Azomite modulates gene expression during photomorphogenesis via phyB-dependent and -independent pathways

Overview

• Study investigates how Azomite, a volcanic ash-based fertilizer rich in micronutrients, affects gene expression during early Arabidopsis seedling photomorphogenesis. It examines both phyB-dependent and phyB-independent pathways.

• Builds on prior work showing Azomite can enhance tomato fruit production, shift tomato root endosphere microbiota and exudates, and promote Arabidopsis seedling growth under photosynthetically active radiation (PAR). The work aims to connect complex micronutrient mixtures with transcriptional changes and growth outcomes.

• Key finding: Azomite elicits broad gene-expression changes that overlap with phyB-regulated genes, with effects that depend onAzomite dose and light condition (Dc vs Rc). The study identifies specific genes and pathways (including carbon fixation, hormone signaling, and suberin/cutin metabolism) and proposes a dose- and light-dependent model of Azomite action.

Background on Azomite and context

• Azomite is a hydrated calcium sodium aluminosilicate (volcanic ash-based fertilizer) with a silicon base and an average of over 70 minerals and trace elements, including 529.7 ppm rare earth elements. It is produced in different grades of fineness; this study uses the Micronized grade.

• Manufacturer-provided composition includes minimal N-P-K (0–0–0.2) with ~1.8% Ca, 0.5% Mg, 0.1% Cl and Na; potassium present as soluble K2O; other elements include P, Na, Fe, Mn, Zn, Cu, Mo, Se.

• Prior greenhouse studies in tomato showed growth responses up to 2 g/plant, with diminished fruit production at 3 g/plant. PhylloStart microbial foliar spray could synergize with Azomite and mitigate high-dose negative effects.

• Microbiome studies in tomato indicated Azomite alters root rhizosphere/endosphere communities and shifts core taxa toward carbohydrate-metabolizing functions, implying changes in carbohydrate production and root exudates that recruit microbes.

• Hypothesis: Optimal Azomite micronutrients alter carbohydrate biosynthesis via photosynthesis, potentially changing root exudates and microbial recruitment, thereby promoting growth in a light-dependent manner.

• Previous data showed that 0.5 g/L Azomite increased hypocotyl length and cotyledon area under continuous red light (Rc) but not in darkness (Dc) or continuous far-red (FRc), suggesting PAR dependency for Azomite activity. Growth promotion appeared to be general rather than specific to light signaling sensitivity.

Experimental design

• Organisms: Arabidopsis thaliana ecotype Col-0 (wild type) and phyB-9 (phyB mutant)

• Treatments: Azomite at 0, 0.5, and 1.0 mg/mL added to half-strength MS medium (no added sugars). Plates stratified at 4 °C for 4 days; germination induced by 3 hours white light; plates kept in darkness for 21 hours, then grown for 4 days under either continuous darkness (Dc) or continuous red light (Rc) at 21 °C. Light intensity: Rc at 660 nm, 7 μmol m⁻² s⁻¹; fluence rates monitored with a spectroradiometer.

• Replicates: At least three biological replicates per treatment.

• RNA sequencing: 4-day-old seedlings collected under green safe light, RNA extracted, mRNA purified, fragmented, and converted to cDNA libraries. Three biological replicates per treatment sequenced (Illumina platform).

• Data processing: Reads filtered for ambiguous bases (N > 10%) and low-quality content (>50% bases with quality <5). Clean reads aligned to reference genome with HISAT2 v2.0.5 to generate splice junctions. Transcript assembly and quantification with StringTie v1.3.3b; read counts via featureCounts; gene expression in FPKM calculated. Differential expression analysis with DESeq2 (v1.20.0); P-values adjusted with Benjamini-Hochberg (BH) to control FDR. DEGs defined as adjusted P ≤ 0.05. KEGG pathway enrichment via clusterProfiler.

• Data summaries and statistics: Pre-filtered average reads per sample ~ $5.055 imes 10^{7}$ with SD $ext{SD}=9.5847 imes 10^{6}$; post-filter average ~ $4.981 imes 10^{7}$ with SD $9.552 imes 10^{6}$. Quality metrics: average Q20 97.49%, Q30 92.93%, GC content 44.81%. Genome index built with HISAT2; mapping accuracy benefits from splice-aware alignment.

