Seed-born Burkholderia glumae Infects Rice Seedling and Maintains Bacterial Population during Vegetative and Reproductive Growth Stage

Abstract

Rice world production is affected due to the growing impact of diseases such as bacterial panicle blight, produced by Burkholderia glumae. The pathogen-induced symptoms include seedling rot, grain rot and leaf-sheath browning in rice plants. It is currently recognized the entrance of this pathogen to the plant, from infected seeds and from environmental sources of the microorganism. However, it is still not fully elucidated the dynamics and permanence of the pathogen in the plant, from its entry until the development of disease symptoms in seedlings or panicles. In this work it was evaluated the infection of B. glumae rice plants, starting from inoculated seeds and substrates, and its subsequent monitoring after infection. Various organs of the plant during the vegetative stage and until the beginning of the reproductive stage, were evaluated. In both inoculation models, the bacteria was maintained in the plant as an endophyte between 1 × 10¹ and 1 × 10⁵ cfu of B. glumae.g^-1 of plant throughout the vegetative stage. An increase of bacterial population towards initiation of the panicle was observed, and in the maturity of the grain, an endophyte population was identified in the flag leaf at 1 × 10⁶ cfu of B. glumae.g^-1 fresh weight of rice plant, conducting towards the symptoms of bacterial panicle blight. The results found, suggest that B. glumae in rice plants developed from infected seeds or from the substrate, can colonize seedlings, establishing and maintaining a bacterial population over time, using rice plants as habitat to survive endophyticly until formation of bacterial panicle blight symptoms.

Keywords: Burkholderia glumae; endophyte population; rice plants; sources of inoculum.

Fig. 1

Development of B. glumae disease symptoms in rice seedlings (cvF733), at 7 DAI, through two inoculum sources of the pathogen (A) Seed (vacuum inoculation) and (B) Substrate (pathogenic bacteria incorporated into the supporting substrate). Different letters show statistically significant differences between treatments, with each of the variables (stem length, root length and degree of infection), according to the Duncan test, with a level of 95.0% confidence. The graph corresponds to one of at least two repetitions in time, for both infection models, which showed the same trend and statistical differences.

Fig. 2

Effect of inoculation of B. glumae on stem and root length in relation to non-inoculated plants during vegetative and reproductive phases of the plant. (A) Inoculation under the vacuum seed model and (B) Inoculation in substrate (bacteria incorporated in the seed/plantlet substrate). Different letters show statistically significant differences for each of the plant organs between the vegetative stages, according to the Duncan Test with a level of 95.0% confidence. ^¶ Phases determined according to the extended BBCH scale for rice according to Hack et al. (1992) modified from Lancashire et al. (1991).

Fig. 3

Agarose gel electrophoresis showing PCR products for presumptive B. glumae colonies of inoculated plants at stage of 2–4 tillers. MWM molecular weight marker. Lines: 1, control reaction without DNA sample; 2, Positive control (pure DNA sample of B. glumae 3200-12); 3–6, Presumptive colonies isolated from leaf; 7–10, Presumptive colonies isolated from stem; 11–12, Presumptive colonies isolates from root; 13, Negative control (non-presumptive colony DNA). The size of the amplicon is 286 bp.