3. Introduction
Infection Process And Nodule Formation.
Factors Affecting Symbiosis
APPLICATIONIN AGRICULTURE
Inoculation
FLOW OF PRESENTATION
4. • The symbiosis between leguminous plants and
soil bacteria of the Rhizobiaceae leads to the
formation of nitrogen-fixing nodules, generally
exclusively appearing on the roots.
• A few legume species, however, form nodules
not only on their roots, but also at stem-located
root primordia.
• The first example of this phenomenon was first
reported in 1928 in Aeschynomene aspera L.
by Hagerup.
INTRODUCTION
5. • Only a few legume species bear nodules both on
their roots and stems. They belong to the three
genera Sesbania (one species), Aeschynomene
(about 15 species) and & Neptunia (one species).
• These plants have in common the ability to grow in
waterlogged soils and are potential candidates for
green manuring in paddy fields.
6. • Up to now, 26 different stem-nodulating
legume species have been reported belonging
exclusively to the genera Aeschynomene,
Neptunia, and Sesbania.
7. The most distinctive characteristic of stem-nodulating
legumes is the presence of predetermined nodulation
sites on the stems.
The formation of these sites is independent of infection
with rhizobia They comprise primordia of adventive roots
that are able to grow out under waterlogged conditions .
These primordia can be distributed over the whole
length of the stem or appear only on lower stem portions.
They are arranged in straight vertical rows (Sesbania) or
in spiral-shaped rows winding around the stem
(Aeschynomene), giving the appearance of random
distribution
8. Fig 1. Nodulated stems of 50-day-old Sesbania rostrata (A)
and Aeschynomene afraspera (B).
10. Azorhizobium caulinodans
The fast growing Rhizobium strain ORS571,now called
Azorhizobium caulinodans, was isolated from stem nodules
of the tropical legume Sesbania rostrata. The strain can grow
in the free-living state at the expense of molecular nitrogen.
The nitrogenase, purified from cells grown in a fermenter,
was shown to be composed of two components, a MoFe-
protein and a Fe-protein.
The enzyme activity was found to be subject to"switch-off"
when ammonia was added to a N2-fixing culture.
11. Electron micrograph of negatively stained Azorhizobium caulinodans ORS
571T grown in liquid medium.
12. For stem inoculation, shoots can be sprayed with
a suspension containing about 108 bacteria/ml,
using either a liquid culture of rhizobia, a
colloidal suspension obtained by mixing
entrapped rhizobia in a phosphate buffer (0.06m,
pH 6.8), or a suspension of crushed stem nodules
with water passed through a filter.
Inoculation
13. Fig 2. Typical appearances of stem nodules formed by mutants
categorized by type.
14. Table:2 Effects of A. caulinodans inoculation on N, P and K
accumulation of S. rostrata on 60 DAS
Tukey–Kramer HSD test (P<0.05).
15. Infection Process, Nodule Formation, and Fine Structure of Stem
Nodules
• Stem-nodulating rhizobia are true soil bacteria;
without the appropriate macrosymbiont they
thrive saprophytically on soil organic matter.
• Due to their unipolar flagellum they are highly
mobile in the aquatic phase.
• Besides the soil, the phyllosphere of the host
plant seems to be an alternate ecological niche
for nonsymbiotic growth of stem-nodule
rhizobia.
16. • Inoculum for stem infection can come from two
sources: the soil and the phyllosphere. In both
cases, rain seems to play major role for stem
inoculation.
• soil splash due to rain may bring rhizobia to the
lower stem.
• ants as a possible vector to inoculate primordia
on the stem of S. rostrata under nonflooded
conditions.
• Wind may be another important factor; often a
high nodulation rate can be observed on stems of
stem nodulating legumes along dusty roads.
17. • Wind may transport contaminated soil particles
on above-ground plant parts thus favoring stem
inoculation.
• After the rhizobia have reached the nodulation
site, they penetrate into the cortical tissues of
the stem.
18. • Upon penetration of rhizobia in the intercellular
space, infection pockets are formed into the
basal layers of the root primordium, which then
resume their meristematic activity. As a result,
cell division starts in the infective center and the
nodule begins to form.
• intracellular infection threads are formed.
• At the infective stage, rhizobia change in shape
and size. They become bacteroids and are
surrounded by a membrane envelope.
19. • As the growing infective center forms the
nodule, the majority of the cells in stem
nodules are infected.
• The formation of stem nodules becomes
macroscopically visible within 5 to 7 days
after inoculation and the nodules reach their
full size in 15 to 20 days.
• In S. rostrata, stem nodules are spherical
protrusions, 5 to 12 mm in diameter. They can
easily be detached from the stem, as the basal
portion of the nodule forms a narrow neck.
20. • In Aeschynomene flattened hemispheric nodules
are formed. Because no neck formation is found,
the nodules are hard to remove.
21. Factors Affecting Symbiosis
Survival of Rhizobia : Like root-nodule
bacteria, stem-nodulating bacteria are true soil
bacteria. A carbon source is critical for their
survival in the soil.
Phyllospheric or Epiphytic Survival :
contain nutritional substances like amino acids
and carbohydrates.
22. Mineral Nitrogen :
• Normally, crop plants respond to fertilizer
nitrogen in the field since most of the cultivated
soils worldwide are deficient in nitrogen.
• For reasons of energy legumes prefer soil or
fertilizer nitrogen rather than biologically fixed
nitrogen.
• Mineral N can reduce both nodulation and N2
fixation in stem nodulating legumes; the degree
of inhibition varies with the compound and its
concentration.
24. Agronomic Use of the Stem-Nodulating
Legumes
Leguminous GM crops are potential N sources
with relatively high efficiency, but also have
diverse non-N effects such as the improvement of
soil properties and control of weeds, pests, and
diseases. Positive effects on soil properties
include,
maintenance or improvement of soil organic
matter.
conservation and recycling of nutrients.
26. Potential Rice Environments for GM Use
• The irrigated lowland environment, characterized by
an assured water supply, has a high cropping
intensity.
• The main problem of this environment is crop
competition for time, space, and labour. In areas
where the gap is very short or where early-season
waterlogging occurs, fast-growing Sesbania or
Aeschynomene would fit between the main crops. S.
rostrata may be used as GM before wet season rice,
during the long-day period (April/May) and A.
afraspera before the dry season rice
(November/December) during the short-day period.
27. Growing GM as an Intercrop and on
Bunds/Wastelands.
Optimum Time for GM Incorporation and
Rice Transplanting.