During a survey of endophytic diazotrophic bacteria associated with different rice varieties in Tamilnadu, some "endophytes" were obtained. Thirteen bacterial isolates from surface-sterilized roots and shoots were obtained in pure culture, which produced indole acetic acid (IAA) and reduced acetylene to ethylene. Polymerase chain reaction (PCR) amplification confirmed the presence of nif-H gene in all the isolates. Morphological, biochemical, and molecular characteristics indicated that all of them belonged to the genus Burkholderia One of them, MGK3, was consistently more active in reducing acetylene, and 16S rDNA sequences of isolate MGK3 confirmed its identification as Burkholderia vietnamiensis. Colonization of rice root was confirmed by strain MGK3 marked with gusA gene. The inoculated roots showed a blue color, which was most intense at the points of lateral root emergence and at the root tip. Transverse sections of roots, 15 days after inoculation, revealed beta-glucuronidase (GUS) activity within many of the cortical intercellular spaces next to the stele and within the aerenchyma. Nitrogen fixation was quantified by using (15)N isotope dilution method with two different cultivars grown in pot and field experiments. Higher nitrogen fixation was observed in variety Ponni than in ADT-43, where nearly 42% (field) and 40% (pot) of the nitrogen was derived from the atmosphere (% Ndfa). Isolate MGK3 was used to inoculate rice seedlings in a comparison with four other diazotrophs, viz., Gluconacetobacter diazotrophicus LMG7603, Herbaspirillum seropedicae LMG6513, Azospirillum lipoferum 4B LMG4348, and B. vietnamiensis LMG10929. They were used to conduct two pot and four field inoculation experiments. MGK3 alone, and combined with other diazotrophs, performed best under both pot and field conditions: combined inoculation produced yield increases between 9.5 and 23.6%, while MGK3 alone increased yield by 5.6 to 12.16% over the uninoculated control treatment.

During a survey of nitrogen-fixing Burkholderia associated with sugarcane in Tamil Nadu, some endophytes were isolated on PCAT medium. Isolation was based on the use of the selective PCAT medium. Four isolates were studied, all belonging to the genus Burkholderia. One of them, MG43 was consistently more active in reducing acetylene and was identified as Burkholderia vietnamiensis. This isolate was used to inoculate micro-propagated sugarcane plantlets in a comparison with two other diazoptrophs, viz. Gluconacetobacter diazotrophicus T and Herbaspirillum seropedicae T . Inoculated plants and uninoculated controls were used in a pot experiment followed by two field experiments under different rates of nitrogen fertilisers. MG43 and G. diazotrophicus performed best in sugarcane, their natural host. Biomass increase due to MG43 inoculation reached 20% in the field. Inoculated plants were heavily colonised by the inoculated bacterium (up to 115,000 CFU g )1 root fresh weight). Inoculation by a combinaison of the three strains performed less well than inoculation by a single MG43 suspension. Ecological implications are discussed, as well as the potential of these bacteria to provide a feasible alternative to higher N fertilisers rates in a low input and long term sustainable rural economy.

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