The control of prickle formation in Rubus

This study identifies the WOX1 gene as the genetic basis for pricklelessness in blackberries and raspberries and demonstrates that CRISPR-mediated knockout of WOX1 in elite prickled varieties successfully produces prickleless cultivars without compromising other agronomic traits.

The Gist

Imagine you are picking a blackberry. You reach out, and instead of just grabbing a juicy fruit, you get scratched, snagged, and poked by dozens of tiny, sharp thorns. For farmers, this isn't just annoying; it's a nightmare. It slows down harvesting, hurts workers, and drives up the cost of the fruit in the grocery store.

For over a century, breeders have tried to fix this by cross-breeding plants to find the "thornless" gene. But it's been like trying to find a needle in a haystack while wearing blindfolds. The "thornless" trait is recessive (meaning you need four copies of the gene in these plants to work), and it's stuck in a genetic "traffic jam" with other bad traits, like sour fruit or plants that are too weak to survive winter.

The Big Discovery: Finding the "Thorn Switch"

This paper is the story of how a team of scientists finally found the exact "switch" that turns thorns on and off, and how they used modern technology to flip that switch in the best-tasting blackberries without all the old-fashioned breeding headaches.

Here is the breakdown of their journey:

1. The Detective Work: Finding the Culprit

The scientists gathered a massive library of 268 different blackberry and raspberry plants, ranging from wild, thorny varieties to smooth, thornless ones. They treated this like a giant crime scene investigation.

• The Clues: They used three different high-tech detective tools:
  • GWAS (Genome-Wide Association Study): Like scanning a crowd of people to see who shares a specific feature.
  • BSA (Bulked Segregant Analysis): Like mixing all the "thornless" suspects into one bucket and all the "thorny" suspects into another, then comparing their DNA to see what's different.
  • IBD (Identity-by-Descent): Tracing the family tree to see exactly which piece of DNA was passed down from the original "thornless" ancestors.

The Verdict: They narrowed it down to a tiny neighborhood on Chromosome 4. Inside that neighborhood, they found a single gene called WOX1. Think of WOX1 as the foreman of a construction site. Its job is to tell the plant's cells, "Hey, build a sharp prickle here!" or "Build a sticky gland here!"

2. The "Broken Blueprint"

The scientists discovered that in nature, thornless plants exist because their WOX1 gene is broken.

• In some plants, a tiny piece of DNA was inserted into the gene, acting like a typo in a recipe that ruins the whole cake.
• In others, a piece was deleted.
• In the famous "Burbank Thornless" blackberry (a historic variety), the plant actually had two different broken versions of the gene, making it double-thornless.

When the WOX1 foreman is broken, the construction site stops building prickles and glandular hairs (the sticky, smelly hairs that also protect the plant). The result? A smooth, safe-to-harvest cane.

3. The "Magic Wand": Gene Editing

For years, breeders had to cross-breed a thorny, high-quality blackberry with a thornless, low-quality one, and then spend 10-15 years trying to get the good fruit back while keeping the thorns gone. It was slow and messy.

This team used Gene Editing (CRISPR/Cas12a) as a "magic wand."

• They took a modern, elite blackberry plant that had great fruit but was covered in thorns.
• Instead of cross-breeding, they went straight to the plant's DNA and used the "wand" to break the WOX1 gene, just like the natural mutants had.
• The Result: The plant immediately became thornless. It also lost the sticky glandular hairs (which is fine for harvesting), but everything else stayed the same. The fruit was still sweet, the plant was still strong, and the leaves were still green.

4. Why This Matters

This is a game-changer for agriculture.

• Speed: Instead of waiting a decade for a new variety, they can create a thornless version of an existing favorite in a single generation.
• Safety: No more "linkage drag." In the past, when you bred for thornless, you accidentally dragged along bad traits (like sour fruit). Gene editing allows you to fix only the thorn problem without touching the rest of the plant's DNA.
• The Future: This means we could soon see blackberries and raspberries that are easy to pick by hand, cheaper to produce, and just as delicious as the prickly ones.

In a Nutshell:
Scientists found that a specific gene (WOX1) is the "boss" that tells blackberry plants to grow thorns. They figured out how nature breaks this boss to make thornless plants, and then used gene editing to break that boss in the best-tasting commercial blackberries. The result? A smooth, safe, and delicious berry without the scratch.

Technical Summary

1. Problem Statement

Prickles (sharp, non-vascular epidermal outgrowths) on blackberry (Rubus subg. Rubus) and red raspberry (Rubus idaeus) canes pose significant challenges in commercial agriculture. They hinder manual harvesting and pruning, increase labor costs, and cause physical injury to workers.

• Genetic Complexity: The prickleless trait is recessive. In tetraploid blackberries, four copies of the recessive allele are required for the phenotype.
• Linkage Drag: Historically, breeding for prickleless cultivars relied on the s locus (specifically the sru allele in blackberry and sri in raspberry). However, this locus is linked to undesirable agronomic traits, including trailing growth habits, late harvest seasons, large seed size, high fruit acidity, and susceptibility to freeze damage.
• Breeding Bottlenecks: Breaking the linkage between the prickleless trait and these deleterious traits has been difficult due to extensive linkage disequilibrium (LD) around the s locus and the recessive nature of the trait, which requires extensive backcrossing and inbreeding, leading to a loss of genetic diversity.

2. Methodology

The authors employed a multi-faceted approach combining genomics, transcriptomics, and gene editing to identify the causal gene and validate its function.

