Chen WJ, Kuo TY, Hsieh FC, Chen PY, Wang CS, Shih YL, Lai YM, Liu JR, Yang YL, Shih MC.

Sci Rep. 2016 Sep 8; 6:32950. doi: 10.1038/srep32950.

Abstract

Rice bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most destructive rice diseases worldwide. Therefore, in addition to breeding disease-resistant rice cultivars, it is desirable to develop effective biocontrol agents against Xoo. Here, we report that a soil bacterium Pseudomonas taiwanensis displayed strong antagonistic activity against Xoo. Using matrix-assisted laser desorption/ionization imaging mass spectrometry, we identified an iron chelator, pyoverdine, secreted by P. taiwanensis that could inhibit the growth of Xoo. Through Tn5 mutagenesis of P. taiwanensis, we showed that mutations in genes that encode components of the type VI secretion system (T6SS) as well as biosynthesis and maturation of pyoverdine resulted in reduced toxicity against Xoo. Our results indicated that T6SS is involved in the secretion of endogenous pyoverdine. Mutations in T6SS component genes affected the secretion of mature pyoverdine from the periplasmic space into the extracellular medium after pyoverdine precursor is transferred to the periplasm by the inner membrane transporter PvdE. In addition, we also showed that other export systems, i.e., the PvdRT-OpmQ and MexAB-OprM efflux systems (for which there have been previous suggestions of involvement) and the type II secretion system (T2SS), are not involved in pyoverdine secretion.

See: http://www.ncbi.nlm.nih.gov/pubmed/27605490

Figure 4: Complementation assay.

(a) ClpV is an essential component of T6SS. T6SS activity was detected by western blotting using VgrG antibody from culture supernatants and cell lysates of wild-type, clpV::Tn5 and clpV::Tn5/clpV. The level of RpoA, which was used as a loading control, was detected by western blotting in cell lysates with a specific antibody. (b) Overexpression of ClpV in clpV::Tn5 mutant is able to complement pyoverdine secretion. The complemented vector pCPP30 harboring a clpV fragment was analyzed for pyoverdine secretion in the clpV::Tn5 mutant by MALDI-IMS. MALDI-IMS imaging of pyoverdine from incubation of wild-type, clpV::Tn5 mutant and clpV::Tn5/clpV P. taiwanensis with Xoo were monitored.