The interactions between two groups of rice endophytic bacterial strains and
several rice cultivars were investigated. Various strains of
Rhizobium leguminosarum bv.
trifolii, originally isolated from rice plants grown in
Egypt, comprise one group. The second group of bacterial strains was isolated
from rice cultivars grown in the Philippines. Inoculation experiments with
rice seedlings showed that specific isolates of these rice-associating
bacteria could either promote, inhibit, or have no influence on rice plant
growth. Furthermore, these growth effects were greatly influenced by the
environmental growth conditions used. Studies to examine root colonisation
patterns, using Rhizobium strains into which a plasmid
expressing the green fluorescent protein has been placed, showed that the
bacteria preferentially colonise rice seedling surfaces mainly in clumps. This
occurs along grooves on the rice root surface, or at the emerging lateral root
zones and at the root tips. However, rhizobia could also colonise
intercellularly in lateral roots formed on the main roots near the culm region
of the seedling. Under the growth conditions used, this occurred most
frequently with strain R4 which multiplied and migrated to form long lines of
individual bacterial cells along the inside of growing lateral roots. A
bioassay to measure bacterial multiplication in rice leaves showed that the
rice-associating strains can multiply and survive at different rates within
these tissues. They were not, however, detected migrating into other parts of
the leaf from the original site of pressure-infiltration, indicating that the
bacterial ability to migrate within the lateral roots is not matched by a
similar capacity in rice leaves. We suggest that some of these
rice-associating bacteria possess important genes that enhance their ability
to intimately colonise niches on and within rice tissues, and promote rice
plant growth.
Varietal differences in number of eggs of the whitebacked planthopper and egg mortality caused by ovicidal response of rice plant among major high-yielding rice cultivars for new demand