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.

硼 (B) 是植物发育的必需营养素，在许多作物中经常观察到硼的缺乏。木栓状裂脉（CSV）是柑橘缺硼的典型症状，但缺硼下形成的机制尚不清楚。我们的工作强调了木质素生物合成在 CSV 形成中的作用。两年大的“纽霍尔”脐橙在缺乏 B 的条件下生长，直到出现 CSV 症状。通过静脉分离和解剖分析观察CSV的形态，结果显示维管束细胞畸形，细胞壁严重增厚。有趣的是，在维管束周围清楚地观察到了木栓层，这在以前没有报道过。此外，在原始维管束周围发现了许多小的、新的和不完整的维管束。这些结果表明，柑橘中 B 缺乏导致 CSV 的形成可能归因于木质素在静脉维管束中的积累。木质素含量测定表明，在缺硼胁迫下，叶片木质素含量显着增加。进一步的 FTIR 光谱分析表明，B 缺陷叶片中特征木质素峰对应的相对吸光度高于对照植物，但木质素单体的比例没有变化。这些结果表明，在 B 缺乏胁迫下木质素积累显着增加，并促进了 CSV 症状的形成。对木质素生物合成的关键酶进行了分析，结果表明，B 缺乏激活了柑橘叶片中除 4CL 外的 PAL、C4H、CCR 和 CAD 酶的活性。相似地，CsPAL、CsC4H、Cs4CL2、Cs4CL3、CsCCR1、CsCAD2、CsCAD3和CsCAD5。这些结果意味着木质素代谢在 B 缺乏下 CSV 的形成中起着至关重要的作用。综上所述，我们的结果为柑橘中缺硼胁迫引起的 CSV 形成机制提供了重要的见解。