Grafting improves drought stress memory by increasing the P5CS1 gene expression in Brassica rapa

Plants accumulate proline as a drought-tolerance strategy. However, the effect of grafting on adaptive proline accumulation remains to be discovered. Physiological methods were used to compare drought tolerance of turnip (B. rapa subsp. rapa) and rapeseed (B. rapa subsp. oleifera). Chromatin Immunoprecipitation (ChIP) and gene expression analysis were used to reveal epigenetic modifications and transcriptional regulation of the Δ1-pyrroline-5-carboxylate synthetase 1 (P5CS1) gene within grafting experiments. Turnip is more drought tolerant than rapeseed, accompanied by higher expression of P5CS1 gene and more proline accumulation in turnip than in rapeseed. Grafting rapeseed onto turnip improved its drought tolerance. Repeated drought stress induced stress memory in proline accumulation and markedly enhanced P5CS1–2 expression in the rapeseed scion of rapeseed/turnip (scion/rootstock) plants compared to self-grafted rapeseed plants. This accumulation required renewed histone H3K4me3 modification at the P5CS1–2 chromatin. Grafting rapeseed onto turnip enhanced drought resistance in Brassica rapa through potential signaling between turnip rootstock and rapeseed scion altering epigenetic modification on the P5CS1–2 locus and increasing its transcript level during the stress memory response.