Phosphate solubilization by Penicillium spp. closely associated with wheat roots

In this study we found that Penicillium spp. exhibiting P-solubilizing activity are common both on and in the roots of wheat plants grown in southern Australian agricultural soils. From 2,500 segments of washed and surface-disinfested root pieces, 608 and 223 fungi were isolated on a selective medium, respectively. All isolates were screened for P solubilization on solid medium containing hydroxyapatite (HA); 47 isolates (5.7%) solubilized HA and were identified as isolates of Penicillium or its teleomorphs. These isolates were evaluated for solubilization of Idaho rock phosphate (RP) in liquid culture. Penicillium bilaiae strain RS7B-SD1 was the most effective, mobilizing 101.7 mg P l(-1) after 7 days. Other effective isolates included Penicillium simplicissimum (58.8 mg P l(-1)), five strains of Penicillium griseofulvum (56.1-47.6 mg P l(-1)), Talaromyces flavus (48.6 mg P l(-1)) and two unidentified Penicillium spp. (50.7 and 50 mg P l(-1)). A newly isolated strain of Penicillium radicum (KC1-SD1) mobilized 43.3 mg P l(-1). RP solubilization, biomass production and solution pH for P. bilaiae RS7B-SD1, P. radicum FRR4718 or Penicillium sp. 1 KC6-W2 was determined over time. P. bilaiae RS7B-SD1 solubilized the greatest amount of RP (112.7 mg P l(-1)) and had the highest RP-solubilizing activity per unit of biomass produced (up to 603.2 mug P l(-1) mg biomass(-1) at 7 days growth). This study has identified new isolates of Penicillium fungi with high mineral phosphate solubilizing activity. These fungi are being investigated for the ability to increase crop production on strong P-retaining soils in Australia.