Utilization of the plant hormone indole-3-acetic acid for growth by Pseudomonas putida strain 1290

Abstract

We have isolated from plant surfaces several bacteria with the ability to catabolize indole-3-acetic acid (IAA). One of them, isolate 1290, was able to utilize IAA as a sole source of carbon, nitrogen, and energy. The strain was identified by its 16S rRNA sequence as Pseudomonas putida. Activity of the enzyme catechol 1,2-dioxygenase was induced during growth on IAA, suggesting that catechol is an intermediate of the IAA catabolic pathway. This was in agreement with the observation that the oxygen uptake by IAA-grown P. putida 1290 cells was elevated in response to the addition of catechol. The inability of a catR mutant of P. putida 1290 to grow at the expense of IAA also suggests a central role for catechol as an intermediate in IAA metabolism. Besides being able to destroy IAA, strain 1290 was also capable of producing IAA in media supplemented with tryptophan. In root elongation assays, P. putida strain 1290 completely abolished the inhibitory effect of exogenous IAA on the elongation of radish roots. In fact, coinoculation of roots with P. putida 1290 and 1 mM concentration of IAA had a positive effect on root development. In coinoculation experiments on radish roots, strain 1290 was only partially able to alleviate the inhibitory effect of bacteria that in culture overproduce IAA. Our findings imply a biological role for strain 1290 as a sink or recycler of IAA in its association with plants and plant-associated bacteria.

FIG. 1.

Complete, partial, or no degradation of IAA by different representative bacterial isolates in various media. Shown is the fraction of IAA that remained in the medium supernatant after a 75-h incubation of bacteria (100-fold diluted from an overnight King’s B-grown culture) with 9 mM IAA at 30°C in either M9 minimal medium, phosphate buffer supplemented with 0.4% fructose, or plain phosphate buffer. Isolate 1290 is the only representative of group 1 (see Results), isolate 1292 and 1294 are the only ones of groups 2 and 3, respectively, and isolate 1296 represents a total of three isolates in group 4.

FIG. 2.

Growth of P. putida 1290R at the expense of IAA in M9 minimal medium (A) or phosphate buffer (C). Symbols: ▪, OD₆₀₀ as a measure for bacterial biomass; ○, IAA concentration in the medium supernatant. Open triangles (▵) show the fate of IAA in the absence of bacteria. (B) No growth or IAA degradation was observed with E. herbicola 299R. The results shown are representative of several repetitions of the experiment.

FIG. 3.

Catechol 1,2-dioxygenase activity in cell extracts of P. putida 1290R cells grown on M9 minimal medium containing 0.1% glucose, 0.1% glucose, plus 5 mM IAA, 5 mM IAA, or 8.5 mM benzoate.

FIG. 4.

(A) Production of IAA from 4.5 mM tryptophan in M9 minimal medium, supplemented with 0.1% glucose, by P. putida 1290R (▴) or E. herbicola 299R (▵). (B) Corresponding growth curves of P. putida 1290R (▪) and E. herbicola 299R (□). The results shown are representative of several repetitions of the experiment.

FIG. 5.

Effect of IAA, P. putida 1290R, and combinations thereof on the elongation of radish roots in growth pouches. Different letters indicate significantly different (P < 0.05) root lengths. −, No bacteria were added initially; +, addition of 5 × 10⁸ cells of P. putida 1290R per ml.

FIG. 6.

Effect of coinoculation of P. putida 1290R and bacterial IAA producers CD14, CD15, and CD32 on radish root elongation. Different letters indicate significantly different (P < 0.05) root lengths. Inoculation in the absence (−) or the presence (+) of P. putida 1290R was as indicated.