Symbiotic Fungi: Principles and Practice (Soil Biology)

6 A. Das and A. Varma
1.3.2 Symbiotic Association of Bacteria
with Nonleguminous Plants
Many species of as many as thirteen genera of nonleguminous angiosperms, which
are all woody and dicots, bear root nodules that can fix nitrogen, e.g., Casuarina,
Alnus. Some members of the family Rubiaceae develop nodule-like structures on
the leaves, which contain nitrogen-fixing bacteria. Over 400 species of three genera
of Rubiaceae and one genus of Myrsinaceae reportedly have bacterial leaf nodules.
Light and/or electron microscope studies of a few species have shown that bacteria
exist in spaces within buds filled with mucilage secreted by glands. These bacteria
enter substomatal chambers (Rubiaceae) or marginal hydathodes(Myrsinaceae) and
establish short-lived colonies, in intercellular spaces, that die out almost before full
leaf expansion. Bacteria occur in seeds between endosperm and embryo, but only
two studies have followed bacteria into flowers and ovules. Previous work on the
physical relations of bacteria and host plants is discussed critically. Reviewing
work done on isolation and identification of presumed endophytes leads to the
conclusion that there is no agreement as to whether one or several bacterial taxa are
the endophyte, and there are no unambiguous identifications, although four genera
are suggested as possibilities (Lersten and Horner 1976).
1.3.3 Establishment of the Mutualistic Relationship
Between Rhizobia and Legumes
1.3.3.1 Presymbiosis Stage
A diverse group of rhizobia may inhabit the rhizosphere (root and soil area) of a
specific legume, but only one or a limited number of species will interact with the
legume. Assessment of rhizobial diversity indicates rhizobia are highly heterogeneous,
as they differ in growth rates, biosynthetic pathways, habitats, catabolic
activities, etc. (Gonzalez et al. 2008). The soil contains various bacteria and
microorganisms. Each species of legume generally excretes a spectrum of flavonoids,
stachydrines and aldenic acids into the rhizosphere, making the biochemical
environment in which the rhizobia grow unique for each type of legume. This
unique concentration gradient and spectrum of flavonoids is used to attract the
appropriate rhizobial species to colonize the root and produce nodules. Some of
these root exudates are also used as nutrients by some of the rhizobia, e.g.,
homoserine released by pea plants is the preferred nutrient (carbon and nitrogen
source) for R. leguminosarum biovar vicieae, which forms a symbiotic relationship
with pea plants (Van Egeraat 1975). Hence, a combination of preferential rhizobial
population growth and movement, possibly by chemotaxis and/or electrotaxis
(Miller et al. 1986) results in the appropriate rhizobial species colonizing the
legume root and producing effective nodules.