Arbuscular mycorrhizal fungi induce Zn uptake and antioxidant efficiency in broccoli (Brassica oleracea L.) exposed to Zn deficiency

Zinc (Zn) is an essential micronutrient for plant growth and development, and its deficiency in soil can be a significant problem for broccoli (Brassica oleracea L.) production. While the genetic efforts to breed Zn-efficient broccoli are complex, harnessing microbiomes is an emerging way to induce mineral efficiency in crops. In this study, broccoli, a non-host mycorrhizal species, was cultivated with or without zinc deficiency in the presence or absence of arbuscular mycorrhizal fungi (AMF), confirmed by the colonization efficiency in roots. Zn deficiency caused a significant decrease in morphological parameters and photosynthetic attributes being consistent with the decreased Zn levels in root and shoot relative to Zn-sufficient plants. However, the broccoli plants inoculated with AMF showed a substantial improvement in morphological and photosynthetic parameters in Zn-deficient conditions due to the significant increase in Zn levels. In addition, AMF-inoculated plants under Zn deficiency showed a significant decrease in cell death (%), electrolyte leakage, and H2O2 levels compared to Fe-deficient plants, which further suggests the improvements in cellular status due to the AMF colonization. The real-time PCR experiments showed a significant induction in the expression of BoZIP1 and BoNRAMP1 genes in the roots of Zn-deficient broccoli inoculated with AMF, suggesting that AMF may be associated with the induction of these genes responsible for Zn-uptake and transport. We further observed the induction of POD (peroxidase), APX (ascorbate peroxidase) and SOD (superoxide dismutase), and S-metabolites (cysteine and glutathione) predominantly in the roots of Zn-starved broccoli inoculated with AMF which may confer tolerance to Zn-deficiency induced oxidative damage. This is the first report on the role of AMF in mitigating Zn-deficiency in broccoli which may promote the microbiome-aided improvement of plants suffering from Zn-deficiency in broccoli and other vegetable crops.