Short-term drought effect on biochemical processes and microbial growth in the rhizosphere of two maize genotypes.

The rhizosphere is a highly dynamic biological interface where most decomposition processes of soil organics are performed by actively growing microorganisms producing extracellular enzymes. As the rate of enzymatic reactions and affinity of enzymes to the substrate are influenced by plant genotype and water content in soil, we hypothesized to boost genotype effect of wild and root hair deficient maize plants after a short-term drought due to resources limitation. We further hypothesized that (1) maximum enzymatic rates (V_max) for ß-glucosidase, leucine-aminopeptidase, acid phosphatase, and N-acetylglucosaminidase will decrease due to low accessibility to substrates; and (2) microbial growth will be retarded due to limited nutrients availability. We tested these hypotheses on the Zea mays L. (WT) and a root hair deficient mutant (rth3) grown in soil columns. Drought effect was compared between the brushed soil from roots called root-affected soil, and the rhizosphere soil obtained after the subsequent washing of roots. Microbial growth induced by glucose and nutrients application was determined by microcalorimetry.

Only two of four enzymes tested were sensitive to drought: ß-glucosidase and phosphatase. Maximum enzymatic rates of ß-glucosidase and phosphatase in the rhizosphere were, respectively, 73 and 47 % slower under drought treatment, compared to the well-watered plants. In the rhizosphere of rth3, only ß-glucosidase activity was reduced by 32 % under drought treatment compared to well-watered plants In root-affected soil, drought decreased ß-glucosidase activity by 72 and 57% for WT and rth3 plants, respectively. In the rhizosphere of WT plants, higher affinity for substrates was revealed for ß-glucosidase and phosphatase, respectively, as 31 and 42% lower Michaelis-Menten affinity constant (K_m) under drought versus optimal watering. In the root-affected soil of rth3 mutant, only ß-glucosidase showed a 39 % lower K_m under drought compared to well-watered plants. Higher enzymatic affinity under drought versus optimal moisture indicated a different set of enzymes either of microbial or plant origin. On the other hand, plant genotype effect was visible under drought for ß-glucosidase activity in rhizosphere soil, when maximum rate was 54 % lower for WT plants compared to rth3, suggesting that ß-glucosidase activity hotspots were not associated to root-hairs.

Glucose-induced microbial growth was retarded for 12 to 14 hours under drought compared to well-watered treatment. A prolonged lag phase could be due to the smaller fraction of active microorganisms, which is driven by a non-optimal moisture of the soil. Moisture appeared to be a more determinant factor for microbial growth and enzymatic activity compared to plant genotype, whose effect was reinforced under drought.