Introduction

Climate change is increasingly driving extreme weather events worldwide, with rising temperatures and drought episodes posing serious threats to plant growth and production. These abiotic stresses are now recognized as the most critical environmental constraints threatening plant growth, development, and productivity worldwide (Sato et al. 2024). Rising temperatures and irregular precipitation patterns are projected to intensify further throughout the 21st century, posing serious challenges to sustainable agricultural and horticultural production systems.

Floricultural crops, including cut flowers, potted plants, and garden ornamentals, occupy an important position in the global horticultural industry. Unlike food crops, where productivity and biomass yield are the primary concerns, the commercial value of floricultural crops is largely determined by aesthetic qualities such as flower color, shape, fragrance, and vase life. Even moderate levels of abiotic stress can cause visible deterioration in these traits, leading to substantial economic losses.

Although the molecular mechanisms underlying heat and drought stress responses have been extensively studied in the model plant Arabidopsis thaliana and major food crops, comparatively less attention has been given to floricultural species. In recent years, however, advances in transcriptomics and functional genomics have enabled significant progress in understanding stress responses in ornamental plants. This review focuses on recent molecular, transcriptomic, physiological, and functional studies on heat and drought stress responses in four major floriculture crops, chrysanthemum, rose, lily, and petunia, while drawing on model plant studies to explain conserved signaling pathways. By integrating fundamental mechanistic knowledge with crop-specific findings, this review seeks to offer insights useful for researchers and breeders working toward the development of stress-tolerant floricultural cultivars.

Molecular Mechanisms of Heat Stress Response

Heat stress perception

The ability of plants to rapidly detect temperature changes is the first critical step in mounting an effective heat stress response. Heat stress perception occurs simultaneously at multiple cellular sites. At the plasma membrane, elevated temperatures increase membrane fluidity and activate cyclic nucleotide-gated calcium channels (CNGCs), leading to a rapid influx of Ca²⁺ into the cytoplasm (Sato et al. 2024). Elevated cytosolic Ca²⁺ is perceived by calmodulin 3 (CaM3), which activates calmodulin-binding protein kinase 3 (CBK3), subsequently phosphorylating and enhancing the DNA-binding activity of HSFA1. Concurrently, heat-induced protein misfolding in the cytoplasm and endoplasmic reticulum (ER) releases HSFA1 from its inhibitory complex with HSP70 and HSP90. Additionally, heat stress stimulates reactive oxygen species (ROS) overproduction in chloroplasts and mitochondria, further promoting HSFA1 trimerization and nuclear translocation (Bakery et al. 2024;Fortunato et al. 2023) (Fig. 1A).

The HSF-HSP signaling cascade

Heat shock transcription factors (HSFs) serve as the central regulators of the heat stress response. HSFA1 functions as the master regulator, orchestrating the early transcriptional reprogramming required for thermotolerance. Upon activation, HSFA1 trimerizes and translocates to the nucleus, where it binds to heat shock elements (HSEs) in the promoters of heat-responsive genes. Primary targets include genes encoding heat shock proteins (HSPs) and secondary transcription factors such as HSFA2, dehydration-responsive element-binding protein 2A (DREB2A), and multiprotein bridging factor 1c (MBF1c). HSPs function as molecular chaperones that prevent protein denaturation and facilitate refolding of misfolded proteins (Bakery et al. 2024) (Fig. 1A). HSFA2 plays a particularly important role in sustaining the heat stress response over prolonged periods and contributes to the establishment of heat stress memory through epigenetic modifications (Staacke et al. 2025). Transcriptomic analysis in Lilium longiflorum further revealed that the HSF-HSP and ROS pathways are activated simultaneously and with similar temporal kinetics upon heat onset, with small HSPs (sHSPs; HSP20 family) emerging as the most rapidly and dramatically induced molecular node linking chaperone activity to ROS signaling. Notably, sHSP family members dominated the intersection of the HSF-HSP, ROS, and protein processing pathways, establishing sHSPs as central integrators of the heat stress response in lily (Zhou et al. 2022).

