Plant Meristems: A Layered Defense Against Mutations

The potential impact of new mutations on an organismal function and evolution depends on the developmental fate of the affected cells. Stem cells in the shoot apical meristem form much of the plant body, but only those in the L2 can form gametes. This raises the question of whether the mutation rate is optimized for this developmental plan.

To test this, researchers compared stem cell mutations accumulated over decades of clonal propagation in two potato varieties with those arising during leaf-cell regeneration. By sequencing DNA from layer-enriched cell fractions, they found that variant allele frequencies in whole-leaf DNA can predict a mutation's layer of origin, revealing that mutation rates are far from uniform. Notably, mutations accumulated independently in the L1 layer but jointly in the L2 and L3 layers. In the cultivar Desiree, the L1 layer exhibited mutation rates fourfold higher than in deeper layers, suggesting germline-progenitor protection. A similar, though subtler, bias was observed in Red Polenta, with L1 rates 1.6-fold higher.

The contrast became even starker when examining mutations during the regeneration of differentiated cells. These mutations were fiftyfold more frequent and bore a pronounced 8-oxoguanine signature, confined to intergenic regions. This suggests different mutation pressures and repair mechanisms at play in different cellular contexts. Furthermore, across all samples, the genic mutation rate was less than half that of intergenic regions, a pattern consistent with either enhanced repair or purifying selection.

These compelling findings underscore that plants are not passive recipients of genetic errors. Instead, as lead author Amundson notes in the paper, "These findings indicate that plants employ mechanisms to restrict mutations according to cellular context and genomic region and suggest that the layered organization of angiosperm meristems evolved to balance genetic fidelity with adaptability." This delicate balance, crucial for ensuring the integrity of the germline while allowing for necessary genetic variation, enables plants to maintain stable core functions and adapt to changing environments.