Transcriptomic and Structural Insights into Leaf Variegation in Ilex Solar Flare

Study Overview and Bibliographic Information

• Title of Study: Transcriptomic and Structural Insights into Leaf Variegation Development in Ilex × ‘Solar Flare’.

• Journal: International Journal of Molecular Sciences (Int. J. Mol. Sci.) 2025, 26, 3999.

• Key Dates:

  • Received: 14 March 2025

  • Revised: 10 April 2025

  • Accepted: 22 April 2025

  • Published: 23 April 2025

• Correspondence Authors:

  • Hong Chen (Nanjing Botanical Garden Memorial Sun Yat-Sen).

  • Donglin Zhang (University of Georgia).

• Core Subject: Ilex × ‘Solar Flare’, a variegated holly cultivar showing stable, heritable yellow margins surrounding green central regions, used as a model for woody plant variegation.

Taxonomic Classification and Background of Leaf Variegation

• Categorization of Variegation: Variegated leaves are essentially classified into two principal categories:

  • Pigment-Related Variegation: Resulting from impaired chloroplast development or mutations in chlorophyll metabolism. I. × ‘Solar Flare’ is categorized specifically under the chlorophyll deficiency type.

  • Structural Variegation: Resulting from physical properties like epidermal modifications, air spaces between mesophyll cells, or appendages.

• Ecological Functions: Variegation serves beyond ornamental appeal, assisting in:

  • Plant adaptation and responses to abiotic factors.

  • Reproduction and protection against herbivores.

  • Regulation of leaf temperature via differential light absorption.

• Model Mutants Mentioned:

  • Arabidopsis mutants: YELLOW VARIEGATED (var)1, var2, and var3 (encoding FtsH proteins vital for chloroplast function) and immutans (im).

  • Hordeum vulgare (barley): HvCMF7 mutations in albostrians barley.

  • Camellia sinensis: ‘Anji Baicha’ (temperature-sensitive albino phenotype).

Phenotypic Color Indices and Quantitative Assessment

Comparative analysis between the Yellow sector (VY) and the Green sector (VG) revealed the following colorimetric data:

• Lightness ($L^*$): VY ($75.17 \pm 0.30$) was significantly higher than VG ($46.76 \pm 2.20$), indicating VY is much lighter.

• Red-Green Axis ($a^*$): VY ($3.37 \pm 1.07$) was positive, while VG ($-9.16 \pm 0.61$) was negative, reflecting a shift from green to reddish/yellowish hue.

• Yellow-Blue Axis ($b^*$): VY ($53.99 \pm 5.50$) was substantially higher than VG ($27.02 \pm 4.10$), confirming strong yellow pigmentation.

• Color Saturation ($C$): VY was markedly greater ($54.10 \pm 5.56$) than VG ($28.56 \pm 3.80$).

• Hue Angle ($H^°$): VY ($86.48 \pm 0.76$) was significantly lower than VG ($104.40 \pm 11.70$), indicating a shift toward intense yellow.

Biochemical Profile: Pigment and Precursor Levels

• Chlorophyll Content:

  • Chlorophyll a, chlorophyll b, and total chlorophyll (a + b) were significantly higher in VG (19.51, 25.90, and 20.79 times higher, respectively).

  • The chlorophyll a/chlorophyll b ratio showed no statistically significant variation.

• Carotenoid and Flavonoid Content:

  • VG contained 4.79 times more carotenoids and 1.16 times more flavonoids than VY.

  • Carotenoid/Chlorophyll Ratios: These were significantly elevated in yellow tissues (VY), increasing by 4.07 (Car/Chl a), 5.76 (Car/Chl b), and 4.34 (Car/Chl t) times.

• Chlorophyll Biosynthetic Intermediates:

  • Early precursors: Levels of 5-aminolevulinic acid (ALA), porphobilinogen (PBG), uroporphyrinogen III (Urogen III), and coproporphyrinogen III (Coprogen III) were similar in both sectors.

  • Later intermediates: VY showed a significant depletion of Protoporphyrin IX (Proto IX), Magnesium Protoporphyrin IX (Mg-Proto IX), and Protochlorophyllide (Pchlide).

  • Specific values for Mg-Proto IX: $24.42$ (VY) vs. $38.77$ (VG) ($p < 0.05$).

  • Specific values for Pchlide: $9.16$ (VY) vs. $20.97$ (VG) ($p < 0.05$).

Cytological and Ultrastructural Analysis

• General Anatomy: Both VG and VY sectors featured sun-adapted morphology with a double-layered epidermis, dual-layered palisade cells, and nine layers of spongy cells with considerable airspaces.

• Scanning Electron Microscopy (SEM) Observations:

  • Yellow tissues exhibited fewer cuticle surface ridges than green tissues.

  • Stomatal distribution was limited to the lower epidermis (abaxial).

  • Stomatal Density: VY showed significantly lower density than VG.

  • Stomatal Closure: VY exhibited a substantially higher percentage of closed stomata ($84.47\%$) compared to VG ($41.65\%$).

• Chloroplast Ultrastructure (TEM):

  • VG Chloroplasts: Spindle-shaped, well-organized thylakoid systems (granal and stromal lamellae), containing starch granules ($1.19$ per cell), and limited plastoglobuli.

