Gypsy Retrotransposon Insertion Causes Stay-Green Chinese Cabbage by Blocking Chlorophyll b Breakdown

Chinese cabbage may have found a new way to remain green long after its leaves would normally begin to yellow. A study published in Theoretical and Applied Genetics reports that a mobile DNA sequence, known as a Gypsy long terminal repeat retrotransposon, disrupted a gene responsible for chlorophyll breakdown and produced a stay-green phenotype. The discovery links a naturally occurring genome rearrangement to a visible trait with direct implications for the storage quality and market value of leafy vegetables.

The research focused on a Chinese cabbage line called 5BTL, whose leaves retain their green color during aging and after harvest. In most plants, leaf senescence involves the gradual dismantling of chloroplasts, the cellular structures that carry out photosynthesis. Chlorophyll molecules are degraded as part of this process, causing leaves to turn yellow. Although this transition is biologically programmed, it can reduce the visual quality and freshness of vegetables during transportation and storage. Plants with the stay-green trait delay this color change because one or more steps in chlorophyll degradation are impaired.

To identify the genetic cause of the 5BTL phenotype, the researchers combined bulked segregant analysis sequencing, or BSA-seq, with fine-mapping and molecular validation. BSA-seq compares genomic regions enriched in groups of plants with contrasting traits. By pooling individuals with either green-retaining or normally yellowing leaves, researchers can detect DNA variants associated with the phenotype without first analyzing every plant independently. This approach narrowed the candidate region to a segment on chromosome A04 of Brassica rapa, the species that includes Chinese cabbage.

Within that interval, the team identified the gene BraA04g008820.3C, which they named BrNYC1-A04 because of its strong similarity to NON-YELLOW COLORING 1, or NYC1, in Arabidopsis thaliana. NYC1 belongs to the short-chain dehydrogenase/reductase family and is involved in the first stages of chlorophyll b degradation. Chlorophyll b is a light-harvesting pigment found in photosynthetic antenna complexes. Before these complexes can be dismantled during senescence, chlorophyll b must be converted into chlorophyll a through a reaction involving chlorophyll b reductase activity.

Sequence analysis revealed a large insertion in the first intron of BrNYC1-A04 in the stay-green line. The inserted DNA was identified as a Gypsy LTR retrotransposon, a class of mobile genetic element that spreads through a copy-and-paste mechanism involving an RNA intermediate. LTR retrotransposons contain repeated sequences at their ends and often carry genes encoding the enzymes needed for reverse transcription and integration. In plant genomes, these elements are abundant and have contributed extensively to genome evolution, but they can also interfere with nearby genes when they insert into coding regions or regulatory sequences.

In the case of 5BTL, the retrotransposon did not simply increase or decrease the activity of the gene’s promoter. Instead, it disrupted the way the gene’s precursor RNA was processed. The insertion altered normal splicing of the first intron, causing the mature messenger RNA to contain an abnormal sequence and a premature termination codon. Such a codon signals that protein production should stop too early. The resulting transcript is predicted to encode a shortened, nonfunctional protein, and it may also be recognized and eliminated by the plant’s nonsense-mediated mRNA decay pathway, a surveillance system that removes faulty messenger RNAs.

The functional consequences were visible at both the biochemical and cellular levels. Plants carrying the defective BrNYC1-A04 allele showed reduced chlorophyll b degradation and changes in chloroplast structure. Because chlorophyll b is associated with light-harvesting complex II, failure to process this pigment can prevent the orderly dismantling of photosynthetic membranes during senescence. The researchers also observed an effect on chlorophyll b reductase activity, supporting the conclusion that the mutation blocks a specific step in pigment turnover rather than merely slowing overall leaf aging.

The study further connected BrNYC1-A04 to the broader regulatory network controlling senescence. Under dark-induced senescence conditions, the expression of BrNYC1-A04 was regulated by BrORE1, a Chinese cabbage homolog of the Arabidopsis NAC transcription factor ORE1. ORE1 is a major promoter of leaf senescence and activates genes involved in the degradation of cellular components. This result suggests that BrNYC1-A04 operates downstream of a recognized senescence-control pathway. When the gene is structurally damaged by the retrotransposon, signals that would normally activate chlorophyll breakdown can no longer produce the full biochemical response.

To test the gene’s role independently of the naturally occurring insertion, the researchers used virus-induced gene silencing, or VIGS, in the wild-type Chinese cabbage line HN19-G. VIGS temporarily reduces the expression of a chosen gene by using a plant virus to trigger sequence-specific RNA silencing. Silencing BrNYC1-A04 in HN19-G significantly delayed the loss of chlorophyll b, recreating a central feature of the 5BTL stay-green phenotype. Together with genetic mapping, the insertion analysis, transcript evidence and biochemical observations, this experiment provides strong support for a causal relationship between loss of BrNYC1-A04 function and delayed leaf yellowing.

The finding adds to a growing body of research showing that transposable elements can act as powerful sources of agricultural variation. Earlier studies in Chinese cabbage and related crops have linked mutations in chlorophyll metabolism genes to stay-green phenotypes, but the new work identifies a mobile DNA insertion specifically targeting a chlorophyll b reductase gene. The result also highlights why introns cannot be dismissed as biologically irrelevant DNA: a sequence inserted into an intron can derail RNA processing and effectively switch off an otherwise intact gene. For breeders, the Gypsy insertion may serve as a molecular marker for selecting plants with improved postharvest color retention. However, a persistent green appearance does not automatically mean that a leaf remains fully photosynthetically active or nutritionally superior, so future work will need to examine yield, stress tolerance, flavor, nutrient composition and storage performance. The authors’ model proposes that the retrotransposon-induced loss of BrNYC1-A04 reduces chlorophyll b reductase activity, preserves light-harvesting complexes and slows chloroplast dismantling, allowing Chinese cabbage leaves to stay green for longer. The study therefore turns a striking visual trait into a precise molecular story: a wandering piece of DNA entered a plant gene, disrupted its splicing, and unexpectedly helped preserve the color that consumers associate with freshness.