Characterisation of Endo-Polygalacturonases Activities of Rice (Oryza sativa) Fungal Pathogens in Nigeria, West Africa

Abstract

Rice (Oryza sativa) is cultivated in swampy geographical locations of tropical Nigeria, West Africa. Here it is infected by a host of fungal pathogens on the field or contaminated at postharvest. This has led to its loss and reduction in its production in both the national and global market. Lasiodiplodia theobromae and Rhizoctonia solani have recently been identified as the major fungal phytopathogens causing the deterioration of this grain on the field and at postharvest and affecting its production in Nigeria leading to gross capital loss. Hence the need to determine physiological control measures for the eradication of both phytopathogens on the field and at postharvest. In this study, tropical strains of Lasiodiplodia theobromae and Rhizoctonia solani obtained from deteriorated rice (Oryza sativa) were grown in a growth nutrient medium composed of MgSo4.7H20, K2HPO4, FeSO4.7H20, potassium nitrate and pectin at 30°C. Endo-Polygalacturonase activities were produced by the fungal isolates in the growth medium within ten days. The endo-polygalacturonases from both fungi were purified by a combination of ammonium sulphate precipitation, dialysis, gel filtration (on Sephadex G-100 column) and ion-exchange chromatography (on CM-Sephadex C-50 and CM-Sephadex C-25 columns). The molecular weight of endo-polygalacturonase from the Lasiodiplodia theobromae using Sephadex G-100 was estimated as 124,000 Daltons while that of the Rhizoctonia solani was estimated as 92,000 Daltons. The purified endo-polygalcuronase from the Lasiodiplodia theobromae exhibited optimum activity at 30°C and at pH 4.5 while that from the Rhizoctonia solani exhibited optimum activity at 32°C and at pH 5.0. The purified endo-polygalacturonases from both fungi exhibited optimum activities at 0.2% pectin concentration. They were stimulated by Ca2+ but inhibited by ethlylenediamine tetracetic acid (EDTA) and 2,4-dinitrophenol. The purified endo-polygalacturonase from the Lasiodiplodia theobromae lost 80% of its activity within 20 minutes of heat at 80°C. While the purified endo-polygalacturonase from the Rhizoctonia solani lost 82% of its activity within 20 minutes of heat at 80°C. Potassium nitrate as nitrogen source in the defined growth medium with pectin as carbon source supported highest activity of endo-polygalacturonase by the Lasiodiplodia theobromae while ammonium chloride as nitrogen source in the defined growth medium with pectin as carbon source supported highest activity of endo-polygalacturonase by the Rhizoctonia solani. In conclusion, the conditions inhibiting endo-polygalacturonases from Lasiodiplodia theobromae and Rhizoctonia solani capable of degrading the pectin portion of rice (Oryza sativa) can be adapted as feasible control measures limiting the infection and contamination of rice (Oryza sativa) by these phytopathogens on the field and at postharvest. Temperature and pH extreme from 30°C and pH 4.5 will be feasible inhibitory control measures for the growth of Lasiodiplodia theobromae on rice (Oryza sativa) in Nigeria while temperature and pH extreme from 32°C and pH 5.0 will inhibit growth of Rhizoctonia solani on the grain. These physiological conditions will preserve pectin in rice (Oryza sativa) from degradation by these two fungal phytopathogens.