Genome-wide identification and characterization of the NAC transcription factor family in Cynodon dactylon and their expression during abiotic stresses

This study presents the first comprehensive genome-wide identification and characterization of 237 NAC transcription factor genes in *Cynodon dactylon*, revealing their phylogenetic diversity, tissue-specific expression patterns, and distinct upregulation or downregulation profiles in response to drought, heat, salt, and submergence stresses, thereby establishing a foundation for breeding stress-resilient bermudagrass.

The Gist

Imagine Bermudagrass as the ultimate "survivor" of the plant world. It's the tough, green carpet you see on golf courses and sports fields that can handle scorching heat, dry spells, and even being underwater. Scientists already have the "instruction manual" (the genome) for this grass, but they realized they were missing a crucial chapter: the part that explains how it survives such tough conditions.

This study focuses on a specific team of workers inside the grass's cells called NAC transcription factors. Think of these NAC proteins as the foremen or managers in a massive factory. Their job is to read the instruction manual and tell the factory which machines to turn on or off to keep the plant alive.

Here is what the researchers discovered, broken down simply:

1. The Big Roll Call

The scientists went through the entire genome and found 237 different NAC managers (genes). They organized them into 14 different departments based on what they do:

• The "Growth Crew" (NAM/NAC1): About 40 of these managers are like construction foremen. They focus on building the plant, helping it grow tall, and making flowers.
• The "Emergency Response Team" (SNAC): About 23 of these are like firefighters or paramedics. Their only job is to scream "Code Red!" and activate survival modes when things go wrong.

2. Where They Work

The researchers checked where these managers hang out in the plant.

• Some managers are generalists; they are busy in the roots, leaves, and flowers all at once, keeping the basic lights on.
• Others are specialists. Some only work in the roots (like deep-dive engineers), while others only work in the flowers (like event planners).

3. The Stress Test

The most exciting part was seeing how these managers reacted when the grass was put under pressure. The scientists simulated four different disasters:

• Drought (No water)
• Heat (Scorching sun)
• Salt (Salty soil)
• Submergence (Being underwater)

The Results:

• The All-Stars: Three specific managers (CdNAC122, 149, and 155) from the "Emergency Response Team" were like the ultimate superheroes. They showed up and started working no matter what the disaster was. Whether it was dry, hot, salty, or wet, these three were always on duty.
• The Specialists: Other managers were picky.
  • Some only woke up for drought (like CdNAC37).
  • Some only cared about heat (like CdNAC7).
  • Others were specific to salt or being underwater.
• The Shutdown: Interestingly, 53 managers actually stopped working during stress. These were mostly the "Growth Crew." It's like a factory manager saying, "We don't have enough power right now, so let's stop building new wings and just keep the lights on to survive." The plant sacrifices growth to save its life.

Why Does This Matter?

Before this study, we knew Bermudagrass was tough, but we didn't know who was pulling the levers to make it tough. Now, we have a complete list of the "survival managers."

This is a huge deal for farmers and breeders. Instead of guessing which grass will survive a drought, they can now look for these specific managers. They can breed new, super-tough Bermudagrass varieties that have extra-strong "Emergency Response Teams," ensuring our golf courses and lawns stay green even when the weather turns against them.

In short: This paper is like finding the master switchboard for a tough plant's survival system, identifying exactly which buttons to press to keep it alive in a crisis.

Technical Summary

Technical Summary: Genome-wide Identification and Characterization of the NAC Transcription Factor Family in Cynodon dactylon

1. Problem Statement

• Context: Common bermudagrass (Cynodon dactylon) is a globally significant grass species valued for its resilience in turf, forage, and soil stabilization applications.
• Knowledge Gap: Despite the availability of the C. dactylon genome sequence, there is a lack of systematic characterization regarding the specific genes responsible for its stress resilience.
• Specific Focus: The NAC (NAM, ATAF1/2, and CUC2) transcription factor (TF) family is known to play critical roles in plant growth, development, and abiotic stress responses. However, the composition, classification, and stress-responsive mechanisms of the NAC family in bermudagrass had not been previously explored.

2. Methodology

The study employed a multi-faceted genomic and transcriptomic approach:

• Genome-wide Identification: Systematic screening of the C. dactylon genome to identify all NAC transcription factor genes.
• Phylogenetic Analysis: Classification of identified genes into evolutionary groups to infer functional relationships based on known NAC families in other plants.
• Tissue-Specific Expression Profiling: Utilization of RNA-seq data to analyze expression patterns across different plant tissues (e.g., roots, inflorescence) to determine functional specificity.
• Stress-Responsive Expression Analysis: Quantitative assessment of gene expression under four distinct abiotic stress conditions:
  • Drought
  • Heat
  • Salt
  • Submergence

3. Key Contributions

• First Comprehensive Characterization: This study provides the inaugural systematic analysis of the NAC transcription factor family in Cynodon dactylon.
• Functional Classification: It establishes a phylogenetic framework for 237 identified genes, linking specific clades to growth/development versus stress response functions.
• Stress-Specific Marker Identification: The research pinpoints specific candidate genes that serve as potential biomarkers for different types of environmental stress, distinguishing between general stress responders and stress-specific regulators.

4. Key Results

• Gene Identification and Classification:
  • A total of 237 CdNAC genes were identified.
  • These were phylogenetically grouped into 14 distinct classes.
  • NAM/NAC1 class: Contains 40 members, primarily associated with plant growth and development.
  • SNAC class: Contains 23 members, strongly associated with stress responses.
• Tissue-Specific Expression:
  • Approximately 25% of CdNAC genes showed constitutive expression across all tissues, suggesting essential housekeeping roles.
  • 13 genes exhibited higher expression in roots, while 9 genes were upregulated in inflorescence, indicating specialized tissue functions.
• Abiotic Stress Responses:
  • Upregulation: A significant number of genes were induced by stress:
    • Drought: 35 genes (e.g., CdNAC37, 130, 145, 199).
    • Heat: 43 genes (e.g., CdNAC7, 12, 18, 29).
    • Salt: 10 genes (e.g., CdNAC46, 151).
    • Submergence: 42 genes (e.g., CdNAC9, 31).
  • Universal Stress Responders: Members of the SNAC class, specifically CdNAC122, 149, and 155, were consistently upregulated across all tested stress conditions.
  • Downregulation: 53 genes were downregulated under stress, predominantly belonging to the NAM/NAC1, TERN, and OsNAC7 classes. This suggests a mechanism where stress suppresses photosynthesis and developmental processes to conserve energy.

5. Significance

• Foundational Resource: This work creates a critical database for the C. dactylon NAC family, serving as a reference for future functional genomics.
• Breeding Applications: By identifying specific candidates (such as the SNAC class members) that confer tolerance to drought, heat, salt, and submergence, the study provides direct targets for molecular breeding programs.
• Mechanistic Insight: The findings elucidate how bermudagrass balances growth suppression with stress activation, offering a deeper understanding of the molecular basis of its cosmopolitan resilience.
• Future Directions: The study lays the groundwork for functional validation (e.g., overexpression or knockout studies) to develop stress-resilient bermudagrass varieties for turf and forage industries.