Understanding the basis of Phytophthora tolerance is essential for targeted improvement of citrus. Here, we determined the short term and long-term plant (leaf and root tissues) response of Phytophthora tolerant (Swingle citrumelo; SC) and susceptible (rough lemon; RL) citrus genotypes by examining important defense related genes belonging to structural reinforcement, receptors, salicylic acid (SA) & jasmonic acid (JA) pathways. We also estimated downstream activity of the enzymes lipoxygenase (LOX), phenylalanine ammonia lyase (PAL) and total phenols. The plants were artificially inoculated with mycelium and zoospores of P. nicotianae in root zone and studied at 34- and 105- days post inoculation (dpi). SC roots strongly upregulated receptor like genes viz., LecRK-I.9, BIR2, MPK4, MKK1 and TTG2 at 34-dpi; RL in contrast showed delayed (at 105-dpi) upregulation of LecRK-I.9, MKK1 and TTG2. SC also showed higher PAL activity, higher phenolic content and stronger expression of GSL5 in roots at 34-dpi to create barrier against invading pathogen. SC roots upregulated SA pathway genes like ICS1, EDS1, PAD4, CBP60g, PR-1 at 34-dpi, whereas, RL showed higher expression of JA-related genes (LOX2 and AOS) at 105-dpi, suggesting their role in favoring necrotrophic phase. In general, the SC leaf at 105-dpi mimicked the defense response of its roots at 34-dpi. Study results suggest that early perception, preformed barriers and restricting the pathogen in biotrophic phase are the key tolerance mechanisms of Swingle citrumelo to P. nicotianae. The differentially expressing receptor genes (LecRK-I.9, BIR2) and SA mediated defense gene, PR1 in Swingle citrumelo should further be explored for utilization in citrus rootstock improvement.

中文翻译：

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基因表达和代谢物分析揭示了更快的病原体识别和基于减少模式触发的免疫反应对柑橘中烟草疫霉耐受性的重要性


了解疫霉菌耐受性的基础对于有针对性地改善柑橘至关重要。在这里，我们确定了耐疫霉菌（Swingle citrumelo;SC） 和易感（粗柠檬;RL）柑橘基因型通过检查属于结构加固、受体、水杨酸（SA）和茉莉酸（JA）途径的重要防御相关基因。我们还估计了脂氧合酶 （LOX） 、苯丙氨酸解氨酶 （PAL） 和总酚酶的下游活性。在根区用 P. nicotianae 的菌丝体和游动孢子人工接种植物，并在接种后 34 天和 105 天 （dpi） 进行研究。SC 根强烈上调受体样基因，即 LecRK-I.9、BIR2、MPK4、MKK1 和 TTG2，分辨率为 34-dpi;相比之下，RL 显示 LecRK-I.9 、 MKK1 和 TTG2 的延迟 （105-dpi） 上调。SC 还显示出更高的 PAL 活性、更高的酚类含量和更强的 GSL5 在 34 dpi 时在根中的表达，以形成抵御入侵病原体的屏障。SC 根在 34 dpi 时上调 ICS1 、 EDS1 、 PAD4 、 CBP60g 、 PR-1 等 SA 通路基因，而 RL 在 105 dpi 时表现出较高的 JA 相关基因 （LOX2 和 AOS） 表达，表明它们在有利于坏死期中的作用。一般来说，105 dpi 的 SC 叶模仿了其根在 34 dpi 的防御反应。研究结果表明，早期感知、预先形成的屏障和在生物营养期限制病原体是 Swingle 柑橘对烟草假单胞菌的关键耐受性机制。应进一步探索 Swingle citrumelo 中差异表达受体基因 （LecRK-I.9， BIR2） 和 SA 介导的防御基因 PR1，以用于柑橘砧木改良。