Colonization process of Arabidopsis thaliana roots by a green fluorescent protein-tagged isolate of Verticillium dahliae

Colonization process of Arabidopsis thaliana roots by a green fluorescent protein-tagged isolate of Verticillium dahliae Dear Editor, Verticillium wilt, caused by the soil-borne fungus Verticillium dahlia Klep., poses a major threat to a broad host range of more than 400 plant species, including economically important cotton, and causes serious loss of lint yield in China, the USA and may other countries (Bell, 1992;Bejorano-Alcazar et al., 1995;Zhang et al., 2012).The colonization of cotton roots by V. dahliae in soil naturally leads to the colonization of vascular tissues in cotton (Garber and Houston, 1966;Gerik and Huisman, 1988).Recently, the development of confocal laser scanning microscopy (CLSM) has significantly increased our knowledge of the colonization process of V. dahliae on various plant roots (Eynck et al., 2007;Vallad and Subbarao, 2008;Prieto et al., 2009;Zhang et al., 2012).However, the comprehensive observation of the cotton living root colonization process by V. dahliae under CLSM is not likely to be practicable due to the large size of cotton roots.In this study, to better understand the colonization process of V592, which was isolated from cotton in Xinjiang, China, we first inoculated cotton and Arabidopsis with GFPlabeled V592 (V592-GFP1) for an infectivity test in which it was compared with its wild-type isolate, V592.For each isolate, twelve cotton and Arabidopsis seedlings were inoculated with conidia from V592-GFP1 and V592 by unimpaired root dip-inoculation (Supplemental material).Leaf wilt was visually apparent on the cotton leaves at two weeks post-inoculation (wpi), and the whole leaf was dried out, was epinastic and was scorched with stem vascular displaying discoloration (Fig. 1B) by 3 wpi without an obvious difference between the V592-GFP1 and V592 infection (data not shown).Similar disease severity on Arabidopsis caused by V592-GFP1 and V592 was also observed (Fig. 1C).Leaf wilt was observed at 10 days post-inoculation, and the whole leaf was scorched by approximately 3 wpi.These results revealed that the pathogenicity caused by V592-GFP1 did not significantly differ from the wild-type isolate.Given that its pathogenicity was unaffected, V592-GFP1 was used to monitor the infection processes of V. dahliae.CLSM observation started at 6 h after inoculation (hpi).The root surface covered by the conidia at random positions was observed on either the main or lateral roots of both the cotton