaite, M., Bell,
N.L. and Stewart, A. (2009). New Zealand Plant Protection
62: 28-33.
7th Australasian Soilborne Diseases Symposium 46