氮调控甘蔗锰毒和抗性的机制研究

Excessive manganese-induced chlorosis in sugarcane plantlets grown on acid soils is a serious problem limiting the production of cane sugar in our country and threatening to sugar safety of the country. In preliminary works, we found that ammonium-form nitrogen alleviated significantly excessive manganese toxicity to sugarcane and confer resistance to manganese in sugarcane, while corresponding mechanism has not been documented. To elucidate the mechanisms for nitrogen form impacts on manganese toxicity and tolerance in sugarcane and explore a way to prevent and control manganese-induced chlorosis in sugarcane by nitrogen nutrition management, we will carry following study including:(1) regulation of manganese toxicity to plant and sugarcane manganese acquisition by various nitrogen sources; (2)impact of nitrogen forms on composition of polysaccharides in the cell wall of sugarcane, polysaccharides molecular modification and manganese immobilization in the cell walls; (3) the effects of various nitrogen sources on internal detoxifying to manganese and coordinated detoxifying in symplast and apoplast. These studies will provide the basis for elucidating the mechanisms for manganese resistance in sugarcane, which is the most important to resolve manganese-induced chlorosis in sugarcane grown on acid soils and safeguard sustainable development of our country's sugar industry.

酸性土壤上过多的锰引起的甘蔗幼苗黄化是限制我国蔗糖生产、威胁我国食糖安全的重大问题。我们在前期的工作中发现，铵态氮能有效减轻甘蔗锰毒害提高甘蔗锰毒抗性，而相关机制国内外尚未见报道。为揭示氮素形态对甘蔗锰毒及抗性的影响机制、探索氮素营养管理防控锰引起的甘蔗幼苗黄化病的途径，本项目拟开展以下三方面研究：（1）不同形态的氮对甘蔗锰毒害及锰吸收的调控；（2）不同形态的氮对细胞壁多糖组成、结构修饰及锰固持的影响；（3）不同形态的氮在锰内部解毒效应及共质体-质外体协同解毒上的作用。本研究将为甘蔗锰毒抗性机制的阐明提供科学依据，对于锰引起的甘蔗幼苗黄化问题的解决及我国蔗糖产业的可持续发展意义重大。

在此前的研究已报道甘蔗幼苗锰毒黄化问题是制约我国蔗糖生产的重大问题。本研究揭示了氮源对甘蔗幼苗锰毒黄化的影响及相关机制：（1）培养液中的铵态氮显著抑制过多的锰引起的甘蔗幼叶黄化，且在酸性土壤上施用铵态氮肥的甘蔗生长量显著高于施用硝态氮的甘蔗。（2）铵盐为氮源的甘蔗锰吸收量、植株锰含量和培养液的pH值显著降低，而培养的pH值从7.0降低至低pH值后甘蔗根尖锰含量随着pH降低而显著减少；相反，在酸碱缓冲体系中，根系锰含量在铵态氮和硝态氮源处理间的差异并不显著。此外，锰诱导的根系分泌的尼克酰胺，以铵态氮为氮源的甘蔗显著高于硝态氮源的植株，而介质中尼克酰胺抑制锰的吸收并减轻甘蔗锰毒。（3）以铵态氮为氮源的甘蔗根系细胞壁中锰的分配比重、果胶中锰含量及果胶中糖醛酸含量显著增加；并且，铵态氮处理后根尖的葡萄醛酸脱羧酶、果胶脂酶、膨胀素等果胶及细胞壁代谢相关基因表达显著上调。另一方面，铵态氮为氮源的植株，锰引起的NO积累、细胞壁果胶去甲酯化程度及细胞壁对锰的吸附量显著低于以硝酸盐为氮源的处理；NO供体可逆转铵态氮对NO积累及细胞壁甲酯化，而NO清除剂及硝酸还原酶抑制剂则显著抑制硝酸盐对NO积累、果胶甲酯化及锰吸收的影响。（4）铵态氮处理显著增加锰在根系细胞汁液（液泡）中的分配而减少细胞壁中的分配；并且，根尖中Mn毒抗性相关的基因Nramp2、植物耐铜蛋白基因HMA5等基因明显上调；此外，铵态氮为氮源的植株，锰在根系及地上部间的转运量显著低于硝态氮为氮源的植株。这些结果说明，铵态氮源有利于减轻甘蔗锰毒、提高甘蔗锰毒抗性，其机制在于铵态氮源的植株酸化根际并分泌尼科酰胺减少锰的吸收，铵也通过NO信号等途径调控细胞壁对锰的固持和向地上部的运输能力并增加锰在液泡中的分配从而提高甘蔗锰毒黄化的抗性。相关结果在国内外未见报告，研究结果对甘蔗幼苗锰毒黄化问题的解决有重要意义。项目研究成果将陆续发表。