Abstract
Zoospore exudates play important roles in promoting zoospore communication, homing and germination during plant infection by Phytophthora. However, it is not clear whether exudates affect plant immunity. Zoospore-free fluid (ZFF) and zoospores of P. nicotianae were investigated comparatively for effects on resistance of Arabidopsis thaliana Col-0 and mutants that affect signaling mediated by salicylic acid (SA) and jasmonic acid (JA): eds16 (enhanced disease susceptibility16), pad4 (phytoalexin deficient4), and npr1 (nonexpressor of pathogenesis-related genes1). Col-0 attracted more zoospores and had severe tissue damage when flooded with a zoospore suspension in ZFF. Mutants treated with ZFF alone developed disease symptoms similar to those inoculated with zoospores and requirements of EDS16 and PAD4 for plant responses to zoospores and the exudates was apparent. Zoospore and ZFFs also induced expression of the PR1 and PDF1.2 marker genes for defense regulated by SA and JA, respectively. However, ZFF affected more JA defense signaling, down regulating PR1 when SA signaling or synthesis is deficient, which may be responsible for Arabidopsis mutant plants more susceptible to infection by high concentration of P. nicotianae zoospores. These results suggest that zoospore exudates can function as virulence factors and inducers of plant immune responses during plant infection by Phytophthora.
Highlights
Phytophthora species use a variety of strategies to infect plants, causing severe losses for food crops and ornamental plant production, as well as damage in forest ecosystems [1]
sterile distilled water (SDW)-treated leaves/sustained little damage in trichomes or leaf tissue and supported limited hyphal growth with no sporangia production (Fig 1B, 1D, 1F and 1H). These results indicate that Zoospore-free fluid (ZFF) promoted susceptibility of Arabidopsis plants to P. nicotianae
Zoospore exudates of Phytophthora have been shown as chemical signals for Phytophthora quorum sensing leading to zoospore aggregation, homing and infection initiation as well as communications between zoosporic oomycetes and with bacteria [3, 5, 7, 39, 42]
Summary
Phytophthora species use a variety of strategies to infect plants, causing severe losses for food crops and ornamental plant production, as well as damage in forest ecosystems [1]. P. parasitica) can colonize foliar structures and roots of over 200 plant species including Arabidopsis [1,2,3,4] This pathogen can use exudates from zoospores and bacteria to promote successful infection [3, 5, 6]. The exudates from zoospores of Phytophthora and Pythium, termed zoospore-free fluids (ZFF), function as quorum-sensing signals promoting zoospore homing and triggering infection by single zoospores [3, 5, 7]. It is not clear whether ZFF affects plant immunity
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