Physiological characteristics (SDH and ALP activities) of arbuscular mycorrhizal colonization as affected by Bacillus thuringiensis inoculation under two phosphorus levels

The effect of Bacillus thuringiensis (B.t.) inoculation on plant growth and on the intra- and extraradical mycorrhizal development of lettuce roots colonized by Glomus mosseae or Glomus intraradices was examined in an inert, soil-less substrate. Histochemical determination of succinate dehydrogenase (SDH) and alkaline phosphatase (ALP) activities which indicate active fungal metabolism was carried out at two phosphorus (P) levels. The presence of B.t. increased extra- and intraradical colonization [measured as frequency (%F), intensity (%I) and percentage of arbuscules (%A)] for both arbuscular mycorrhizal fungi (AMF) rather than plant growth or nutrition regardless P level. Under the lowest level of P fertilization, B.t. enhanced to a similar extent the extra- and intraradical development of both endophytes, but the proportion of fungal tissue showing SDH or ALP was increased in G. intraradices-colonized plants. [SDH: 458% (M) and 512% (A); ALP: 358% (M) and 300% (A)]. P supply decreased G. intraradices colonization to a higher extent than G. mosseae. Nevertheless, the totality of G. intraradices structures developed in P-amended medium showed intraradical o extraradical activity, while in G. mosseae-colonized roots, SDH and ALP activities highly decreased relative to fungal tissue determined by TB staining as affected by P. Our results show that bacterial inoculation compensates the negative effect of P on the intraradical fungal growth and vitality. P amendment reduced in a higher extent G. intraradices infection intensity (non-vital and vital staining) and G. mosseae activity (ALP staining). Thus, big differences in the proportion of SDH-active infection showing ALP activity in mycelium developed by each endophyte were noted at the highest P level. Physiological plant parameters such as photosynthetic activity did not explain specific changes on each arbuscular-mycorrhizal fungus as affected by P or B.t. inoculation. The increased extraradical mycelium development and metabolic fungal activity as a result of B.t. inoculation positively affected N and P plant content and photosynthetic rate in G. intraradices-colonized plants under the lowest P conditions. In general, the increased metabolically active fungal biomass in co-inoculated plants was irrespective of P level and was not related to the P plant uptake from the inert soil-less substrate. These results show the bacterial effect increasing the physiological and metabolic status of AM endophytes, which not only confirms but also extends previous findings on arbuscular mycorrhizae-bacteria interactions. The present study emphasizes the ecological and practical importance of rhizosphere free-living bacteria as mycorrhizae-helper microorganisms.