Mechanisms Of Infection And Colonization Of The Dominant Endophyte Pantoea Agglomerans YS19 On Host Rice Plant

This is the first report about mechanisms of infection and colonization of adorminant endophytic diazotroph Pantoea agglomerans YS 19 on host plant rice(Oryza sativa) Yuefu.The results showed that there were two growth phases for F agglomeransYS 19 as cultivated in liquid medium, including the single cell growth phase beforeexponential growth phase and the symplasmata forming phase beginning at thedeceleration growth phase.A rifampicin resistant mutant strain P agglomerans YS 1 9B was screened andused to conjugate with the gfr gene donator strain Escheichia coli ccli 8 A /pFAJ 1820.The conjugons could produce bright green fluorescence. The maximal growth rate andbiomass of the labeled strain reduced 12.4% and 6% respectively in comparison withthe wild type strain. Meanwhile the generation time increased by 14.0%. A dynamicsequation model describing the growth of YS19B::gfr companied with the loss of gfrwas established and the loss rate of the gfr was calculated as 9.756 X 10 ~. Thefluorescence spectra of P agglomerans YSI9B::gfr showed that the maximalemission wavelength was 508 nm when excited at 400 nm. The GFP production of Fagglomerans YS 1 9B :gfr reached its maximal value when cultivated for 20 hours inLB medium. The labeled strain might die in starved cultivating conditions, howeverthe fluorecence of the living cells were conserved. It was suggested that YS19B::g/j,was suitable to be used to visualize the interaction with the host plant.Microscopic observation of colonization of P agglomerans YS19B::gfr on riceroots using laser scanning confocal microscope showed that the young roots werepreferably colonized by endophytic bacteria in comparison with the older roots. Andthe elongation region of young roots was the dominant habit. The lateral rootjunctions and the root caps were also important infection and colonization sites for Pagglomerans YS 1 9B: :gfr.Microscopic studies of infection and colonization of P agglomeran.s YS 19 onrice plants cultivated under gnotobiotic conditions showed that P agglomerans cells3transformed from long bacillary bacteria into short bacillary bacteria when thesymplasmata formed in medium and of which the cell volume decreased to 30% oforiginal form. The symplasmata also formed inside roots, stems and leaves when riceplants were infected by P agglomerans YS 19. The volume of component cells of thesymplasmata decreased to 20----30% in comparison with the cultivated form. It wassuggested that the symplasmata might be an adaptive structure of this strain forendophytic living inside host plants. It could be favorable for endophytic nitrogenfixation according to the calculation of the model of oxygen spreading in thesymplasmata. More P agglomerans YS 19 were found colonized on the leaf surfacesthan on the roots. It was also identified that the lateral root junctions and root hairswere important sites for P agglomerans YS 19 infection. P agglomerans YS 19 mostlycolonized in root cortex parenchyma tissues, stem parenchyma tissues and centralbasal tissues; while only a few were found in vascular tissues. The endophyticbacteria might transform their habitats mainly depend on plant vascular tissues andstomata, hydathodes on leaf sur&ces.Absorbing isotherm and absorbing velocity at 25t of P agglomeraris YS19absorbed on rice seedlings roots were measured. As dominant endophytic bacteriaassociated with rice, P agglomerans YS 19 showed much higher absorbing velocitythan that of the control strain. The quantity of single layer fill absorption q, was 1.43X iO'~ mg' and the absorbing coefficient was 2.35 X 10'. It was suggested that theselectivity and specificity of an endophyte had been showed at the absorption stageduring the infection process. The absorbing coefficient could be...