Extrachromosomal circular DNA are functional, heritable units that expand genomic plasticity and confer resilience.

Although far less well-known and understood than chromosomal DNA, extrachromosomal circular DNA (eccDNA) are a pervasive and dynamic component of eukaryotic genomes. eccDNA are nuclear-localized, double-stranded DNA circles that exist independently of the main chromatin body. They share many sequence features with chromosomal DNA, including encoding functional genes; however, unlike chromosomes, eccDNAs are highly heterogenous, capable of autonomous replication and ultra-high gene expression, and do not necessarily segregate evenly or follow Mendelian inheritance during cell division. Although several recent reviews have focused on their roles in human health, emerging research in plants shows that eccDNAs are intricately associated with rapid adaptation to stress, particularly in weedy and invasive plants. This plant-centric review synthesizes evidence that eccDNAs carry full-length genes, regulatory elements, and transposable sequences, that collectively enable gene amplification, novel protein variants, and context-specific expression. We propose that eccDNAs function as "genomic shock absorbers": stress-inducible, non-Mendelian reservoirs of genetic diversity that buffer genomes against environmental challenges such as nutrient limitation and xenobiotic exposure. Drawing parallels with bacterial plasmids, we argue that eccDNA facilitate novel and important genome-environment interactions beyond those mediated by chromosomes. Harnessing these elements as non-Mendelian vehicles for genetic innovation could offer a route to translate weed-derived resilience into novel crop improvement strategies, enabling the design of climate-ready, stress-resilient agriculture grounded in weed inspired mechanisms of adaptability and tolerance.