Drought-induced anatomical changes in roots and hypocotyls of Westar summer rape Brassica napus L. plants reduce Verticillium longisporum disease severity.

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conclusionSevere drought stress reduces the susceptibility of B. napus plants to Verticillium longisporum infection. Plants often face combined abiotic and biotic challenges impairing growth and productivity. The effects of water stress on infectivity of Verticillium longisporum, a host-specific vascular fungus, were investigated in the susceptible summer rape (Brassica napus L.) cultivar Westar. Plants were root inoculated with V. longisporum and subsequently grown under control water conditions (field capacity, FC), or under moderate (60% FC) or severe (30% FC) water stress. Severe, but not moderate, water stress decreased V. longisporum disease symptoms and delayed the acropetal progression of the fungus from the roots to the leaves. These effects were associated with the formation of apoplastic barriers which included localized thickening of cell walls and suberinization of the epidermis and endodermis in the roots, as well as precocious development of a suberized peridermal layer in the hypocotyl. Microscopic visualization of Verticillium colonization revealed that these barriers hindered the progression of the fungus within the host tissues. Unlike FC conditions, where the fungus advanced uniformly through root and hypocotyl tissues before invading the vasculature and occluding vessel elements, colonization of 30% FC plants was delayed by the suberized epidermis, endodermis, and periderm. Consequently, access of the pathogen to the xylem and its spread to the above-ground organs were limited. The importance of suberin during V. longisporum colonization was further tested in transgenic Arabidopsis lines with varying levels of root suberin. Disease symptoms and presence of fungal DNA in shoots were generally exacerbated in suberin-deficient lines and mitigated in those over-accumulating suberin. Collectively these results suggest that severe water stress reduces disease symptoms in B. napus plants inoculated with V. longisporum, and that formation of suberized barriers might be an adaptive strategy to limit disease progression.