• Goals: (i) characterize Azomite-induced transcriptional changes during photomorphogenesis; (ii) determine overlap with phyB-regulated gene expression; (iii) understand dose- and light-dependent responses and identify key regulators (transcription factors and signaling components).

Key methods and analytical notes (technical details)

• Gene-level analysis workflow:

  • Align reads with HISAT2; assemble transcripts with StringTie; count reads per gene with featureCounts; compute FPKM for gene expression.

  • Identify DEGs between conditions using DESeq2; adjust for multiple testing with BH method; significance threshold: $p_{ ext{adj}} \,\le\, 0.05$.

  • Map DEGs to KEGG pathways and assess enrichment with clusterProfiler.

• Data interpretation caveats:

  • 0.5 g Azomite often yielded more robust growth-promoting transcriptional changes under Rc and more pronounced effects in Col-0 than in phyB mutants, consistent with phyB-dependent and -independent components.

  • 1.0 g Azomite tended to produce broader changes, sometimes reducing growth phenotypes observed at 0.5 g, echoing dose-dependent ectopic effects seen in tomato and previous Arabidopsis work.

• Dose rationale: Based on prior observations of dose-dependent effects, two concentrations (0.5 g/L and 1.0 g/L) were selected to compare with untreated controls and to probe dose responsiveness across Dc and Rc in Col-0 and phyB-9.

• Genotype considerations: The inclusion of the phyB-9 mutant enables dissection of phyB-dependent gene regulation from Azomite-driven, phyB-independent responses. Tepperman et al. (2004) provides a reference for phyB-regulated early light-responsive genes to compare with current data.

Results: phyB-regulated gene expression (basal, light-responsive context)

• Comparison to Tepperman et al. (2004): 4-day-old seedlings under Dc show partial overlap with Rc-regulated gene responses observed in earlier timepoints. Specifically:

  • 50% overlap in genes repressed in phyB mutants versus wild-type across Rc exposure, and 33% overlap in genes induced in phyB mutants (Supplemental Fig. 1).

• Focus on 17 early- and late-light responsive genes identified by Tepperman et al. that show robust Rc responsiveness differences between WT and phyB: in this study, 8 of these 17 genes showed altered expression in the phyB mutant under 4 days of Rc. Except for GI, the other 7 genes followed the direction of phyB regulation consistent with Tepperman et al. (i.e., repressed or induced consistent with phyB activity). GI expression differed: it was repressed in phyB absence under Rc, whereas Tepperman et al. found a different pattern in the earlier time course. This shift likely reflects temporal differences (early Rc response vs 4-day exposure) and circadian oscillator attenuation in phyB vs Col-0 under Rc. Overall, results align with prior findings but reflect timepoint-dependent differences in phyB-regulated gene sets.

Results: Azomite-regulated gene expression

• Across Col-0 and phyB-9, 4-day-old seedlings grown in Dc or Rc showed differential expression in response to both 0.5 g and 1.0 g Azomite.

• 0.5 g Azomite:

  • Most genes with decreased expression under 0.5 g were in phyB (Dc), phyB (Rc), and Col-0 (Dc) with percentages: 29.6%, 28.5%, and 17.9%, respectively (Fig. 1a).

  • Most genes with increased expression under 0.5 g were in Col-0 (Dc), phyB (Rc), and phyB (Dc) with percentages: 36.9%, 19.7%, and 17.9%, respectively (Fig. 1b).

  • Under Rc, Col-0 de-etiolated seedlings showed 54 downregulated genes (11.9%) and 10 upregulated genes (2.2%), indicating phyB activity is required for Azomite-induced expression changes in these Rc-responsive genes.

  • 20 downregulated (4.4%) and 3 upregulated (0.7%) genes were unique to Col-0 in Rc, showing phyB-dependent Azomite effects.

• 1.0 g Azomite:

  • Compared to 0.5 g, more genes showed decreased expression in Col-0 Rc, while phyB Dc and Rc showed similar but lesser changes, indicating dose-dependent breadth, with stronger effects in Col-0 Rc than in phyB mutants (Fig. 2a vs 2b).

