Differential photoperiodic control of morning and evening expressed transcripts in tomato

Through high-resolution RNA-seq time-course analysis of tomato under varying photoperiods, this study reveals that morning-phased transcripts act as transcriptional sensors of day length to serve as zeitgeber references for evening transcripts, which measure photoperiod via coincidence with external cues to drive seasonal adaptation.

The Gist

The Big Picture: Plants Have an Internal Clock, But It Needs to Be Reset

Imagine a tomato plant is like a person living in a house with no windows. Inside, they have a 24-hour biological clock (the circadian rhythm) that tells them when to wake up, eat, work, and sleep.

In nature, this clock doesn't run on its own; it needs to be reset every day by the sun. The sun rising is the "alarm clock" (dawn), and the sun setting is the "bedtime signal" (dusk).

The length of the day changes with the seasons. In summer, the sun is up for a long time (Long Days). In winter, it's up for a short time (Short Days). The big question this study asked is: How does a tomato plant adjust its internal schedule when the day gets longer or shorter?

The Experiment: A High-Speed Camera for Plant Genes

To figure this out, the scientists didn't just look at the plants once a day. They used a "high-speed camera" approach on the plant's DNA.

• The Setup: They grew tomato plants in three different "time zones":
  1. Short Day: 6 hours of light, 18 hours of dark (like winter).
  2. Neutral Day: 12 hours of light, 12 hours of dark (like spring/autumn).
  3. Long Day: 18 hours of light, 6 hours of dark (like summer).
• The Sampling: They took a "snapshot" of the plant's genetic activity (which genes are turned on or off) every two hours for a full 24-hour cycle.
• The Cast: They used different versions of tomato plants. Some were the standard "domesticated" supermarket tomatoes, and others were "wild" tomatoes that still had their original, ancient genes. This helped them see how humans changed the plant's clock during farming.

The Discovery: Two Teams of Genes

The scientists found that the plant's genes don't all wake up at the same time. They split into two distinct teams, like a Morning Crew and an Evening Crew.

1. The Morning Crew (MPTs)

• When they work: They peak right around sunrise.
• What they do: They handle "growth stuff." Think of them as the construction workers. They build proteins, fix cell walls, and get the plant ready to photosynthesize.
• How they react to day length: They are super flexible. If the day gets longer, these genes change their "shape" and timing. They stretch out their workday to match the new schedule. They act like the managers who look at the calendar and say, "Okay, today is a long day, let's adjust our start time."

2. The Evening Crew (EPTs)

• When they work: They peak about 12 hours after the Morning Crew (around midday or dusk).
• What they do: They handle "maintenance stuff." Think of them as the night shift janitors and security guards. They fix DNA, repair errors, and prepare the plant for sleep.
• How they react to day length: They are stubborn. They try to stick to their 12-hour schedule regardless of how long the day is.
  • The Problem: In a short winter day, they finish their work right when the sun goes down. But in a long summer day, they finish their work while it's still bright outside! They get "misaligned" with the sunset.

The "Aha!" Moment: How the Plant Measures Time

Here is the clever part of the discovery. The plant doesn't have a ruler to measure how long the day is. Instead, it uses a coincidence trick.

1. The Morning Crew changes its schedule based on the length of the day.
2. The Evening Crew stays on a fixed 12-hour loop.
3. Because the Morning Crew shifts but the Evening Crew doesn't, the gap between them changes depending on the season.

The plant uses this gap as a signal. It's like a shadow clock.

• Short Day: The shadow is short. The Morning and Evening crews are close together.
• Long Day: The shadow is long. The crews are far apart.

The Evening Crew acts like a sensor. It waits to see if it is still "daytime" when it finishes its 12-hour shift. If it's still bright, the plant knows, "Ah, it's summer!" If it's dark, the plant knows, "It's winter."

The Role of the "Domestication" Genes (EID1 and LNK2)

The study also looked at two specific genes that humans changed when they first started farming tomatoes: EID1 and LNK2.

• LNK2: This is like the conductor of the orchestra. When this gene is mutated (changed by humans), the whole orchestra (the circadian clock) gets a bit out of tune, especially in long days. It affects how well the plant keeps time.
• EID1: This is more like a specialized switch for light. It helps the plant decide when to flower (bloom). When humans changed this gene, it helped tomatoes grow in places where the days are very long or very short, allowing them to be grown all over the world, not just near the equator.

The Takeaway

This paper tells us that plants are incredibly smart engineers. They don't just passively wait for the sun to set.

• They have a Morning Manager that stretches and shrinks the schedule based on the season.
• They have an Evening Worker that stays on a fixed loop.
• By comparing the two, the plant knows exactly how long the day is, allowing it to know when to grow, when to repair itself, and when to flower.

This helps us understand how crops like tomatoes adapted to move from their native tropical homes to cold northern countries, and it gives scientists the tools to breed better crops for a changing climate.

Technical Summary

1. Problem Statement

Photoperiod (day length) is a critical environmental cue that regulates plant growth, development, and adaptation to different latitudes. While the circadian clock is known to synchronize internal rhythms with external light-dark cycles, the precise molecular mechanisms by which plants perceive and integrate photoperiodic information at the transcriptomic level remain incompletely understood. Specifically, there is a need to understand:

• How global gene expression dynamics shift across varying photoperiods (Short, Neutral, and Long days).
• The distinct roles of morning-phased versus evening-phased transcripts in sensing day length.
• How domestication-related mutations in key circadian regulators (EID1 and LNK2) in tomato (Solanum lycopersicum) alter these photoperiodic responses, potentially explaining the crop's expansion from equatorial origins to higher latitudes.