  • VY Chloroplasts: Swollen/rounded morphology, absent or indistinct grana, dense populations of vesicles and plastoglobuli, and zero starch granules.

  • Morphometric Data:

    • Chloroplast number per cell: VG ($8.67 \pm 1.15$) vs. VY ($5.00 \pm 0.82$).

    • Dimensions: VG ($6.78 \times 4.01\,\mu\text{m}$, ratio 1.69) vs. VY ($4.77 \times 4.29\,\mu\text{m}$, ratio 1.12).

Photosynthetic and Fluorescence Capacity

• Gas Exchange Parameters:

  • Net Photosynthetic Rate ($P_n$): VY ($-2.43 \pm 0.25\,\mu\text{mol}(CO_2)\,m^{-2}\,s^{-1}$) vs. VG ($2.87 \pm 0.15\,\mu\text{mol}(CO_2)\,m^{-2}\,s^{-1}$). The negative value in VY indicates CO2 loss via respiration exceeds photosynthetic uptake.

  • Transpiration Rate ($E$): Reduced by $44.25\%$ in VY.

  • Stomatal Conductance ($G_s$): Reduced by $40.14\%$ in VY.

  • Intercellular $CO_2$ Concentration ($C_i$): Increased by $89.66\%$ in VY, suggesting internal CO2 is not being utilized efficiently for assimilation.

• Chlorophyll Fluorescence:

  • Maximal Quantum Yield of PSII ($F_v/F_m$): VY ($0.16 \pm 0.07$) vs. VG ($0.70 \pm 0.03$), a $77.14\%$ reduction indicating severe photodamage.

  • Nonphotochemical Quenching (NPQ): Reduced by $90.48\%$ in VY.

  • Nonregulatory Energy Dissipation ($Y(NO)$): Increased by $154.55\%$ in VY, reflecting heightened photoinhibition and oxidative stress.

Transcriptomic Insights and Gene Expression Analysis

• Sequencing Data: 256,757,618 high-quality clean reads; mapping rate to I. latifolia reference genome: $84.72\%$ to $86.25\%$.

• Differential Expression: 3510 Differentially Expressed Genes (DEGs) identified: 1573 upregulated and 1937 downregulated in VY.

• Top Enriched Gene Ontology (GO) Terms:

  • Cellular Component: “thylakoid” (GO:0009579), “photosystem” (GO:0009521), “photosynthetic membrane” (GO:0034357).

  • Molecular Function: “tetrapyrrole binding” (GO:0046906), “iron ion binding” (GO:0005506).

• Top Enriched KEGG Pathways: “flavonoid biosynthesis” (ko00941), “photosynthesis–antenna proteins” (ko00196), “porphyrin metabolism” (ko00860), and “carotenoid biosynthesis” (ko00906).

Specific Molecular Mechanisms of Variegation

• Chlorophyll Metabolism Regulation:

  • Impaired Synthesis: Upregulation of CHLD accompanied by downregulation of CHLH and CHLG. Magnesium chelatase (MgCh) requires the coordinated action of CHLD, CHLI, and CHLH subunits; this imbalance disrupts Mg insertion into Proto IX.

  • Accelerated Degradation: Upregulation of PAO (pheophorbide a oxygenase) in yellow sectors leads to increased chlorophyll breakdown.

  • Compensatory Response: Upregulation of HEMB and PORA transcript levels was observed, likely as a response to the synthesis blockage.

• Carotenoid and Flavonoid Biosynthesis:

  • Downregulated: PSY (phytoene synthase), LUT5, NCED1, NCED2, LCYB, and CYP707A1.

  • Upregulated: AAO3.

• Chloroplast Development and Transcription Factors:

  • Golden 2-like (GLK): GLK1 and GLK2 were both significantly downregulated in VY. These factors are crucial for initiating plastid differentiation and regulating genes for chloroplast biogenesis.

  • Thylakoid Membrane Genes: 18 DEGs related to thylakoid function were downregulated, including PsbC, PsbO, PsbR, PsaD, and PsaH.

• Photosynthetic Machinery Disruption: 40 genes across multiple components (PSII/PSI reaction centers, Cytochrome b6/f, electron transport, F-type ATPase) were downregulated.

Experimental Methodologies

• Samples: Four-year-old cutting-grown liners of Ilex × ‘Solar Flare’ in 3-gallon containers.

• Pigment Quantification Equations:

  • $Ca = 13.95 \times A_{665} - 6.88 \times A_{649}$

  • $Cb = 24.96 \times A_{649} - 7.32 \times A_{665}$

  • $Cx+c = (1000 \times A_{470} - 2.05 \times Ca - 114.8 \times Cb) / 245$

• Fixation and Staining: Formalin, alcohol, and glacial acetic acid (90:5:5) fixative; dual staining with $1\%$ aqueous safranin and $0.5\%$ fast green.

• EM Technology: Quanta 200 scanning electron microscope (FEI) and JEM 1400 transmission electron microscope (JEOL Ltd.).

• Validation: qRT-PCR used Actin as an internal reference for 12 selected genes, confirming RNA-seq results via the $2^{-\Delta\Delta Ct}$ method.