Screening maize inbred lines and crosses for resistance to aflatoxin and fumonisin accumulation

Abstract

Maize forms a primary diet in Africa and the demand exceeds its local production. In Kenya, it is equally a subsistence and commercial crop, grown by small scale (75%) and large scale farmers (25%). Production of this crop is usually constrained by ear rot fungi in the Aspergillus and Fusarium genus. In Sub-Saharan Africa, 13-70% of maize yield is lost due to ear rots. Further still, these fungi produce mycotoxins, some of which are poisonous to both humans and animals. Over 300 mycotoxins are known but in this study aflatoxins and fumonisins are reported due to their health concerns in Africa. Outbreaks of aflatoxicosis, resulting into deaths, have occured in Kenya yearly since 1981, in Eastern Kenya, due to consumption of maize contaminated with aflatoxins. Aspergillus flavus has been the cause of the contamination. Chronic exposure to aflatoxins is known to cause cancer while consumption at high levels result to death in both humans and other animals. Aflatoxin is classified as class 1 carcinogen by Internationaion Centre for Cancer Research. Among the Fusarium toxins, is fumonisin, which is also a commonly reported contaminant in maize in Eastern Kenya and is documented to cause oesophageal cancer in humans. The major cause of fumonisins in maize is F. verticilloides. Knowledge and management of these fungi is therefore an important step in control of human exposure to these toxins. This study aimed at determining mating types among A. flavus populations and identifying maize inbred lines and hybrids resistant to Aspergillus ear rot (AER), Fusarium ear rot (FER), aflatoxins and fumonisins accumulation. Forty four A. flavus isolates from the University of Nairobi mycology culture collection were grouped by mating types (MAT) using multiplex polymerase chain reaction (PCR) assay. Field trials for screening for resistance were conducted in Kiboko and Katumani during 2013 planting season. Twenty three inbred lines were planted in Fusarium and Aspergillus blocks. In each block, one seed was sown per hill and a total of 33 seeds planted per row. Inoculation with 2ml of inoculum made from selected three toxigenic species of A. flavus and F. verticilloides was done respectively in the two blocks when the silks were about an inch long. Hybrids developed from the same inbred lines were planted and inoculated in the same manner. The experimental blocks were managed using common agronomic practices including irrigation as necessary. Timely harvesting was done at four weeks after physiological maturity and ear rot evaluation performed immediately after harvest. The maize cobs were dried in the sun until about 13% moisture content and the grains screened for aflatoxins and fumonisins accumulation using Enzyme Linked Immunosorbent Assay (ELISA). The isolates with bands corresponding to 270 base pairs (bp) were grouped as MAT1-2 and those corresponding to 396 bps as MAT1-1. Aspergillus flavus isolates used for spore inoculation which included strains 201365 had MAT1-1 and MAT1-2 at a single MAT locus while 100095 and 100130 belonged to MAT1-2. Presence of mating type genes in the isolates was important because through mating type analysis of the isolates, gene flow among the fungi can be traced and appropriate biocontrol strategy incorporating gene flow aspect developed. Lines CML 495 (1.091μg/kg), CB 222 (1.1μg/kg), CML 390 (1.23μg/kg), and CML 444 (1.362μg/kg) had least aflatoxins and can be used for breeding for resistance to aflatoxin. However, lines P502 (38.4μg/kg) and R119W (37.6μg/kg) accrued high aflatoxin levels. Lines CML 390 (1.08mg/kg), CML 247 (1.43mg/kg) and CKL05003 (1.78mg/kg) can be used for breeding for resistance to fumonisin though R2565Y (6.76mg/kg) and VO617Y-2 (6.21mg/kg) accumulated the most fumonisins. Hybrids CKLO5022 (F) x CB 248 (M) (15.877μg/kg) and CB 248 (F) x CKLO5022 (M) (17.545μg/kg) had aflatoxins below 20μg/kg. The other crosses ranged between 22.455-99.263μg/kg. Seventy three percent of the hybrids accumulated fumonisins below 2mg/kg (0.224-1.865). Hybrid CKLO5015 (F) x CML 444 (M) had the least fumonisin (0.224mg/kg) and CML 444(F) x CKLO5015(M), CML 444(F) x CML 495(M), CML 495(F) x CML 444(M), P502(F) x CML 444(M), CKLO5015(F) x R119W(M) and CML 444(F) x R119W(M) have shown resistance to Fusarium infection thus recommended for inclusion in hybrid programs.