Drought is a common abiotic stress affecting plant growth and development (Sun et al., 2022). Many physiological and biochemical processes of plants are affected by drought stress, resulting in reactive oxygen species accumulation, membrane structure damage, ion imbalance, and inhibition of enzyme activity, photosynthesis, and respiration (Anjum et al., 2017; Shinozaki and Yamaguchi-Shinozaki, 2007). Plants have evolved various strategies to adapt to drought stress, such as morphological, cell physiological, metabolic, and molecular changes (Chaves et al., 2009; Li et al., 2022). Because global climate change has increased the frequency of drought, the mechanisms of plant responses to drought and resistance to drought have attracted extensive attention of researchers (Rahimi et al., 2019; Zhao et al., 2022).
Plants have the ability to adapt to environmental stress by regulating physiological, molecular, and metabolic processes (Shinozaki and Yamaguchi-Shinozaki, 2007). To reduce the osmotic and oxidative stress caused by drought, plants accumulate osmotic-regulating substances (soluble sugar, proline, etc.), increase the activity of protective enzymes (superoxide dismutase, catalase, peroxidase, etc.), and increase the content of non-enzyme free radical scavengers (reduced glutathione, vitamin C, etc.) (Lu et al., 2019; Miao et al., 2006). Plant hormones are also crucial in increasing drought tolerance. Plants accumulate abscisic acid (ABA) under drought stress, which causes stomatal closure of guard cells in order to reduce water loss and to regulate the expression of genes that enhance drought tolerance (Fujii et al., 2009; Liu et al., 2022). The ABA-mediated signaling pathway contains ABA receptors (Pyrabactin resistance/Pyrabactin resistance-like, PYRs/PYLs), positive regulators (ABA-activated SNF1-related protein kinases 2, SnRK2s), negative regulators (type 2C protein phosphatases, PP2Cs), and transcription factors (ABA-responsive element binding factors, ABFs) (Raghavendra et al., 2010; Yoshida et al., 2015). The SnRK2 protein kinases control stomatal closure, and overexpression of ABF3 in Arabidopsis can reduce the transpiration rate to enhance drought tolerance (Kang et al., 2002; Yoshida et al., 2002). Jasmonate (JA) also mediates the response of plants to drought stress and participates in the biosynthesis of secondary metabolites involved in plant defense (Chini et al., 2021; Howe et al., 2018; Wasternack and Feussner, 2018).
With the development of high-throughput sequencing technology, several recent studies have used RNA sequencing (RNA-seq) to reveal the expression patterns and signal regulation pathways of drought stress-related genes, and to identify the key genes and molecular mechanisms of plant responses to drought stress (Joshi et al., 2016; Sun et al., 2022). In Tamarix psammophila, for example, researchers found 1618 and 2716 differentially expressed genes (DEGs) after 1 and 2 weeks of drought stress, respectively; these DEGs were mainly involved in the mitogen-activated protein kinase (MAPK) signaling pathway, tryptophan and α-linolenic acid metabolism, and biosynthesis of flavonoids and phenylpropanoids (Sun et al., 2022). Analysis of drought-stressed Seriphidium transiliense seedlings by the weighted gene co-expression network (WGCNA) method indicated that transcription factors mainly belonging to WRKY, bHLH, NAC, LEA, AP2/ERF, MYB, GRAS, C2H2, MADS, and bZIP families were important in the response to drought (Liu et al., 2022). In other recent studies, transcriptomic analysis of Sophora davidii (Zhao et al., 2022), Dendrobium sinense (Zhang et al., 2021), Hordeum vulgare (Hong et al., 2020), and Ginkgo biloba revealed the genes whose expression changed in response to drought stress (Chang et al., 2020).
Transcriptome analysis can identify the genes that respond drought stress responses but cannot indicate how metabolites respond to stress (Kim et al., 2007). Metabonomic analysis, in contrast, can reveal how the synthesis, decomposition, and transformation of metabolites change in response to external environmental stress (Gargallo-Garriga et al., 2015; Sun et al., 2022). Metabolic analyses have found that the changes in plant primary and secondary metabolites help plants adapt to and resist drought stress (You et al., 2019). For example, the contents of lipids and organic acids in T. psammophila increased under drought (Sun et al., 2022). Arabidopsis responded to drought stress by increasing the contents of flavonoids, amino acids, and lipids (Pires et al., 2016; Tarazona et al., 2015). The important metabolites differing in drought-tolerant vs. drought-sensitive sesame plants under drought stress were ABA, amino acids, and organic acids (You et al., 2019). Metabolomic analysis showed that phenylalanine, oxidized glutathione, and ABA were related to the drought tolerance of S. davidii (Zhao et al., 2022). Moreover, the biosynthesis and accumulation of unsaturated fatty acids were found to be involved in the response of maize and peanut to drought stress (Gundaraniya et al., 2020; Yang et al., 2018).
Casuarina equisetifolia is a tall evergreen tree that is widely planted in the coastal zone of South China. In addition to being resistant to drought and wind and tolerant of high salt levels, C. equisetifolia is able to grow rapidly in barren environments (Wang et al., 2021b). As a consequence, it can be used to prevent wind damage and sand erosion during the restoration of plant communities in tropical and subtropical coastal zones. To survive in sand, which has poor water retention, C. equisetifolia has evolved deep roots. However, the molecular mechanisms of C. equisetifolia responses to drought stress are unknown, and C. equisetifolia responses to drought stress have not been studied by combining transcriptomics and metabolomics. In this study, we used transcriptomic and metabolomic analyses to study the drought response of C. equisetifolia branchlets. We identified several key genes and metabolic pathways involved in the drought response of C. equisetifolia. This study thereby increases our understanding of the mechanisms of C. equisetifolia response to drought and may also provide useful information for the management and restoration of zonal vegetation.