Pseudomonas fluorescens 2P24 is a soilborne bacterium that synthesizes and excretes multiple antimicrobial metabolites. The polyketide compound 2,4-diacetylphloroglucinol (2,4-DAPG), synthesized by the phlACBD locus, is its major biocontrol determinant. This study investigated two mutants defective in antagonistic activity against Rhizoctonia solani. Deletion of the gidA (PM701) or trmE (PM702) gene from strain 2P24 completely inhibited the production of 2,4-DAPG and its precursors, monoacetylphloroglucinol (MAPG) and phloroglucinol (PG). The transcription of the phlA gene was not affected, but the translation of the phlA and phlD genes was reduced significantly. Two components of the Gac/Rsm pathway, RsmA and RsmE, were found to be regulated by gidA and trmE, whereas the other components, RsmX, RsmY, and RsmZ, were not. The regulation of 2,4-DAPG production by gidA and trmE, however, was independent of the Gac/Rsm pathway. Both the gidA and trmE mutants were unable to produce PG but could convert PG to MAPG and MAPG to 2,4-DAPG. Overexpression of PhlD in the gidA and trmE mutants could restore the production of PG and 2,4-DAPG. Taken together, these findings suggest that GidA and TrmE are positive regulatory elements that influence the biosynthesis of 2,4-DAPG posttranscriptionally.

Acidovorax citrulli causes seedling blight and bacterial fruit blotch (BFB) of cucurbits, a serious disease threat to cucurbit seed and fruit production worldwide. Despite its economic importance, current knowledge on the biology and pathology of A. citrulli is limited. This work studies a mini-Tn5 mutant, MJ22-3, that showed reduced virulence on melon seedlings. The gene disrupted by Tn5 was identified as leuB, which encodes a putative 3-isopropylmalate dehydrogenase, an enzyme that appears to be essential for leucine biosynthesis. The leuB gene in-frame deletion mutant MDleuB, as well as the Tn5-inserted mutant MJ22-3, failed to grow in minimal medium without leucine and exhibited reduced swimming motility; however, the mutations had no effect on biofilm formation or induction of the hypersensitive response on tobacco (Nicotiana tabacum). When inoculated at a low cell density (10 4 CFU mL À1), the leuB gene mutants showed a significantly slower growth rate and attenuated virulence on melon cotyledons compared to the wildtype MH21. However, inoculations with the mutants at high concentration (10 8 CFU mL À1 ), or low concentration (10 4 CFU mL À1 ) inoculations supplemented with 0Á01% leucine resulted in growth rates and disease severities similar to that of the wildtype MH21. The results suggest that leucine biosynthesis is essential for both in vivo growth and full virulence of A. citrulli MH21 on melon seedlings.