2. Methodology

The study employed a high-resolution, multi-factorial experimental design to dissect transcriptomic rhythms:

• Plant Material: Four isogenic lines of S. lycopersicum var. lycopersicum were used:
  1. MoneyMaker (MM): Cultivated variety with mutant alleles of EID1 and LNK2 (eid1/lnk2).
  2. EID1 line: Carrying wild-type EID1 from S. pimpinellifolium (EID1/lnk2).
  3. LNK2 line: Carrying wild-type LNK2 from S. pimpinellifolium (eid1/LNK2).
  4. LE line: Carrying wild-type alleles for both genes (EID1/LNK2).
• Experimental Conditions: Plants were grown under three photoperiod regimes at controlled temperatures (21°C day/18°C night):
  • Short Day (SD): 6h light / 18h dark.
  • Neutral Day (ND): 12h light / 12h dark.
  • Long Day (LD): 18h light / 6h dark.
• Sampling Strategy: Leaf samples were collected every 2 hours over a 24-hour cycle (ZT1 to ZT23) for two biological replicates per genotype and condition.
• Sequencing & Analysis:
  • RNA-seq: Illumina NovaSeq 6000 sequencing (approx. 32.7M reads/sample).
  • Oscillation Detection: Used ARSER (Autoregressive Spectral Analysis) to identify rhythmic transcripts (FDR < 0.01).
  • Dynamic Modeling: Applied Functional Data Analysis (FDA) to compute first (d1, rate of change) and second (d2, curvature) derivatives of expression profiles to analyze waveform geometry.
  • Comparative Genomics: Differential expression analysis (DESeq2) and motif enrichment analysis (HOMER) to link regulatory elements to specific gene groups.

3. Key Contributions

• High-Resolution Dataset: Generated the most comprehensive temporal transcriptome dataset in tomato to date, utilizing 2-hour sampling intervals (improving upon previous 4-hour studies) to capture fine-grained phase shifts.
• Bimodal Phase Distribution: Confirmed a robust bimodal distribution of rhythmic transcripts, with peaks clustering tightly around ZT0 (dawn) and ZT12 (dusk), regardless of photoperiod length.
• Differential Regulation Model: Proposed a novel model where Morning-Phased Transcripts (MPTs) act as primary transcriptional integrators of photoperiod, while Evening-Phased Transcripts (EPTs) act as post-transcriptional sensors via external coincidence mechanisms.
• Domestication Insights: Dissected the specific contributions of EID1 and LNK2 mutations to the attenuation of circadian rhythms under long-day conditions.

4. Key Results

A. Global Transcriptome Dynamics

• Oscillation Prevalence: 90.69% of expressed transcripts exhibited oscillatory behavior in at least one condition.
• Photoperiod Effects:
  • Phase Shifts: As day length increased, the global transcriptome shifted phase. MPTs remained tightly aligned with dawn (ZT0), while EPTs shifted relative to dusk.
  • Misalignment: In Long Days (LD), EPTs peaked during the light period, and in Short Days (SD), they peaked during the dark. This creates a systematic misalignment between EPTs and the actual timing of dusk, suggesting EPTs measure photoperiod via "external coincidence" (coincidence of expression with light).
  • Amplitude Dampening: Under LD conditions, the number of detected oscillating transcripts decreased, and those that remained showed significantly reduced amplitude and Daily Expression Integral (DEI). This suggests rhythmicity is attenuated but not abolished in long days.

B. Distinct Waveform Responses (MPTs vs. EPTs)

• MPTs (Morning): Showed significant waveform plasticity. Their expression profiles changed shape (specifically the rising phase) depending on photoperiod. Functional Data Analysis revealed MPTs have "open" trajectories in phase space, indicating they are sensitive to external forcing and adjust their activation rates based on day length.
• EPTs (Evening): Showed robust, stable dynamics. Their waveforms remained largely unchanged across photoperiods, maintaining "closed" limit cycle trajectories. They peak ~12 hours after MPTs, suggesting they rely on the timing set by MPTs rather than directly sensing the photoperiod transcriptionally.

C. Role of Domestication Genes (EID1 and LNK2)

• LNK2: The wild-type LNK2 allele was crucial for maintaining rhythmicity under Long Days. The cultivated mutant (lnk2) resulted in a broader reduction of oscillating genes and altered the expression of core clock genes (e.g., reduced PRR5/TOC1, increased CCA/LHY).
• EID1: Mutations in EID1 primarily affected photoperiod-dependent flowering time regulators (e.g., SP5G, SP11B.1) and showed a specific enrichment of the REF6 binding motif in genes that oscillate in wild-type lines but not in eid1 mutants. This links EID1 to PHYB-mediated regulation.
• Interaction: The study suggests LNK2 has a broader impact on the core circadian clock, while EID1 functions more specifically as a phytochrome regulator influencing flowering time.

5. Significance and Conclusion

This study provides a mechanistic model for how tomato adapts to varying latitudes:

1. Transcriptional Sensing: Morning transcripts (MPTs) sense photoperiod length transcriptionally, adjusting their waveform and phase in response to day length.
2. Temporal Reference: MPTs serve as a "zeitgeber" reference for evening transcripts (EPTs).
3. External Coincidence: EPTs, which are robust to waveform changes, peak ~12 hours later. Their encoded proteins integrate day-length information by coinciding with light availability (external coincidence), thereby regulating downstream outputs like flowering.
4. Domestication: The domestication mutations in EID1 and LNK2 fine-tuned this system, likely allowing tomato to expand from its equatorial origins to higher latitudes by modulating the sensitivity of the circadian clock to long days.

The findings highlight that photoperiodic adaptation is not a uniform shift of the entire clock but a differential regulation where morning and evening components play distinct, complementary roles in measuring time and light.