柑橘响应低pH高铝互作的分子生理机制

Acid (low pH)- and aluminum (Al)-toxicities coexist in acidic soils. Only when plants tolerate both acid- and Al-toxicities can they really adapt to acidic soils. So far, few studies on the effects of pH on plant Al-toxicity have focused mainly on the herbaceous plants, and the results are inconsistent. Citrus belong to evergreen subtropical woody fruit trees. In the major citrus production regions, citrus are cultivated mainly in acidic and strong acidic soils and are usually affected by low pH and high Al. It is imperative to study systematically and deeply the effects of low pH and high Al interactions on citrus and their adaptive mechanisms. Based on on-line retrieval, acid-toxicity and Al-toxicity are always investigated seperatively as independent factors by programme applicant and other researchers. In this study, we will use Al-sensitive ‘Sour pummelo’ (C. grandis) and Al-insensitive ‘Xuegan’ (Citrus sinensis) seedlings as the materials and take the lead in investigating the effects of low pH and high Al interactions on growth, photosynthesis, Al and nutrient uptake; Al-induced secretion of root organic acid anions and organic acid metabolism in roots and leaves; reactive oxygen species and sulfur metabolisms in roots and leaves; callose content in roots and the contents of cell wall polysaccharides and their compositions in roots and leaves, and the transcriptome in roots and leaves revealed by RNA-Seq. The objectives of this study are to elucidate the molecular and physiological mechanisms of citrus in response to low pH and high Al interactions at physiological, biochemical, and transcriptomic levels, finially enhancing the tolerance of citrus plants to acidic soils, and provide scientific basis for citrus cultivation of high yield and good quality.

由于酸性土壤同时存在有酸毒（低pH）和铝毒，植物只有耐酸和耐铝同时具备时才能真正适应酸性土壤，目前少数有关pH对植物铝毒影响的研究多集中于草本植物，且结果不尽一致。柑橘为亚热带常绿木本果树，主产区多为酸性或强酸性土壤，易受低pH高铝的危害，因此，很有必要系统深入研究低pH高铝互作对柑橘的影响及其适应机制。据检索，申请人和其他研究者均把柑橘铝毒和酸毒分开来研究。为此，本项目拟以铝敏感的‘酸柚’（Citrus grandis）和不敏感的‘雪柑’（C. sinensis）实生苗为试材，率先研究低pH高铝互作对生长、光合、铝和营养吸收，铝诱导的根有机酸分泌和根叶有机酸代谢，根叶活性氧和硫代谢，根胼胝质含量和根叶细胞壁成分，以及基于RNA-Seq的根叶转录组的影响，旨在从生理生化和转录组水平明确柑橘响应低pH高铝互作的分子生理机制，最终提高柑橘适应酸性土壤的能力，为柑橘高产优质栽培提供科学依据。

本项目以‘雪柑’（Citrus sinensis）实生苗为试材，率先研究了低pH高铝互作对生长、光合、铝和营养吸收，铝诱导的根有机酸分泌和根叶有机酸代谢，根叶活性氧和硫代谢、胼胝质含量和细胞壁成分，以及基于RNA-Seq的转录组的影响，旨在从生理生化和转录组水平明确柑橘响应低pH高铝互作的分子生理机制。主要结果如下：.a) 将营养液pH从2.5增加到4.0可减轻铝毒对生物量，叶光合作用，根叶细胞壁和有机酸代谢，根茎叶元素吸收，元素含量，以及元素在根茎叶中的分配等的影响；铝毒加剧了低pH对上述生理参数的影响。.b) pH升高可增加铝处理根中碳向有机酸代谢的流入，从而提高根有机酸代谢和苹果酸和柠檬酸含量以及根苹果酸和柠檬酸的分泌，因而降低根茎叶铝含量；pH升高也可增单株营养元素（即：N、P、K、Ca、Mg、S和B）吸收以及根茎叶中这些元素的含量。这可能是pH升高缓解铝毒导致的雪柑生长和光合受抑，提高雪柑耐铝性的原因。无铝毒下，低pH可增加雪柑实生苗根苹果酸和柠檬酸分泌，从而提高其耐酸性。.c) pH升高可提高铝毒叶和根的活性氧（ROS）和丙酮醛（MG）清除能力，使ROS和MG产生和清除保持平衡，从而保护它们免受氧化损伤。无铝毒下，低pH诱导的叶和/或根中参与ROS和MG清除的酶活性和含硫化合物的上调不能保护低pH处理的叶和根免受氧化伤害；铝毒下低pH诱导的氧化伤害更显著。.4) 本研究从pH 4.0 + 1 mM铝处理的根（P4AR）vs pH 4.0 + 0 mM铝处理的根（P4R）鉴定到的差异表达基因（DEGs）远多于从pH 3.0 + 1 mM铝处理的根（P3AR）vs pH 3.0 + 0 mM铝处理的根（P3R）鉴定到的DEGs，表明增加pH可提高根系对铝毒的代谢适应性。相反，本研究从pH 3.0 + 1 mM铝处理的叶（P3AL）vs pH 3.0 + 0 mM铝处理的叶（P3L）鉴定到的DEGs远多于从pH 4.0 + 1 mM铝处理的叶（P4AL）vs pH 4.0 + 0 mM铝处理的叶（P4L）鉴定到的DEGs，表明增加pH缓解了铝毒对基因表达的影响，这可能与增加pH减少了叶中铝积累有关。