ROS accumulation and antioxidant defense

Reactive oxygen species, including superoxide anion (O₂^•−), hydrogen peroxide (H₂O₂), and hydroxyl radicals (•OH), are generated as byproducts of heat-disrupted electron transport. At moderate concentrations, H₂O₂ acts as a signaling molecule activating mitogen-activated protein kinase (MAPK) cascades and promoting HSF activation, while excessive ROS accumulation causes oxidative damage to proteins, lipids, and nucleic acids, reflecting the dual role of ROS in heat stress responses (Fortunato et al. 2023). To maintain cellular redox homeostasis, plants activate enzymatic antioxidants including superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), which collectively constitute the primary ROS-scavenging machinery. Importantly, the effectiveness of this antioxidant response is not linearly proportional to stress intensity. In L. longiflorum, moderate heat (37–42°C) stimulated coordinated upregulation of SOD, POD, CAT, APX, and GR activities, together with elevated ascorbic acid (AsA) and glutathione (GSH) concentrations, resulting in effective ROS suppression and minimal membrane damage. However, severe heat (47°C) caused a rapid initial induction followed by a sharp decline in POD and CAT activities, leading to H₂O₂ overaccumulation, elevated malondialdehyde (MDA) content, and irreversible electrolyte leakage, demonstrating a critical temperature threshold beyond which the antioxidant system collapses (Yin et al. 2008). Among SOD isoforms, Cu/ZnSOD showed heat stability while MnSOD was suppressed at 47°C, indicating isoform-specific thermal sensitivity. In Clematis florida, comparative proteome-transcriptome analysis similarly showed that heat-tolerant lines maintained elevated expression of POD4 and glutathione-S-transferase tau 1 (GSTU1) together with HSP18 and HSP70, and that ROS generation via the photorespiratory pathway driven by glycolate oxidase 1 (GO1) was coupled with enhanced ROS scavenging capacity, establishing a coordinated balance between photorespiratory ROS generation and antioxidant scavenging capacity in heat-tolerant ornamental plants (Jiang et al. 2020). Furthermore, ethylene (ET) has emerged as an important amplifier of heat-induced ROS accumulation. Excess ET produced under heat stress binds to ER-localized receptors, activating a transcriptional cascade that promotes further ROS generation. In petunia, reducing ET levels through 1-aminocyclopane-1-carboxylic acid ACC deaminase (ACCD) activity significantly attenuates ROS accumulation and alleviates heat stress symptoms (Baek et al. 2024).

Molecular Mechanisms of Drought Stress Response

Drought stress perception and ABA biosynthesis

Plants perceive soil water deficit primarily through the root system. Two parallel signaling pathways relay drought signals to the shoot. A hydraulic signal is transmitted rapidly through the vascular system, causing partial stomatal closure within minutes. Simultaneously, chemical signals originating from water-stressed roots travel through the xylem to guard cells via two complementary routes. In the classical pathway, abscisic acid (ABA) synthesized in roots is transported through the xylem to guard cells, where it directly triggers stomatal closure. In addition, water-stressed roots synthesize the peptide CLAVATA3/EMBRYO SURROUNDING REGION-related peptide 25 (CLE25), which travels through the xylem to guard cells and activates 9-cis-epoxycarotenoid dioxygenase 3 (NCED3) expression, a key ABA biosynthetic enzyme, via the receptor kinases BARELY ANY MERISTEM 1 and 3 (BAM1 and BAM3), resulting in elevated ABA concentrations locally in guard cells (Liu et al. 2022).

ABA core signaling: the PYL-PP2C-SnRK2 module

The core ABA signaling module consists of PYR/PYL/RCAR receptors, PP2Cs, and SnRK2s. Under well-watered conditions, PP2Cs maintain SnRK2s in an inactive state. Upon ABA binding to PYR/PYL/RCAR receptors, PP2Cs are inhibited, releasing and activating SnRK2s. Activated SnRK2s phosphorylate the anion channel, slow anion channel-associated 1 (SLAC1), and inhibit the inward K⁺ channel, K⁺ uptake permease 1 (KAT1), decreasing guard cell turgor and driving stomatal closure (Liu et al. 2022). Simultaneously, SnRK2s activate NADPH oxidase (respiratory burst oxidase homolog F, RBOHF), generating apoplastic H₂O₂ that is sensed by hydrogen peroxide-induced Ca²⁺ increases 1 (HPCA1), triggering Ca²⁺ influx and further reinforcing stomatal closure through a positive feedback loop. In parallel, SnRK2s phosphorylate ABA-responsive element-binding proteins (AREB)/ABRE-binding factors (ABF) transcription factors, initiating a broader drought-adaptive gene expression program (Kim et al. 2024) (Fig. 1B).

