Molecular and Physiological Mechanisms of the Cadmium Response in Seedlings of Two Theobroma cacao L. Genotypes

Aims Understanding the mechanisms of cadmium (Cd) accumulation in cacao plants is critical for mitigating health risks associated with Cd exposure through chocolate consumption and for guiding plant breeding strategies. A general and genotype-specific molecular and physiological responses were characterized. Methods The seedlings of the two cacao genotypes, PA121 and TSH660, were exposed to 0 and 10 ppm Cd in hydroponic conditions. Leaf and root samples were collected at 0, 24, and 48 h (RNAseq) and at 60 days post-treatment (ICP-OES). Gene expression profiles of Cd-treated and untreated plants were compared using differential gene expression (DEG) and gene ontology analyses. Gas exchange and abscisic acid (ABA) measurements were conducted on greenhouse-grown seedlings of genotype PA121. Results The number of DEGs recorded in roots was nearly twice as high as in leaves at 48 h after Cd exposure. Shared and genotype-specific DEGs related to detoxification, reactive oxygen species, and hormone pathways were upregulated in roots, and carbohydrate, tricarboxylic acid cycle, fatty acid, and terpenoid synthesis DEGs were activated in leaves. Additionally, genes from Cd-transport families, such as ZIP/IRT and NRAMP, were downregulated in roots. More significantly, ABA-associated biosynthetic and signaling transcripts and ABA abundance increased in roots after Cd treatment. PA121 seedlings exposed to 12 ppm Cd exhibited reduced stomatal conductance without a significant decline in photosynthesis. Conclusions These findings are consistent with a model in which Cd triggers ABA-linked root signaling that reduces stomatal conductance and mass flow and reduces expression of active Cd2+ transport, thereby limiting Cd uptake.