Phosphate Solubilization and Growth Promotion of Maize (Zea mays L.) by the Rhizosphere Soil Fungus Penicillium oxalicum.
Wansik Shin1, Jeounghyun Ryu1, Yongjoo Kim2, Jinchul Yang2, Munusamy Madhaiyan1, and Tongmin Sa1. (1) Chungbuk National Univ, 12, Gaesin-dong, Heungduck-ku, Cheongju, South Korea, (2) KG Chemical Corp., 429-1, Okkil-dong, Sosa-Gu,, Bucheon, South Korea
Phosphorus (P), one of the major plant nutrients whose availability is conditioned by various factors, is acquired from soil solution as phosphate anions (predominantly HPO42- and H2PO4-), and these anions are extremely reactive and may be immobilized through precipitation. The availability of phosphate anions in soil is attributed to the presence and activity of soil microorganisms. The existence of soil microorganisms capable of transforming soil P to forms available to the plants has earlier been reviewed. A microbial plant inoculant able to dissolve sparingly soluble inorganic soil P during plant growth would have both economic and environmental advantages. One such soil fungus is the Penicillium fungus, which is considered to be a key group of soil microflora involved in P cycling. We have reported already the mineral phosphate solubilizing activity of Penicillium oxalicum CBPS-3F-Tsa, a soil fungus isolated from rice rhizosphere from Youngnam province of Korea. In this study, we examined (i) the in vitro mineral phosphate solubilizing (MPS) activity of this fungus and possible mechanisms of solubilization in terms of organic acid production and (ii) its impact on growth, and nitrogen (N) and phosphorous (P) accumulation in maize plants in the presence of fused and rock phosphates. The fungus solubilized 129.1, 118.8 and 54.1 mg P L-1 of tri-calcium phosphate [Ca3(PO4)2], aluminium phosphate (AlPO4) and ferric phosphate (FePO4), respectively, after 72 h of incubation. Malic, gluconic and oxalic acids were detected in the flasks supplemented with various phosphate sources together with large amount of malic acid [240, 146, 145 mM AlPO4, FePO4, and Ca3(PO4)2 respectively] followed by the other two. The effects of inoculation of P. oxalicum CBPS-3F-Tsa on maize plants were studied under pot culture conditions. P. oxalicum CBPS-3F-Tsa was inoculated to maize plants alone or along with inorganic phosphates in the form of fused phosphates (FP) and rock phosphates (RP). Inoculation of P. oxalicum CBPS-3F-Tsa increased the plant growth, and N and P accumulation in plants compared to control plants and also had positive effects when applied with RP. The results of this study report that the fungus P. oxalicum strain CBPS-3F-Tsa could solubilize different insoluble phosphates by producing organic acids, particularly malic acid and also improved the efficiency of RP applied to maize plants. The increase of dry weight and N and P accumulation in shoots and roots, highlights the importance of adding P at seeding time and the influence of fungal isolate in playing a role in the release of phosphates in soil. P. oxalicum CBPS-3F-Tsa actively solubilized inorganic RP and increased the accumulated P in maize plants. But this effect could not be seen when applied with FP. This could most likely be due to the repression of P solubilization in microbes, when exposed to phosphate rich environments. It is also evident that the fungus requires an optimum amount of P available soil for it to function efficiently. The presently described results clearly suggest that, under greenhouse conditions, it is possible to benefit from the phosphate solubilizing activity of P. oxalicum CBPS-3F-Tsa when rock phosphate is used. More field trials are required before the recommendation of fungus as a beneficial bioinoculant for maize crop. Further studies on the root colonization and yield improvement would shed more light to the prospects of fertilizing the maize crop with P. oxalicum CBPS-3F-Tsa.