Downstream drought tolerance mechanisms

Beyond stomatal regulation, plants employ several cellular strategies to tolerate the osmotic and oxidative consequences of drought. Compatible solutes including proline, soluble sugars, and trehalose accumulate to lower cellular osmotic potential and stabilize proteins and membranes. Late embryogenesis abundant (LEA) proteins function as molecular chaperones, protecting proteins and membranes from dehydration-induced damage. Decreased cell wall elasticity under drought conditions lowers the turgor loss point to a more negative leaf water potential, extending the range of conditions over which turgor can be maintained. At the transcriptional level, DREB/CBF, NAC, MYB, and WRKY transcription factors coordinate these adaptive responses through both ABA-dependent and ABA-independent pathways (Kim et al. 2024). These mechanisms have direct practical relevance in ornamental crop production. In potted carnation (Dianthus caryophyllus), gradual drought induced osmotic adjustment of approximately 0.3 MPa at full turgor, accompanied by significant increases in cell wall rigidity, which together maintained turgor under deficit irrigation conditions (Álvarez et al. 2009). Together, these findings suggest that osmotic adjustment and cell wall remodeling operate as an integrated cellular defense that complements ABA-mediated stomatal regulation under drought.

Heat and Drought Stress Responses in Floricultural Crops

Chrysanthemum

At the physiological level, heat-tolerant chrysanthemum cultivars are characterized by elevated proline and sucrose contents and enhanced POD activity, collectively contributing to flower bud initiation and development under high stress conditions (Chumber and Jhanji 2022). High temperature also directly impairs ornamental quality by suppressing anthocyanin biosynthesis, as transcriptomic and metabolomic analyses revealed that 35°C conditions significantly reduced cyanidin and pelargonidin glucoside accumulation in heat-sensitive cultivars through differential regulation of genes in the MAPK signaling and phenylpropanoid biosynthesis pathways, providing a molecular basis for heat-induced flower color fading in chrysanthemum (Li et al. 2024). At the molecular level, small heat shock proteins play a direct protective role, as overexpression of CmHSP17.9 in transgenic chrysanthemum prevented irreversible protein aggregation and enhanced thermotolerance under high-temperature stress (Ling et al. 2024).

Among transcription factors, CmHSFA4 has been characterized as a key regulator of ionic and ROS homeostasis in chrysanthemum. Li et al. (2018) showed that overexpression of CmHSFA4 enhanced tolerance by upregulating ion transporters CmSOS1 and CmHKT2 under salt stress, and Wang et al. (2024b) subsequently demonstrated that the CmHSFA4-CmMYBS3-CmTPL complex represses CmMYB121 through histone deacetylation, fine-tuning ionic homeostasis. Although these studies primarily addressed salt stress, the osmotic and oxidative components of salt stress substantially overlap with those of drought stress, and HSF family members, including HSFA4, are increasingly recognized as broad stress-responsive regulators that respond to shared signals such as ROS accumulation and osmotic imbalance across multiple abiotic stresses (Bakery et al. 2024). These findings collectively suggest that CmHSFA4 functions as a broad stress-responsive regulator beyond its canonical role in thermotolerance, with potential relevance to drought-associated osmotic and oxidative stress responses in chrysanthemum.

Exogenous melatonin application (100 μM) significantly improved drought stress tolerance in chrysanthemum by maintaining higher net photosynthetic rates, elevated antioxidant enzyme activities (SOD, POD, and CAT), increased osmolyte accumulation (soluble sugars and soluble protein), and enhanced photosystem II efficiency (Luo et al. 2023a). Transcriptomic analysis further revealed that melatonin broadly modulated multiple transcription factor families including MYB, WRKY, NAC, and AP2/ERF, while simultaneously suppressing drought-induced ABA and jasmonate accumulation, suggesting that its protective effects involve comprehensive transcriptional and hormonal reprogramming (Luo et al. 2023b). At the postproduction stage, exogenous s-ABA applications (125–1000 mg·L⁻¹) significantly reduced stomatal conductance and delayed visible wilting by 1.2–4.0 days in finished chrysanthemum plants, with the response varying among cultivars. Treated plants fully recovered after rewatering, whereas untreated drought-stressed plants developed leaf chlorosis and necrosis (Waterland et al. 2010). These findings demonstrate that the core ABA-stomatal closure pathway is functionally conserved and practically exploitable in chrysanthemum, and that exogenous ABA application represents an immediately applicable tool for reducing postproduction drought losses in floricultural supply chains.