  • Overall, 1.0 g produced broader transcriptional changes than 0.5 g, consistent with earlier phenotypic observations of stronger ectopic effects at higher Azomite doses.

• Across treatments, there is a notable interaction between Azomite concentration and light condition: Rc tends to magnify the number of genes affected by Azomite in Col-0, whereas phyB mutants show a dampened response, suggesting light sensitivity modulates Azomite efficacy.

• Intersection with phyB-regulated responses under Rc: 29 genes showed overlap between Azomite (0.5 g, Rc) and phyB-regulated expression changes (Rc). Of these 29, about half (15) were regulated in the same direction by both Azomite and phyB status, while 14 showed opposite directions (Fig. 3, Table 2).

• About two subgroups among these overlapping genes:

  • 11 of the 14 genes induced in phyB mutants under Rc were repressed by Azomite under Rc in Col-0, and 3 genes repressed in phyB mutants showed increased expression with Azomite in Col-0 (Fig. 3, Table 2).

• Transcription factors and nutrient-response genes frequently appear in the Azomite-phyB intersection, indicating that light-responsive transcriptional networks are modulated by Azomite (Table 2).

Gene-level insights: notable genes and their potential roles

• SUBERMAN (MYB39; AT4G17785): a MYB-family transcription factor regulating suberization of the root endodermis. Downregulated by Azomite in Rc and in phyB mutant contexts, suggesting Azomite may suppress suberin deposition to promote nutrient uptake under certain light conditions or in the absence of phyB.

• AtNIGT1/HRS1 (AT1G13300): a regulator that integrates nitrate and phosphate signals; downregulated by Azomite and in phyB mutants. In previous work, HRS1 represses primary root growth under P deficiency when nitrate is present; its suppression could promote root growth and nutrient uptake in Azomite-treated seedlings, especially without phyB activity.

• JAL41 (AT5G35940) and JAL20: Jacalin-related lectins induced by Azomite and suppressed in phyB mutants. Function not fully defined; related proteins (e.g., JAL10) are involved in root cap and nutrient response/stress signaling; possible roles in nutrient availability responses under Azomite treatment.

• PLT3 (Polyol Transporter 3, AT2G18480): downregulated by Azomite under Rc in Col-0 Rc, indicating potential shifts in carbohydrate transport. PLT family members are transporters for polyols; downregulation may reflect a shift from certain sugar alcohols toward other carbon substrates (e.g., sucrose) in Azomite-treated roots.

• CYP77B1: fatty acid epoxygenase; upregulated by Azomite under Rc in Col-0; may contribute to cutin synthesis and reduced water loss, indicating a link between Azomite treatment and cuticle fortification under red light.

• NAS1 (Nicotianamine Synthase 1, AT5G04950): downregulated by Azomite in both Col-0 and phyB mutants under Rc; nicotianamine is a metal chelator involved in metal homeostasis; downregulation may indicate altered metal uptake strategies in Azomite-treated seedlings.

• FSD1 (Superoxide Dismutase, AT4G25100): upregulated by Azomite under Rc, potentially providing protection against oxidative stress in the presence of trace metals in Azomite.

• GI (GIGANTEA): circadian clock-related gene; shows differential expression in phyB contexts under Rc, with Rc-induced regulation differing from previous studies, likely due to timepoint and circadian effects.

Pathway-level insights (KEGG and functional enrichments)

• 0.5 g and 1.0 g Azomite in Col-0 Rc seedlings enriched the Carbon Fixation in Photosynthetic Organisms pathway, suggesting enhanced carbon assimilation capacity under red light with Azomite treatment (Fig. 4a).

• In Col-0 Rc, hormone signaling pathways, phenylpropanoid biosynthesis, and glutathione metabolism were among the pathways inhibited at 0.5 g, while additional pathways were inhibited at 1.0 g (Fig. 4a). This indicates dose- and pathway-specific adjustments to signal transduction and secondary metabolism under Azomite treatment.

