連作對結球甘藍(Brassica oleracea var. capitata)根部可培養型內生細菌之影響及其功能性研究

Abstract

Continuous cropping obstacle (CCO) is the most common bottleneck that affects crop yield in agricultural plantation. Several important vegetable and melon crops in Taiwan are highly susceptible to CCO, such as cabbage (Brassica oleracea var. capitata), Brassica rapa chinensis, pepper (Capsicum annuum), tomato (Lycopersicon esculentum Mill.), muskmelon (Cucumis melo L.) etc. The annual production of the highly demanding Cruciferae vegetable Brassica oleracea var. capitata is around 320 to 340 thousand metric tons and accounts for 11-12% of the total foliage crops in Taiwan, yet its production in particular is highly hampered by CCO. To characterize CCO effects on the endophytic microflora in cabbage root tissues, three consecutive harvests from continuous cropping (each harvest period of about 50 to 58 days) on a farmland in Yunlin County, Siluo district was performed. Root samples were harvested and culturable endophytic bacteria were isolated and identified. The bacterial colony-forming unit (CFU) in cabbage root tissues were 2.3x104 (CFU/g), and among them, 310 bacterial strains were isolated for 16S ribosomal RNA gene amplification and sequence analysis. A total of 40 different bacterial species were identified, 2 strains of Actinobacteria, 2 strains of Bacteroidetes ,10 strains of the Firmicutes, 3 strains of the α-Proteobacteria, 1 strains of the β-Proteobacteria and 22 strains of the γ-Proteobacteria. A comparison of cabbage root endophytic bacteria revealed that continuous cropping increased gram-negative bacteria numbers (55% to 75%) while gram-positive bacteria decreased relatively (45% to 25%). Previous studies indicated that plant endophytes confer plant growth promoting substances (PGPS) and disease resistance activities, such as the production of IAA (indole-3-acetic acid), siderophores, and the ability to produce degrading enzymes (protease, cellulase, amylase) which could be applied as agricultural biotechnological products. We have tested the 40 different endophytic bacterial strains for the above-mentioned functions as well as their antagonistic effect on five common crop fungal pathogens (Sclerotium rolfsii Sacc., Colletotrichum higginsianum Sacc., Rhizoctonia solani Kuhn, Alternaria brassicae and Phytophthora spp.). The results showed 21 strains with proteolytic ability, 9 strains with starch degrading ability, 5 strains with cellulose degrading ability, 7 strains with phosphate solubilization ability, 26 strains with siderophore producing and 30 strains with IAA producing capabilities. Seven strains showed antagonistic activities against phytopathogenic fungi, in which LS0171, LS0156 and LS0363 strains exhibited antagonistic effects on three different phytopathogenic fungi. It is worth mentioning that LS0363 strain has the ability to synthesis siderophores, IAA and also antagonize three different phytopathogenic fungi. We have selected individual endophytic isolates that were classified as Biosafety Level 1 and possessed multiple PGP promoting activities for their functional studies by inoculating on rice seeds. Inoculation of endophytes in rice showed that LS0161 and LS0126 strains could increase the length of rice roots and LS0183 strain could increase the rice fresh weight. In addition, LS0388, LS0410 and LS0258 strains could increase chlorophyll a content of rice leaf. This study is expected to develop novel environmental friendly biological agents for their PGP and disease resistance activities in attempt to reduce or substitute the use of chemical fertilizers and to reduce environmental pollution.