Rose

Rose is the most commercially important cut flower globally, and both heat and drought stresses significantly compromise flower yield, color intensity, vase life, and essential oil quality. High-temperature stress in rose leads to bud dormancy or even plant death, directly reducing ornamental value and incurring substantial economic losses in the cut flower industry. Transcriptomic and metabolomic analyses of Rosa hybrida under heat stress identified 4,652 differentially expressed genes alongside significant reductions in protein and chlorophyll contents, while proline and MDA contents and POD activity were elevated, collectively indicating a broad reprogramming of protective and oxidative responses (Wang et al. 2024a). At the transcription factor level, RcHsfA7 has been characterized as a positive regulator of thermotolerance in rose, with overexpression enhancing heat resistance through upregulation of downstream HSP genes (Sun et al. 2025).

Drought responses have been more extensively characterized through transcription factor studies. Zhang et al. (2024) showed that RcMYB8, an R2R3-type MYB transcription factor, integrates drought and salinity tolerance by directly activating RcPR5/1 and RcP5CS, promoting proline accumulation for osmotic adjustment through a dual-target regulatory strategy. RcNAC091, a drought-inducible NAC transcription factor, enhances drought tolerance through the ABA-dependent pathway by activating ABA biosynthesis (RcNCED1) and signaling genes (RcABF2), as well as downstream RcMYB8, establishing a hierarchical regulatory network (Geng et al. 2023). At the genome-wide level, transcriptomic analysis of Rosa chinensis under continuous drought revealed that the transcriptional response is predominantly a late-stage phenomenon, with differentially expressed genes increasing dramatically as drought progressed. Weighted gene co-expression network analysis identified hub transcription factors including NAC072, WRKY75, MYB75, and ERF113, several of which share functional overlap with RcNAC091 and RcMYB8, collectively delineating an increasingly coherent drought-regulatory TF network in rose. Multiple phytohormone signaling pathways including ABA, auxin, jasmonate, and brassinosteroid were simultaneously reconfigured, underscoring the multi-hormonal nature of the rose drought response (Jia et al. 2021).

The developmental stage at which drought stress occurs critically determines its impact on rose production quality. Drought imposed during early vegetative stages had no significant effect on flower quality and photosynthetic capacity fully recovered upon rewatering, whereas stress at the floral bud appearance stage resulted in a high incidence of malformed flowers and irreversible photosynthetic depression, establishing this stage as the critical vulnerability window for rose flower quality under drought (Shi et al. 2019). These findings emphasize that water availability during floral bud development must be prioritized in cut rose production to prevent irreversible quality losses.

Lily

Lily is sensitive to heat stress, which can cause flower bud abortion, petal bleaching, and premature senescence. Physiological studies demonstrated that the antioxidant response in lily follows a temperature-threshold pattern: activities of SOD, POD, CAT, APX, and GR are coordinately induced at 37–42°C, but POD and CAT activities collapse at 47°C, resulting in uncontrolled H₂O₂ accumulation and oxidative membrane damage (Yin et al. 2008). Transcriptomic analysis of L. longiflorum identified the HSF-HSP and ROS pathways as the most rapidly activated signaling cascades, preceding the slower responses of the Ca²⁺-CaM and hormone pathways (Zhou et al. 2022). Among HSF family members, LlHsfA2 functions as a central regulator, directly controlling downstream sHSP genes and modulating ROS scavenging enzyme expression, with virus-induced silencing confirming its essential role in thermotolerance. LlHsfA4 has additionally been identified as a positive regulator of heat tolerance through enhancing ROS-scavenging enzyme activity, including APX2, via the HsfA1-independent pathway (Wang et al. 2022). The R2R3-MYB transcription factor LlMYB305 activates LlHSC70, a cytosolic HSP70 chaperone, directly linking MYB-mediated transcriptional regulation to chaperone-dependent protein protection under heat stress (Wu et al. 2021). Collectively, these findings indicate that lily thermotolerance is orchestrated through a multilayered regulatory network that shares the core HSF-HSP framework with other plants while possessing unique regulatory modules reflecting its specific physiological requirements.

Transcriptome and metabolome analyses of oriental lily under drought stress revealed that sugar metabolism pathways, including galactose, sucrose, and glycolytic pathways, were predominantly downregulated, accompanied by increased MDA content and reduced antioxidant enzyme activities, indicating that drought suppresses primary carbon metabolism while simultaneously triggering oxidative damage responses (Cui et al. 2023). At the transcription factor level, LlNAC2, a stress-responsive NAC transcription factor from tiger lily, was induced by drought, cold, salt, and ABA treatment, and its overexpression in Arabidopsis conferred enhanced drought tolerance through both DREB/CBF-COR and ABA-dependent signaling pathways, with LlDREB1 shown to directly bind the LlNAC2 promoter, establishing a regulatory module connecting ABA and dehydration-responsive signals in lily (Yong et al. 2019). Notably, a recent study identified LrWRKY16 in Lilium regale as a dual regulator of both heat and drought tolerance, directly activating LrHSP17.2 expression through W-box binding and enhancing ROS scavenging capacity under both stresses, providing evidence that a single WRKY-HSP regulatory module can confer cross-stress protection in lily (Mei et al. 2025).

