环剥调控荔枝菌根发育的分子生理机制

Girdling is an important measurement of controlling treetops and hastening flower blossom in litchi cultivation. However, the impact of girdling on the mycorrhizal roots of litchi is still unknown. It is important to dissect the molecular and physiological mechanism of girdling regulating mycorrhizal root development in litchi mycorrhizal root cultivation. Jasmonic acid, salicylic acid and ethylene are the main hormones of plant in response to wounding, and both the hormone and carbohydrate regulate plant mycorrhizal root development. We found that mycorrhizal colonization rate of litchi in field was 45% to 67%, and the root mycorrhizal colonization rate and the soluble sugar content decreased significantly after two weeks of girdling. The ‘Feizixiao/Huaizhi’ stock-scion litchi fruit tree will be used as material, and three treatments (wounding, girdling, and control) will be set up in this work. The regulatory network will be constructed by comparing the differences of transcriptome, hormone levels (jasmonic acid, salicylic acid and ethylene), carbohydrate contents (sucrose, glucose and fructose), and mycorrhizal colonization rate among three different treatment roots. Furthermore, the function of hormone induction (jasmonic acid, salicylic acid and ethylene) and carbohydrate starvation in the process of mycorrhizal root development regulated by girdling will be dissected. This work will clarify the molecular and physiological mechanism of girdling regulating mycorrhizal root development, which would provide more fundamental support in the improvement of mycorrhizal roots cultivation in girdling litchi trees.

环剥是荔枝控梢、促花的重要措施，但环剥对荔枝根系（菌根）的影响尚不清楚，明晰环剥调控荔枝菌根发育的分子生理机制对环剥结果树菌根培育有重要意义。环剥导致机械损伤和根系碳水化合物饥饿，茉莉酸、水杨酸及乙烯是植物响应机械损伤的主要激素，且其与根系碳水平对植物菌根发育产生影响。我们发现田间栽培荔枝根系菌根侵染率达到45%～67%。同时，螺旋环剥两周后菌根侵染率、根系可溶性糖含量显著下降。本项目拟以田间栽培‘妃子笑/怀枝’砧穗组合荔枝结果树为材料，实施纵向割伤、螺旋环剥和不环剥三种处理。通过比较各处理根样间转录组，内源茉莉酸、水杨酸、乙烯水平，蔗糖、葡萄糖、果糖含量以及菌根侵染率的差异，构建环剥调控荔枝菌根发育的基因网络，明晰应激激素诱导与碳饥饿在环剥调控荔枝菌根发育过程中的作用途径。此研究初步明晰环剥调控荔枝菌根发育的分子生理机制，为建立环剥荔枝树菌根培育的方法打下基础，具有理论与实践双重意义。

荔枝是木本菌根类果树，根外菌丝和根毛是其养分和水分吸收的重要器官。环剥是荔枝栽培上重要的控梢促花措施，但其对荔枝根外菌丝发育的影响尚未可知。为了理清环剥对菌根、根毛的影响，我们对一年生荔枝实生苗作了环剥、纵向割伤、无创伤对照三个处理。并对三个处理实生苗根系碳水化合物浓度，糖代谢酶活性，菌根侵染率进行了分析。结果显示：纵向割伤和无创伤对照两个处理根系中的碳水化合物能够供给荔枝菌根发育。环剥引起根系己糖饥饿，荔枝实生苗通过提高中性蔗糖转化酶活性，降低蔗糖磷酸酶与蔗糖合成酶（合成方向）活性调控菌根侵染水平。基于碳水化合物含量影响菌根发育的事实，试验测定了环剥和对照荔枝（结果树）菌根中碳水化合物浓度、菌根侵染率、转录组数据解析荔枝菌根耐受碳饥饿的分子机制。RNA-seq鉴定出荔枝源的54,100 unigenes与真菌源的33,120 unigenes在碳水化合物饥饿条件下得到转录。差异表达基因（DEGs）分析显示类黄酮类、α亚麻酸、萘乙酸是荔枝调控菌根发育侵染的主要功能物质。但是其功能存在分化，类黄酮类可能在碳水化合物降低的早期感知时期起作用，α亚麻酸、萘乙酸则在碳饥饿的适应期影响菌根的形成。荔枝通过刺激抗性基因（NADP依赖的苹果酸脱氢酶基因、内切几丁质酶基因PR4，基础抗性蛋白1基因，LRR受体激酶，过氧化物酶基因）的表达，下调共生信号转导基因（热激蛋白82、热激蛋白70kDa、HSP 90-beta-3、转录因子MYC4、蛋白磷酸酶2C24与调控蛋白NPR5）抑制新的共生结构形成。除了调控共生关系形成，碳水化合物饥饿条件下荔枝通过影响诸如AP2, ERF, Myb, WRKY, bHLH等转录因子及植物血凝素基因表达对菌根共生结构的维持发生作用。与荔枝一样，AM 真菌通过上调 E3泛素连接酶与泛素化酶感应碳水化合物的降低。此结果说明泛素化过程在AM真菌感知碳水化合物降低过程中起重要作用。同时，细胞色素b-245与亮氨酸重复蛋白基因大量存在于DEGs数据库中，此结果说明上述两类基因在真菌感知碳水化合物饥饿的过程中起重要作用。