豚草种带内生真菌及其对种子发芽和幼苗生长的影响

Effects of endophytic fungi from common ragweed on seed germination and seedling development of its host plant

内生真菌是一类共生于植物体内,能够不同程度影响宿主植物生态适应性和竞争能力的微生物。分析内生真菌在豚草种子中的分布、种群结构,以及内生真菌发酵液对种子发芽和幼苗生长的作用。结果显示:发生于6个地区的豚草种子均能分离获得内生真菌,分离率在19%—92.63%之间,不同地区之间差异极显著(P<0.01)。内生真菌主要存在于种子的总苞部位,分离率达到65.52%。发生于福建省长乐市松下镇的豚草种带内生真菌种群包含5个属,以链格孢属(Alternaria)真菌为优势菌群,占82.26%;其次为镰孢属(Fusarium)真菌,占9.68%;其它3个属的真菌发生较少,均低于5%。内生真菌主要以水平传播方式在豚草不同世代之间传播。供试的7个内生真菌菌株的发酵液均不同程度地抑制豚草种子发芽,降低幼苗地上部高度、根长度、根数量和总生物量,但不同菌株发酵液之间抑制程度差异明显,显示不同菌株对豚草种子发芽和幼苗生长产生不同的影响。内生真菌发酵液处理后的种子仍然保持较高程度的活力;不同内生真菌发酵液处理后,有活力的种子维持在50%—87.5%之间,均高于(或等于)清水处理的种子,说明内生真菌代谢产物只是抑制种子的发芽,但并不导致种子的腐烂和死亡。这些研究结果初步显示种子携带的内生真菌可能在豚草入侵生物学中发挥重要的作用。

Endophytic fungi are microbes that live asymptomatically inside plant tissues. Accumulating research studies suggest that endophytic fungi have a fundamental influence on the fitness and competitiveness of their host plants. Here, we surveyed the species diversity and community structure of endophytic fungi associated with the seeds of common ragweed （Ambrosia artemisiifolia L. ）, which is a notorious invasive weed belonging to the Asteraceae family in China. The main objective was to document the distribution of the fungal symbionts and to determine at what point endophytic fungi affect seed germination and seedling development in common ragweed. Endophytic fungi were found in all common ragweed populations collected from six study locations in China, with fungal isolation rates ranging from 19% to 92.63%. Yet, the isolation rates of endophytic fungi were significantly different （P 〈 0.01 ） among all six locations, with the highest isolation rate being obtained from seeds collected from Langfang City, Hebei Province. The endophytes in common ragweed seeds were predominantly isolated from involucre, with the isolation rate reaching 65.52%. The fungal isolation rates were 21.74% for seeds and 18.18% for bracks, respectively. Five fungal genera were detected in common ragweed seeds collected from Changle City, Fujian Province. The most frequent genera of the endophytic community were Altemaria and Fusarium, which accounted for 82.26% and 9.68% of isolates, respectively. Three genera were indicated as rare endophytes in common ragweed seeds, none of which exceeded 5% of isolates. Our data indicate that endophytes from common ragweed seeds are horizontally transmitted to the next generation, because no endophytic fungi were detected in newly developed seedlings. After determining the fungal genera and their relative abundance, an experiment was conducted with fermentation broths from seven representative isolates of common endophytic fungi that are present in A. artemisiifolia seeds. The germination of common ragweed seeds and subsequent seedling development were inhibited when seeds were treated with fermentation broths from seven endophytic fungal isolates. The germination of common ragweed seeds noticeably declined （P 〈 0.01 ） when treated with fermentation broths of the seven selected fungal isolates. Fermentation broths of strain AM-17 and AM-8 had the highest inhibition effects on seed germination. The germination percentages of seeds treated with strain AM-17 and AM-8 fermentation broths were 14.92% and 17.11%, respectively, whereas the germination percentage of seeds treated with water （control） reached 86.52%. The experiments also showed that the growth of common ragweed seedlings is inhibited by treatment with fungal fermentation broths. Four out of seven fungal fermentation broths significantly （ P 〈 0.01 ） reduced the aboveground stem length of common ragweed seedlings when compared to the water treatment （control）. Strains AM-8 and AM-17 inhibited seedling growth the greatest. Six of seven fungal fermentation broths significantly （P 〈 0.01 ） reduced the root length of seedlings, while all fungal fermentation broths significantly （P 〈 0.01 ） reduced lateral root number. In striking contrast, lateral roots failed to develop when seedlings were treated with the fermentation broth of strains AM-17 and AM-8. Five out of the fermentation broths significantly （P 〈 0.01 ） reduced the biomass of common ragweed seedlings. Seventy-seven percent of seedlings were dead after treatment with the fermentation broth of strain AM-17 for 5 days. Seed survival was not severely （P 〉 0.05） affected by fungal fermentation broth treatments. Seed viability analysis indicated that most seeds （50%--87.5%） were viable after the various fungal fermentation broth treatments. Our data demonstrate that metabolites produced in fungal fermentation broths do not facilitate seed germination, but did not kill the seeds.