• Under Rc in etiolated Col-0 (Dc-grown) seedlings, 0.5 g showed no significant effects, but 1.0 g inhibited ribosome-related genes and altered multiple KEGG pathways, including plant hormone signaling (Fig. 4b).

• For phyB mutants under Rc, carbon fixation, carbon metabolism, photosynthesis, and ribosome pathways were enhanced by 1.0 g and to a lesser extent by 0.5 g Azomite (Fig. 4c).

• Under Dc in phyB mutants, observed changes were similar to etiolated Col-0 seedlings (Fig. 4d). Across all genotypes and treatments, carbon fixation emerged as the only common pathway enhanced in Rc, pointing to a central role for carbon fixation in the Azomite response to red light.

• Overall, 0.5 g Azomite tended to have a milder effect than 1.0 g, but both doses promoted carbon fixation under Rc, supporting a link between Azomite and photosynthetic carbon economy under red-light photomorphogenesis.

• The authors propose that Azomite acts through a combination of phyB-dependent and independent mechanisms to alter nutrient uptake, carbohydrate transport, and protective barriers (suberin/cutin balance) to support growth and carbon fixation under light conditions.

Dose-dependent and light-dependent interpretation

• 0.5 g Azomite: most changes occurred in seedlings with elongated hypocotyls (Col-0 Dc, phyB Dc, phyB Rc). In Rc, many changes were phyB-dependent (requires phyB activity for the Rc-responsive Azomite changes).

• 1.0 g Azomite: broader transcriptional changes; greater differences between Rc and Dc, especially in Col-0 Rc, suggesting heat-up of light sensitivity to higher Azomite doses. In phyB mutants, differences between Rc and Dc were reduced, implying a reduced light-sensitivity role when phyB is absent.

• The data support a model where Azomite’s action is dose- and light-dependent, with optimal growth promotion at intermediate doses (0.5 g) under Rc in WT plants, and broader pleiotropic effects at higher doses that may limit growth in some contexts (as seen in tomato and Arabidopsis). A working model envisions Azomite modifying carbon fixation, nutrient uptake, and water retention through phyB-dependent and independent pathways, ultimately affecting growth under photomorphogenic conditions.

Intersections with phyB and implications for signaling networks

• A subset of Azomite-responsive genes overlaps with phyB-modulated genes under Rc, highlighting a convergence of light signaling and micronutrient signaling on particular transcriptional programs.

• Some overlaps show congruent regulation (same direction) between Azomite treatment and phyB status, while others show antagonistic regulation (Azomite and phyB status move gene expression in opposite directions). This supports a nuanced model where Azomite can mimic, complement, or counteract aspects of phyB-driven transcriptional control depending on concentration and light conditions.

• Identification of transcription factors and signaling components in the Azolite-phyB intersection underscores potential targets to dissect how micronutrient mixtures influence early seedling development and nutrient uptake under red light.

Proposed mechanistic model and real-world relevance

• Working model (summary): Azomite acts as a dose-dependent, light-modulated modulator of gene expression during photomorphogenesis, with effects on carbohydrate transport and metabolism, carbon fixation, nutrient uptake, and cuticle formation. Some responses require phyB, while others persist in phyB mutants, revealing both phyB-dependent and -independent pathways.

• Mechanistic implications:

  • Enhanced carbon fixation under Rc suggests Azomite supports photosynthetic carbon economy when red-light signaling is active.

  • Regulation of PLT3 and other transporters implies Azomite may shift sugar transport and root exudation patterns, potentially reshaping the root microbiome and nutrient acquisition strategy.

  • Upregulation of CYP77B1 and downregulation of MYB39/HRS1/NAS1 indicate coordinated adjustments in cuticle formation and suberin deposition, balancing water retention and nutrient uptake under Azomite exposure.

• Real-world relevance: The study informs how complex micronutrient-based fertilizers might interact with light signaling to influence early plant growth and nutrient strategies, with implications for agricultural practices that rely on mineral inputs and light environment management.

Conclusions and key takeaways

• Azomite modulates gene expression during Arabidopsis photomorphogenesis through both phyB-dependent and independent pathways, in a dose- and light-dependent manner.

