Colonization of Resistant and Susceptible Lettuce Cultivars by a Green Fluorescent Protein-Tagged Isolate of Verticillium dahliae

Abstract

Interactions between lettuce and a green fluorescent protein (GFP)-expressing, race 1 isolate of Verticillium dahliae, were studied to determine infection and colonization of lettuce cultivars resistant and susceptible to Verticillium wilt. The roots of lettuce seedlings were inoculated with a conidial suspension of the GFP-expressing isolate. Colonization was studied with the aid of laser scanning confocal and epi-fluorescence microscopes. Few differences in the initial infection and colonization of lateral roots were observed between resistant and susceptible cultivars. Hyphal colonies formed on root tips and within the root elongation zones by 5 days, leading to the colonization of cortical tissues and penetration of vascular elements regardless of the lettuce cultivar by 2 weeks. By 8 to 10 weeks after inoculation, vascular discoloration developed within the taproot and crown regions of susceptible cultivars well in advance of V. dahliae colonization. Actual foliar wilt coincided with the colonization of the taproot and crown areas and the eruption of mycelia into surrounding cortical tissues. Advance colonization of stems, pedicels, and inflorescence, including developing capitula and mature achenes was observed. Seedborne infection was limited to the maternal tissues of the achene, including the pappus, pericarp, integument, and endosperm; but the embryo was never compromised. Resistant lettuce cultivars remained free of disease symptoms. Furthermore, V. dahliae colonization never progressed beyond infected lateral roots of resistant cultivars. Results indicated that resistance in lettuce may lie with the plant's ability to shed infected lateral roots or to inhibit the systemic progress of the fungus through vascular tissues into the taproot.

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

“…Only a few hyphae could extend in a directed manner toward the xylem vessels where the rapid proliferation of the hyphae took place. These observations, together with the intercellular but not intracellular colonization, were consistent with the studies of V. dahliae on oilseed rape, lettuce and cotton roots (Eynck et al, 2007;Vallad and Subbarao, 2008;Zhang et al, 2012), which suggested that plant roots were able to impede the progress of V. dahliae hyphae within the cortical tissue either due to intercellular space blockage, active cellular defenses or a combination of both processes. Once the hyphae xylem vessel infection was established, it triggered a typical Verticillium resistance in the above-ground plant organs, even though it ultimately caused wilt disease as reported in a recent study (Yadeta et al, 2011).…”

“…Once the hyphae xylem vessel infection was established, it triggered a typical Verticillium resistance in the above-ground plant organs, even though it ultimately caused wilt disease as reported in a recent study (Yadeta et al, 2011). In contrast with V. dahliae infection on lettuce roots, where the tips of the lateral roots were the first to invade vascular tissues (Vallad and Subbarao, 2008), we observed that colonies established on Arabidopsis and cotton root hair zones appeared to be the most successful, as they were the first to germinate, display hyphal growth and invade vascular tissues. Nevertheless, despite massive hyphae encompassing the root surface, only a few hyphae successfully invaded the root internal tissues.…”

“…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.…”

“…This hampered a comprehensive analysis of each stage of colonization and infection at one particular time point. Vallad and Subbarao (2008) reported the presence of distinct appressoria during the penetration of lettuce roots by V. dahliae. However, no appressoria were observed in a recent study of V. dahliae on cotton roots (Zhang et al, 2012).…”

Proteomics-based analysis reveals that Verticillium dahliae toxin induces cell death by modifying the synthesis of host proteins

“…Verticillium dahliae generally enters the cotton plant either by penetrating roots or via wounds in root tissues (Vallad and Subbarao 2008). After penetrating the cortex of the root, the fungus passes through the endodermis, invades the xylem vessels, eventually reaching the aerial parts of the plant (Tzima et al 2011).…”