Petunia

Petunia is one of the most popular ornamental bedding plants worldwide, with wholesale value estimated at $160 million in the United States alone in 2020 (USDA 2021), and its quality can be significantly diminished by heat and water deficit stress during both production and postproduction periods. Under heat stress, ET overproduction has been identified as the principal mediator of growth inhibition. 1-aminocyclopropane-1-carboxylate synthase 1 (ACS1) upregulation leads to excess 1-aminocyclopane-1-carboxylic acid (ACC) accumulation and elevated ET production, which activates downstream transcription factors (EIL1, ERF1, ERF2) through ER-localized receptors (ETR2, ERS1), initiating a signaling cascade that promotes ROS accumulation and leaf senescence (Baek et al. 2024). Transgenic petunia plants overexpressing acdS, encoding ACC deaminase that degrades the ET precursor ACC, showed markedly reduced ET and ROS accumulation under heat stress, maintaining higher relative water content, chlorophyll content, and leaf area compared to wild-type plants. Notably, attenuated induction of HSF and HSP genes in transgenic plants may reflect reduced upstream stress signaling rather than enhanced downstream protective capacity, establishing the ET-ROS pathway as a critical amplifier of heat stress damage in petunia.

Drought stress responses in petunia have been examined at both physiological and molecular levels. Transcriptomic analysis of Petunia × hybrida ‘Mitchell Diploid’ under water deficit revealed that AP2/ERF transcription factors were the most abundantly induced, with ABA and ET hormone signal transduction pathways enriched as key upstream regulators (Park et al. 2021). Building on this transcriptomic resource, PhERF039, an AP2/ERF member induced at the early stage of water deficit, was functionally characterized through gene silencing (Park et al. 2024). Reduced expression of PhERF039 altered shoot architecture and delayed flowering, while also reducing stomatal conductance and delaying substrate drying under water deficit conditions, suggesting that PhERF039 plays a negative regulatory role in stomatal closure and drought stress responses in petunia. Repetitive water deficit priming has also been shown to enhance drought tolerance through stress memory mechanisms, with higher priming intensity extending plant shelf life by approximately 25 hours compared to unprimed controls (Lee and Park 2024). At the applied level, antitranspirant applications including s-ABA and β-pinene polymer have been shown to extend shelf life and delay wilting under water deficit conditions in petunia and other bedding plant species (Park et al. 2016), offering practical postharvest management strategies complementary to the molecular approaches described above.

Conclusion

Heat and drought stress represent two of the most significant abiotic challenges facing floricultural crop production. Heat stress is primarily sensed through membrane fluidity changes, protein unfolding, and ROS accumulation, activating the HSF-HSP signaling cascade. Drought stress is mediated through ABA accumulation and the PYL-PP2C-SnRK2 module, which drives stomatal closure and downstream osmotic adjustment. Both pathways converge on shared molecular nodes, such as ROS, Ca²⁺, and phytohormone signaling, that enable coordinated adaptation.

In major floricultural crops, species-specific regulatory mechanisms have been identified (Table 1). The CmHSFA4-CmMYBS3-CmTPL complex in chrysanthemum has been primarily characterized under salt stress but is increasingly recognized as a broad cross-stress regulator responding to shared osmotic and oxidative signals across multiple abiotic stresses. Additional crop-specific mechanisms include the RcMYB8 and RcNAC091 hierarchical network in rose, the LlHsfA2-sHSP-ROS regulatory module in lily, and the ET-ROS-mediated heat stress response and drought tolerance mechanisms including ERF-mediated stomatal regulation, stress priming, and antitranspirant application in petunia. A distinctive feature of floricultural crop stress responses is the intersection between stress tolerance and ornamental quality traits, including flower pigmentation and vase life, aspects that merit deeper molecular investigation in future studies. Future research should prioritize investigation of combined heat and drought stress responses, development of efficient transformation systems for floricultural species, and integration of quality trait metrics into stress tolerance evaluations, with the goal of breeding cultivars capable of maintaining both productivity and ornamental value under changing climate conditions.