Abstract
There has been significant interest in the development of formulations of non-toxigenic strains of Aspergillus flavus for control of toxigenic strains to reduce the aflatoxin B1 (AFB1) contamination of maize. In the future, climate change (CC) abiotic conditions of temperature (+2–4°C), CO2 (existing levels of 400 vs. 800–1,200 ppb), and drought stress will impact on the agronomy and control of pests and diseases. This study has examined (1) the effect of two-way interacting factors of water activity × temperature on colonization and AFB1 contamination of maize cobs of different ripening ages; (2) the effect of non-toxigenic strains of A. flavus (50:50 inoculum ratio) on relative control of toxigenic A. flavus and AFB1 contamination of ripening cobs; (3) post-harvest control of AFB1 by non-toxigenic strains of A. flavus in non-GM and isogenic GM maize cultivars using the same inoculum ratio; and (4) the impact of three-way interacting CC factors on relative control of AFB1 in maize cobs pre-harvest and in stored non-GM/GM cultivars. Pre-harvest colonization and AFB1 production by a toxigenic A. flavus strain was conserved at 37°C when compared with 30°C, at the three ripening stages of cob development examined: milk ripe (R3), dough (R4), and dent (R5). However, pre-harvest biocontrol with a non-toxigenic strain was only effective at the R3 and R4 stages and not at the R5 stage. This was supported by relative expression of the aflR regulatory biosynthetic gene in the different treatments. When exposed to three-way interacting CC factors for control of AFB1 pre-harvest, the non-toxigenic A. flavus strain was effective at R3 and £4 stages but not at the R5 stage. Post-harvest storage of non-GM and GM cultivars showed that control was achievable at 30°C, with slightly better control in GM-cultivars in terms of the overall inhibition of AFB1 production. However, in stored maize, the non-toxigenic strains of A. flavus had conserved biocontrol of AFB1 contamination, especially in the GM-maize cultivars under three-way interacting CC conditions (37°C × 1,000 ppm CO2 and drought stress). This was supported by the relative expression of the aflR gene in these treatments. This study suggests that the choice of the biocontrol strains, for pre- or post-harvest control, needs to take into account their resilience in CC-related abiotic conditions to ensure that control of AFB1 contamination can be conserved.
Highlights
There has been significant interest in developing biocontrol agents for aflatoxin B1 (AFB1) control in staple commodities, especially maize
The objectives of this study were to examine (1) the effect of ripening stage of maize cobs on rates of colonization and AFB1 production by A. flavus in relation to interactions between two-way abiotic factors of temperature × water availability; (2) the effect of these two-way interacting factors on the control of AFB1 contamination using 50:50 ratios of non-toxigenic and toxigenic strains in maize cobs of different ripening stages and in stored non-GM and isogenic GM maize cultivars; and (3) the effect of three-way interacting climate change (CC) abiotic factors on resilience of non-toxigenic A. flavus strains in terms of reducing the expression of key biosynthetic genes involved in aflatoxin synthesis, and on phenotypic AFB1 contamination, in these two types of maize cultivars
This study has shown that the relationship between pre-harvest ripening stage of maize cobs and their inherent water availability will influence both colonization and AFB1 production by toxigenic A. flavus strains and influence the potential for effective control of toxin contamination
Summary
There has been significant interest in developing biocontrol agents for aflatoxin B1 (AFB1) control in staple commodities, especially maize. Work on A. flavus colonization of maize grain has shown that interacting conditions of +2–5°C, elevated CO2 (650–1,000 ppm), and drought stress can result in an increase in both regulatory (aflR) and structural genes (aflD) involved in AFB1 biosynthesis as well as other secondary metabolite genes in maize grain and lead to a significant stimulation in AFB1 contamination (Battilani et al, 2016; Medina et al, 2017a; Gilbert et al, 2018) Bearing this in mind, it is surprising that while biocontrol of toxigenic A. flavus using microbial antagonists and non-toxigenic strains of A. flavus has been examined for many years, their resilience has never been examined under expected CC regimes (Cotty, 1994; Abbas et al, 2011; Bandyopadhyay et al, 2016; Weaver et al, 2016; Kagot et al, 2019). It is very important to understand how the potential biocontrol strains targeting AFB1 control in maize may behave under interacting CC abiotic factors and whether they have the necessary resilience to reduce biosynthesis of AFB1 in situ
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