Applied Soil Ecology 15 2000 183–190
Endophytic communities of rhizobacteria and the strategies required to create yield enhancing associations with crops
A.V. Sturz
a ,∗
, J. Nowak
b
a
Prince Edward Island Department of Agriculture and Forestry, PO Box 1600, Charlottetown, PEI, Canada C1A 7N3
b
Department of Plant Science, Nova Scotia Agricultural College, Truro, NS, Canada B2N 5E3 Received 31 May 1999; received in revised form 8 November 1999; accepted 23 March 2000
Abstract
The plant kingdom is colonized by a diverse array of endophytic bacteria which form non-pathogenic relationships with their hosts. When beneficial, such associations can stimulate plant growth, increase disease resistance, improve the plant’s
ability to withstand environmental stresses e.g. drought, or enhance N
2
fixation. Crop sequences can favour the build-up of advantageous associations of bacterial endophyte populations leading to the development and maintenance of beneficial
host-endophyte allelopathies. Utilization of rhizobacteria in sustainable crop production systems will require strategies to create and maintain beneficial bacterial populations within crops endophytes and as well in the soils surrounding those
crops. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Beneficial association; Endophyte; PGPR; Sustainable crop production; Rhizosphere health
1. Introduction
Successive attempts to introduce beneficial bacte- ria into the rhizospheres of agricultural crops have
generally met with varying degrees of failure due to the difficulties of incorporating non-resident bacterial
components into established and acclimated micro- bial communities. For example, despite many years
of attempting to modify naturally occurring soil pop- ulations of Rhizobium, such efforts have not been
very successful Brockwell et al., 1988; Thies et al., 1991.
Where candidate rhizobacteria have been intro- duced as biocontrol agents, their failure to control
disease development has usually been attributed to
∗
Corresponding author. Tel.: +1-902-368-5664; fax: +1-902-368-5661.
E-mail address: avsturzgov.pe.ca A.V. Sturz
poor rhizosphere competence and the problems as- sociated with the instability of bacterial biocontrol
agents in long-term culture Schroth and Hancock, 1981; Weller, 1988. Consequently, root-associated
bacteria as biological control agents have not yet be- come an established part of most pest management
systems Harman and Lumsden, 1990; Powell and Rhodes, 1994.
Considering the biodiversity of indigenous soil bacteria and the population densities involved, it is
not surprising that it has proven difficult to make any long lasting structural changes to the composition of
bacteria within any given soil-community. One strat- egy which may help contribute to the establishment of
pre-selected beneficial organisms in root zone soils, and which has until recently been excluded from the
research equation, is through fostering the early es- tablishment of selected communities of endophytic
microorganisms within root systems.
0929-139300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 0 0 0 0 0 9 4 - 9
184 A.V. Sturz, J. Nowak Applied Soil Ecology 15 2000 183–190
In recent times the term ‘endophyte’ has been ap- plied almost exclusively to fungi Carroll, 1988; Clay,
1988; including the mycorrhizal fungi O’Dell and Trappe, 1992. However, a more comprehensive defi-
nition is one which includes ‘fungi or bacteria, which for all or part of their life cycle, invade the tissues of
living plants and cause unapparent and asymptomatic infections entirely within plant tissues, but cause no
symptoms of disease’ Wilson, 1995.
The recovery of bacterial populations from the en- dodermis and root cortex of plants has been used
to promote the idea that many bacteria in the rhi- zosphere are able to penetrate and colonize root tis-
sues Quadt-Hallman et al., 1997a,b. The inclusion of endophytic bacteria into the bacterial rhizosphere
community was proposed by Darbyshire and Greaves 1973, and supported by Old and Nicolson 1978.
In this model the root cortex becomes part of the soil–root microbial environment, resulting in a con-
tinuous apoplastic pathway from the root epidermis to the shoot, sufficient for movement of microorganisms
into the xylem Petersen et al., 1981. Thus, a con- tinuum of root-associated microorganisms exist which
are able to inhabit the rhizosphere, the root cortex and other plant organs Kloepper et al., 1992.
2. Exo- versus endoroot bacteria
