Differed response to cadmium-induced stress in ancient hybrid populations
The principal component analysis (PCA) plot of the gene expression variations revealed that a substantial proportion of gene expression variation on PC1 separates the genotypes. This suggests that genetic background is still the most crucial factor in determining the genetic response to heavy metal stress in common reed. Both genotypes have genetic background as ancient hybrids. Hybridization can lead to species divergence and the formation of stable phylogenetic lineages, as illustrated by the phylogenetic trees (Gardner et al., 2023). However, individuals within the same genetic lineage exhibited markedly different responses to cadmium-induced stress, indicating potential genetic incompatibilities. However, similar fluctuations in gene expression have also been observed in non-hybrid Brassica rapa populations sampled over different time periods, suggesting that plants may undergo rapid evolutionary changes in gene expression in response to climate change (Hamann et al., 2021). Gene expression patterns were observed to change along with environmental factors, such as elevations (Ye et al., 2023), latitude (Chen et al., 2020) and light(Mural, 1991), showing their flexibilities with local adaptation. In this study, differences in gene expression and photosynthetic capacity between the two populations may be attributed to local adaptations to their respective geographic origins. The Panjin population in Liaoning Province is geographically further north than the Heze population in Shandong Province.
Indeed, a comparison between control samples (Panjin control vs Heze control ) showed that a large number of upregulated genes in Panjin are enriched in biological processes such as response to external biotic stimulus, defense response to bacteria and oomycetes, cell surface receptor signaling, response to salicylic acid, intracellular water homeostasis, and phosphorylation. This indicates that the Panjin genotype may be preadapted and obtained enhanced defense responses to biotic stimuli, potentially signifying a higher level of resistance to certain pathogens or environmental stresses. The involvement of signaling pathways and phosphorylation suggests complex regulatory mechanisms underpinning these responses. Physiologically, Panjin demonstrated significantly higher maximum quantum efficiency of photosystem II (Fv/Fm) in physiological reactions, signifying better performance under the same conditions. Under high Cd levels, photosynthesis parameters such as A and chlorophyll (Chl) showed a significant decrease in Heze but not in Panjin, suggesting that Heze is more adversely affected by cadmium stress than Panjin. Since cadmium content in leaf tissues did not significantly differ between the two genotypes, and considering the transcriptomic data were obtained from leaves, we can conclude that the stronger response in Heze is caused by the cadmium treatment itself rather than the heavy metal transportation and accumulation process.
That elucidates why, upon exposure to varying cadmium levels, Panjin genotype exhibited only a few DEGs between the treatment and control groups. In contrast, Heze genotype showed a high number of upregulated DEGs, indicating its greater sensitivity and lower resistance to Cd stress compared to Panjin. Similarly, more pathways related to structural defense, energy production, and stress signaling and number of DEGs related to pathways responsible for starch and sucrose metabolism were detected in Heze than in Panjin, suggesting the Heze genotype initiated comprehensive defense against the detrimental effects caused by cadmium. Panjin showed more pathways that prioritize maintaining cell integrity and regulating gene expression to adapt to stress, suggesting it has a more efficient or resilient strategy. The cellular responses are also different between the two populations. In Heze genotype, DEGs were predominantly enriched in biological processes such as intracellular water homeostasis, cell volume homeostasis, and cell surface receptor signaling pathway, regardless of exposure to low or high Cd levels. The increase in ion leakage rate (ILR) with rising Cd levels indicates cell membrane impairment under stress, a phenomenon supported by observations of undeveloped ultrastructure in vacuoles of common reed under electron microscopy (Hakmaoui, Ater, Boka, & Baron, 2007). The plant may endeavor to protect itself against the cell membrane damage through the activation of cell wall remodeling (Loix et al., 2017) to maintain the cellular water osmotic pressure.
Photosynthesis is a conserved life activity from Cyanobacteria to green plants. The deleterious cadmium was found to affect photosynthesis in many plant species by ways of decreasing the chlorophyll and carbon fixation, resulting in disruptions in development and growth (Haider et al., 2021). In common reed, exposure to cadmium results in significant damage to four functional traits associated with the photosynthetic apparatus in Heze genotype and the cell membrane in both haplotypes. Similarly, in lettuce, the treatment with high levels of Cd demonstrates a substantial decline in chlorophyll contents, net photosynthetic rate (A) and maximum quantum efficiency of PSⅡ (Fv/Fm) when compared to control plants (Dias et al., 2013).
When treated with low levels of Cd content, both populations showed upregulated DEGs associated with organelle membrane and thylakoid membrane. It is noteworthy, however, that the number of DEGs is relatively small in Panjin genotype. The photosynthetic apparatus in plants is particularly susceptible to the adverse effects of cadmium stress. The photosystems, consist of light-harvesting complexes and reaction-center complexes interconnected by electron transport chains, involve numerous protein complexes and pigmentations. Consequently, any disruption in these essential components due to Cd stress will result in reduced efficiency photosynthesis. A high level of Cd stress will lead to “a disturbed shape, wavy appearance of grana and stroma thylakoids and swollen intrathylakoidal space” for the chloroplast ultrastructure in common reed (Hakmaoui et al., 2007; Parmar, Kumari, & Sharma, 2013).
The DEGs in the comparison of Heze H vs Heze C treatments are enriched in the phenylpropanoid biosynthesis pathway, essential for lignin biosynthesis in secondary cell wall modification and the clearance of harmful reactive oxygen species. The metabolic products may also sequester cadmium to the extracellular region, impeding its mobility and enhancing plant resistance. In addition, these DEGs may promote the production of flavonoids, which act as plant hormones (Ge, Xin, & Tian, 2023). Unlike the passive protection mechanisms observed in Heze genotype, Panjin genotype upregulates genes to capture a broader spectrum of light types and intensities, coupled with chlorophyll biosynthesis, to mitigate the negative impact of the low cadmium level.
A large number of genes were found to be involved in species interaction and plant-pathogen interactions in Heze genotype, suggesting that the plant’s response to cadmium stress may share pathways with pathogen defense particularly against oomycetes. The accumulation of cadmium can be intertwined with pathogen defense and potentially trigger Systemic acquired resistance (SAR) via salicylic acid (SA) and jasmonic acid (JA) mediated pathway. This synergistic effect, observed extensively in Brassicaceae complies with the elemental defense hypothesis (Z. Liu et al., 2022). Moreover, Heze genotype upregulates genes associated with calcium ion binding. This strategic upregulation may involve the occupation of known ion channels to compete with Cd, thereby exerting an antagonistic effect on cadmium absorption (Huang et al., 2017).