Lignin metabolism plays an essential role in the formation of corky split vein caused by boron deficiency in ‘Newhall’ navel orange (Citrus sinensis Osb.)

Abstract Boron (B) is an essential nutrient for plant development, and its deficiency has been frequently observed in many crops. Corky split vein (CSV) is a typical symptom of B deficiency in citrus, but the mechanism for the formation under B deficiency is unclear. Our work highlighted the role of lignin biosynthesis in the formation of CSV. Two-year-old ‘Newhall’ navel oranges were grown under B-deficient conditions until CSV symptoms developed. The morphology of CSV was observed by vein separation and anatomical analysis, and results showed that the vascular bundle cells were malformed, and their cell walls were heavily thickened. Interestingly, a phellem layer was clearly observed around the vascular bundle, which has not been reported previously. Moreover, many small, new, and incomplete vascular bundles were found around the original vascular bundle. These results indicate that the formation of CSV caused by B deficiency in citrus can probably be attributed to the accumulation of lignin in the vascular bundle of the veins. Lignin content measurement showed the leaf lignin content was markedly increased under B deficiency stress. Further FTIR spectral analysis showed that the relative absorbance corresponding to characteristic lignin peaks were higher in B deficient leaf than control plants, but the ratio of monolignols did not change. These results indicate that lignin accumulation increased significantly under B deficiency stress, and it promotes the formation of CSV symptoms. The key enzymes of lignin biosynthesis were analyzed and the results suggest that B deficiency activated the activity of PAL, C4H, CCR, and CAD enzymes in citrus leaves, with the exception of 4CL. Similarly, lignin biosynthesis key genes expression levels increased under B deficiency stress, including CsPAL, CsC4H, Cs4CL2, Cs4CL3, CsCCR1, CsCAD2, CsCAD3, and CsCAD5. These results implying that lignin metabolism plays an essential role in the formation of CSV under B deficiency. Taken together, our results provide important insights into the mechanism of CSV formation caused by B deficiency stress in citrus.