Mycorrhizal symbiosis maintains DNA integrity and cellular homeostasis in lettuce under combined heavy metals and salt stress conditions

Abstract

Combination stresses are the primary obstacle that plants encounter in nature. Consequently, there is an urgent need for environmentally sustainable solutions. Arbuscular mycorrhizal fungi (AMF) constitute one such eco-friendly approach for promoting agricultural sustainability under heavy metals (HMs) contamination, drought, heat, and salinity stress conditions, owing to their well-documented role as biostimulants. Therefore, the present investigation aimed to assess the impact of combined HMs (Cr, Pb, and Cd; each at 100 mg L− 1) and salt stress (100 mM NaCl) on lettuce and to evaluate the potential of AMF inoculation to mitigate these combined stresses. AMF colonization effectively mitigated this adverse effect, as evidenced by increases in shoot fresh weight (12.97%), total pigment (76.74%), relative water content (RWC, 10.98%), glycine betaine (GB, 21.04%), and phenolic content (15.68%). Conversely, a substantial decrease in stress markers was observed, including lipid peroxidation (MDA, 11.03%), H2O2 content (20.66%), and reductions in the antioxidant enzymes (23.56% in POD, 22.38% in PPO, and 14.73% in phenylalanine ammonia-lyase [PAL]). The Start codon targeted (SCoT) analysis demonstrated that all of these were associated with a 51% retention of genome integrity, and the percentage of damaged nuclei was reduced by 25.78%, as confirmed by the comet assay. Both easily extractable (EE, 29.58%) and total extractable (TE, 36.42%) glomalin content were significantly increased by AMF colonization upon stress. Although AMF-colonized roots exhibited a higher concentration of Pb, Cr, and Cd during combined stress, the concentration of these metals in the shoots was lower. This reduction was attributed to the ability of AMF to reduce the root-shoot translocation by 74.39, 61.42, and 56.92% for Cr, Pb, and Cd, respectively. This method could be used to cultivate lettuce in contaminated locations by trapping HMs and Na+ ions in the roots, which are not edible, while allowing the edible shoots to develop. Thus, AMF could be used to protect food safety and agricultural output in contaminated areas. Experimental layout and the main findings of the study. Lettuce (L. sativa L.) seedlings were AMF-inoculated, grown for 20 days after transplanting, and then subjected for 12 days to the combined HMs (100 mg L− 1 of each Pb, Cr, and Cd) and salt (100 mM NaCl) stress. Increase is indicated with an arrow with the top pointing up, and decrease is indicated with a downward-pointing arrow. *C; refers to control, S; stressed plants with 100 mg L− 1 of Cr, Cd, and Pb and 100 mM NaCl, AMF; AMF-inoculated plants, and AMF + S; AMF-inoculated and stressed with a combination of 100 mg L− 1 of Cr, Cd, and Pb and 100 mM NaCl Experimental layout and the main findings of the study. Lettuce (L. sativa L.) seedlings were AMF-inoculated, grown for 20 days after transplanting, and then subjected for 12 days to the combined HMs (100 mg L− 1 of each Pb, Cr, and Cd) and salt (100 mM NaCl) stress. Increase is indicated with an arrow with the top pointing up, and decrease is indicated with a downward-pointing arrow. *C; refers to control, S; stressed plants with 100 mg L− 1 of Cr, Cd, and Pb and 100 mM NaCl, AMF; AMF-inoculated plants, and AMF + S; AMF-inoculated and stressed with a combination of 100 mg L− 1 of Cr, Cd, and Pb and 100 mM NaCl