Chemotactic response to amino acids of fluorescent pseudomonads isolated from spinach roots grown in soils with different salinity levels

Abstract

Chemotactic response to amino acids of fluorescent pseudomonads isolated from spinach roots grown in a low-salinity soil (LR-group) and its corresponding high-salinity soil (HR-group) was investigated. Furthermore, the amount and composition of amino acids in the root exudate under low- and high-salinity conditions were investigated. All the isolates examined showed a chemotactic response exclusively to the amino acid fraction, while most of them did not respond to the sugar and the organic acid fractions. Based on these results, the chemotactic response to 20 amino acids of randomly selected 48 isolates, of which 12 isolates each belongs to the LR- and HR-groups of both Pseudomonas putida and P. fluorescens, was investigated. The results showed that the spectrum of chemotaxis to 20 amino acids of the H-group of each species was markedly different from that of the corresponding L-group. Also, the amino acid composition of the root exudate in the high-salinity culture solution significantly differed from that in the low-salinity culture solution. The total amount of amino acids from the high-salinity solution was 2.83 μmol g-1 fresh roots, which was as small as 42% of that from the low-salinity culture solution. These results indicated that high salinity of soil may exert an effect on root colonization by fluorescent pseudomonads not only directly through the effect in the bacterial chemotaxis but also indirectly through the modification of the root exudation of amino acids qualitatively and quantitatively.