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).