• 0.5 g Azomite generally promotes growth under Rc in Col-0, with broader transcriptional changes in Rc than in Dc; higher dose (1.0 g) yields more extensive transcriptional changes but can dampen certain growth responses, consistent with ectopic effects observed in other systems.

• 29 genes were commonly regulated by Azomite under Rc and by phyB status, indicating intersection with light signaling; these include transcription factors and nutrient-responsive genes such as SUBERMAN (MYB39), AtNIGT1/HRS1, JAL41, PLT3, and CYP77B1, among others.

• KEGG pathway enrichment highlights carbon fixation as a consistently enriched pathway under Rc in both Col-0 and phyB contexts, with other pathways (hormone signaling, phenylpropanoid biosynthesis, glutathione metabolism, ribosome) showing dose- and genotype-specific modulation.

• The study supports a model where complex micronutrient mixes influence plant growth by adjusting carbon metabolism and nutrient handling via both phyB-dependent and -independent pathways, with potential downstream effects on nutrient uptake efficiency, water retention, and stress responses.

Limitations and future directions

• The study analyzes a single early time point (4 days) and two Azomite doses; additional time-course data would help resolve temporal dynamics and the trajectory of phyB-dependent vs. independent responses.

• Functional validation of candidate genes (e.g., PLT3, MYB39, HRS1, JAL41, CYP77B1, NAS1) under Azomite treatment and light conditions would clarify causal roles in growth responses and nutrient uptake.

• Disentangling individual micronutrient contributions within Azomite could identify specific elements responsible for observed transcriptional programs and help optimize formulations for desired traits.

• Further work could integrate microbiome analyses with transcriptomics to link root exudate changes to microbial community shifts under Azomite treatment.

Appendix references and context

• Tepperman et al., 2004: provided baseline phyB-regulated gene expression under red light; used for cross-study comparison of phyB-dependent gene responses.

• McCue et al., 2023: demonstrated PAR requirement for Azomite-induced seedling growth with Rc; used to frame dose-response and light-dependence.

• Mehlferber et al., 2022; 2023: prior work on tomato microbiome shifts and fruit yield, and on phyllosphere microbial associations, used to motivate and contextualize transcriptional analyses in Arabidopsis.

• Supplemental Fig. 1 and Tables 1–3: provide detailed overlaps, gene lists, fold-changes, and enriched pathways; key for interpreting phyB-dependent vs. independent gene sets and dose effects.

Key terminologies and concepts (glossary)

• Photomorphogenesis: light-dependent development from seedling emergence; in Arabidopsis, red light signaling via phytochromes (e.g., phyB) suppresses hypocotyl elongation and promotes de-etiolation.

• phyB-9: a loss-of-function mutant for phytochrome B, used to dissect light signaling pathways.

• Rc and Dc: continuous red light and continuous darkness growth conditions, respectively.

• DEGs: differentially expressed genes identified by RNA-seq analyses, typically defined by adjusted P-value thresholds (e.g., $p_{adj} \le 0.05$).

• Log2 fold change (Log2 FC): logarithmic measure of change in expression; used to rank/up-down regulation strength.

• FPKM: fragments per kilobase of transcript per million mapped reads; a normalized expression metric in RNA-seq analyses.

• KEGG: Kyoto Encyclopedia of Genes and Genomes; a database used for pathway enrichment analyses.

• BH method: Benjamini-Hochberg procedure for controlling the false discovery rate (FDR) in multiple testing.

• SUBERMAN/MYB39: a MYB transcription factor regulating suberin deposition in root endodermis.

• HRS1/AtNIGT1: nitrate and phosphate signaling integrator; influences root growth under nutrient deficiency.

• PLT3: polyol transporter; part of carbohydrate transport in roots.

• CYP77B1: fatty acid epoxygenase; potentially involved in cutin synthesis and water retention.

• NAS1: nicotianamine synthase; involved in metal homeostasis and chelation.

• FSD1: superoxide dismutase; protective enzyme against oxidative stress.

• GI: GIGANTEA; circadian clock component with broader regulatory roles.

Title for the response

Azomite modulates gene expression during photomorphogenesis through phyB-dependent and independent pathways