• Diversity Panel & Phenotyping: A panel of 268 Rubus accessions (224 prickled, 44 prickleless) was phenotyped for prickles, glandular trichomes, and simple trichomes using both visual inspection and dissecting microscopy.
• Genome Assembly: A high-quality, chromosome-scale genome assembly was generated for Rubus ulmifolius cv. 'Burbank Thornless' (a historic diploid prickleless variety) using PacBio long reads and Hi-C scaffolding. This provided a reference for the sru allele source.
• Genetic Mapping: The causal locus was refined using three complementary strategies:
  1. Genome-Wide Association Study (GWAS): Performed on the diversity panel using the R. argutus 'Hillquist' reference genome.
  2. Bulked Segregant Analysis (BSA): A segregating population from a self-crossed blackberry line (triplex for the s allele) was bulked into prickled and prickleless groups for deep sequencing to identify allele frequency differences.
  3. Identity-by-Descent (IBD) Analysis: Pedigree-informed analysis compared homozygous regions across prickled and prickleless lines from both blackberry and red raspberry to narrow the candidate interval.
• Transcriptomics: RNA-seq was performed on meristems of prickled vs. prickleless segregating plants to identify differentially expressed genes within the mapped interval.
• Gene Editing: A transformation protocol was developed for blackberry. The candidate gene was knocked out in an elite, prickled commercial line (PW-D0760) using CRISPR-Cas12a (LbCas12a) to verify causality.
• Comparative Analysis: Orthologs of candidate genes were analyzed in Rosa (rose), Solanum (eggplant), and Arabidopsis to understand evolutionary conservation and functional divergence.

3. Key Contributions

• Gene Identification: The study definitively identified WOX1 (WUSCHEL-LIKE HOMEOBOX 1) as the master regulator of prickle formation in Rubus.
• Discovery of Multiple Alleles: The authors identified independent loss-of-function mutations in WOX1 responsible for the prickleless phenotype in different species and varieties:
  • Ruwox1-1: An 8bp insertion in the homeodomain of R. ulmifolius (blackberry), causing a premature stop codon.
  • Ruwox1-2: A 4bp deletion in the WUS box of R. ulmifolius 'Burbank Thornless'.
  • Riwox1-3: An 8bp insertion in R. idaeus (red raspberry).
• Compound Heterozygosity: The study revealed that 'Burbank Thornless' is a compound heterozygote (carrying both Ruwox1-1 and Ruwox1-2), explaining its prickleless phenotype despite being heterozygous at the locus in a standard sense.
• Developmental Linkage: The research demonstrated that WOX1 controls a joint developmental pathway for both prickles and glandular trichomes. Knocking out WOX1 eliminated both structures, while simple trichomes remained unaffected.
• Gene Editing Validation: The study successfully developed a transformation system for blackberry and used gene editing to convert a prickled elite cultivar into a prickleless one without altering other agronomic traits.

4. Key Results

• Fine Mapping: The integration of GWAS, BSA, and IBD narrowed the S locus to a ~324 kb region on Chromosome 4, containing approximately 62–115 genes.
• Expression Analysis: Within the candidate region, Ra_g19519 (the WOX1 ortholog) showed significantly lower expression in prickleless samples compared to prickled ones, consistent with nonsense-mediated decay of the mutant allele.
• Mutation Confirmation:
  • In blackberry, the sru allele corresponds to an 8bp insertion (Ruwox1-1) creating a premature stop codon in the homeodomain.
  • In red raspberry, the sri allele corresponds to an 8bp insertion (Riwox1-3) downstream of the homeodomain.
• Editing Outcomes:
  • Gene-edited blackberry plants (PW-D0760) carrying Rrwox1-4 (an 8bp deletion in the homeodomain) were completely prickleless and lacked glandular trichomes.
  • Crucially, no other phenotypic changes (e.g., fruit quality, plant architecture, simple trichomes) were observed, confirming WOX1 is a specific target with low pleiotropy.
• Negative Controls: Editing other candidate genes (e.g., MYB16, GL1, TTG2) did not result in a prickleless phenotype, ruling them out as the primary causal factors in Rubus.

5. Significance

• Solving a Century-Old Breeding Challenge: This study provides the molecular solution to a trait that has been difficult to breed for due to recessive inheritance and linkage drag. It proves that gene editing can bypass the need for extensive backcrossing to break linkage with deleterious traits.
• Precision Agriculture: Breeders can now directly edit the WOX1 gene in elite, prickled varieties that possess superior fruit quality and disease resistance, instantly conferring the prickleless trait without compromising other desirable characteristics.
• Economic Impact: Removing prickles will significantly reduce labor costs, improve harvesting efficiency, and enhance worker safety in the Rubus industry.
• Evolutionary Insight: The discovery of parallel evolution (independent mutations in the same gene across different Rubus species) highlights WOX1 as a developmental "hotspot" for prickle loss.
• Broader Implications: The finding that WOX1 regulates both prickles and glandular trichomes suggests a shared developmental pathway for mechanical and chemical defenses in plants. This contrasts with findings in eggplant, where prickle loss is linked to the LOG gene, suggesting different evolutionary paths for similar traits in different plant families.

In conclusion, this paper establishes WOX1 as the causal gene for the S locus in Rubus and demonstrates the feasibility of using CRISPR-Cas12a to create safe, high-quality, prickleless commercial cultivars, marking a paradigm shift from traditional breeding to precision genome editing for caneberries.