Abstract
Fusarium fungi are soil-borne pathogens, and the pathological effects on plant photosystems remain unclear. This study aimed to deeply reveal pathological characterization in apple seedlings infected with Fusarium solani by investigating photosystems performance and interaction. Roots were immersed in conidial suspension for inoculation. Thereafter, prompt and delayed chlorophyll a fluorescence and modulated 820 nm reflection were simultaneously detected. After 30 days of infection, leaf relative water content and dry weight were remarkably decreased by 55.7 and 47.1%, suggesting that the infected seedlings were subjected to Fusarium-induced water deficit stress. PSI reaction center was more susceptible than PSII reaction center in infected seedlings due to greater decrease in the maximal photochemical efficiency of PSI than that of PSII, but PSI reaction center injury was aggravated slowly, as PSII injury could partly protect PSI by restricting electron donation. PSII donor and acceptor sides were also damaged after 20 days of infection, and the restricted electron donation induced PSII and PSI disconnection by blocking PSI re-reduction. In accordance with greater damage of PSI reaction center, PSI oxidation was also suppressed. Notably, significantly increased efficiency of electron transport from plastoquinone (PQ) to PSI acceptors (REo/ETo) after 20 days of infection suggested greater inhibition on PQ reduction than re-oxidation, and the protection for PSI acceptors might alleviate the reduction of electron transport efficiency beyond PQ upon damaged PSI reaction center. Lowered delayed fluorescence in microsecond domain verified PSII damage in infected seedlings, and elevated delayed fluorescence in sub-millisecond domain during PQ reduction process conformed to increased REo/ETo. In conclusion, F. solani infection depressed PSII and PSI performance and destroyed their coordination by inducing pathological wilting in apple seedlings. It may be a pathogenic mechanism of Fusarium to induce plant photosystems damage.
Highlights
Apple replant disease is a serious problem in major applegrowing regions in the world (Mazzola and Manici, 2012)
We aimed to explore whether F. solani could induce water deficit stress in apple seedlings, and investigate the responses of photosystem II (PSII) and photosystem I (PSI) performance and their interaction through simultaneously detecting prompt chlorophyll a fluorescence (PF), delayed chlorophyll a fluorescence (DF), and modulated nm reflection (MR)
Fusarium fungi can impede water transport through xylem by inducing vessel plugging and bring about pathological foliage wilting in plants (Wang et al, 2015)
Summary
Apple replant disease is a serious problem in major applegrowing regions in the world (Mazzola and Manici, 2012). In contrast to abiotic factors such as soil structure and nutrition, the primary origin inducing apple replant disease appears to be soil-borne pathogens, because this disease can be effectively prevented or alleviated by soil disinfection (Yim et al, 2013; Henfrey et al, 2015). Some fungi including Fusarium, Rhizoctonia, and Cylindrocarpon have been defined as the main soil-borne pathogens for apple replant disease (Tewoldemedhin et al, 2011a,b,c). Considerable Fusarium fungi exist in replanted apple soil around Bohai Bay in China, and apple seedlings exhibit great susceptibility to these pathogens (Yin et al, 2017). Yantai city is an important apple planting area around Bohai bay in China. Young apple trees will be in danger of replant disease in these reconstructed orchards. Replant disease with reduced apple growth and yield have already appeared in some reconstructed orchards in Yantai. Many Fusarium fungi were found in the soil of these orchards, and the most abundant species was Fusarium solani
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