Similar to what has been observed in other plant species, supplying rice leaves with G/GO resulted in the sustained production of H2O2within the apoplast , whereas a mixture of xanthine and xanthine oxidase was found to generate both superoxide and H2O2, the latter by dismutation. Treatment with either compoundor with the enzymes alone had no significant effect on disease development compared to buffer-treated control leaves . However, infiltration of G/GO or X/XO dramatically reduced the size of the necrotic lesions incited by M. oryzae infection . By contrast, pre-treatment with G/GO or X/XO mixtures strongly stimulated necrosis induced by R. solani . By 60 hours after infection, the majority of ROS-treated and Rhizoctonia-inoculated leaves showed extensive necrosis and were almost completely deteriorated . Enhanced ROS generation also greatly enhanced lesion formation by C. miyabeanus, suggesting a common pathogenicity mechanism for both these necrotrophs .Although exogenous catalase did not significantly alter lesion development, infiltration of rice leaves with a specific catalase inhibitor, 3-aminotriazole,grow table hydroponic prior to inoculation, was indistinguishable from the G/GO- or X/XO-treated leaves. No lesions were detected in leaves infiltrated with ROS-producing mixtures, catalase or 3-AT alone, as previously reported.
Building on our earlier work with respect to 7NSK2-mediated ISR, we sought to extend our analysis of the proposed dual role of ROS in rice defense by feeding the pro-oxidative pigment pyocyanin to hydroponically grown rice plants and observe any effects on plant resistance. Opposite to the enhanced resistance observed against M. oryzae, pyocyanin feeding favored subsequent infection by both C. miyabeanus and R. solani . Amending the pyocyanin solution with ascorbate, which has long been recognized as a major antioxidant buffer and free-radical scavenger, severely attenuated the pyocyanin-provoked resistance or susceptibility, corroborating our previous findings. Taken together, these results clearly demonstrate that enhanced ROS levels in inoculated leaves positively influence resistance to M. oryzae, while exerting a negative effect on rice defense to C. miyabeanus and R. solani.Despite the emergence of rice as a pivotal model for molecular genetic studies of disease resistance in cereal crops, molecular information regarding chemically and biologically induced defenses is still largely missing. In an effort to broaden our understanding of the rice induced resistance machinery, we analyzed the host defense responses underpinning ISR triggered by the biocontrol agent S. plymuthica IC1270. The results presented in this study demonstrate that root colonization by IC1270 predisposes rice to undergo a massive oxidative burst and related HR-like cell death at sites of attempted pathogen invasion, a process culminating in heightened resistance to the hemibiotrophic blast pathogen, M. oryzae.
The same treatment, however, rendered plants more susceptible to attack by the necrotrophic pathogens R. solani and C. miyabeanus. Besides tagging ROS and HR-like cell death as two-faced players in the rice defense response, these findings strengthen the argument that rice requires distinct mechanisms for defense against M. oryzae and the necrotrophs R. solani and C. miyabeanus. Mounting evidence indicates that generation of systemic resistance does not necessarily require direct activation of defense mechanisms, but can also result from a faster and stronger activation of basal defenses in response to pathogen attack. For instance, unlike pathogen-induced SAR, classic rhizobacteria-mediated ISR in Arabidopsis is not associated with a direct induction of defense mechanisms, but with priming for augmented defense activation upon challenge inoculation. Other ISR-inducing PGPRs also have been found to enhance the plant’s defensive capacity by hyper-activating pathogen-activated defenses, suggesting that priming for enhanced defense is a common mechanism in PGPR-mediated ISR. The results presented in this study add further support to this concept as root colonization by IC1270 did not cause a strong constitutive resistance phenotype, but ratherprimed plants to hyper-respond to subsequently inoculated pathogens, resulting in excessive defense activation and enhanced resistance to M. oryzae.
This priming effect of IC1270 was borne out by the observation that challenge inoculation of IC1270-colonized plants with M. oryzae entailed a rapid accumulation of autofluorogenic phenolic compounds in and around epidermal cells displaying dense cytoplasmic granulation , two features that are considered as hallmarks of an ETIassociated HR. Comparative profiling of pathogenesis-related H2O2 accumulation in blast susceptible, yet ISR-expressing, and genetically resistant leaf sheath cells, further strengthened the parallels between R protein-mediated ETI and IC1270- triggered ISR priming . Hence, IC1270 appears to protect rice from M. oryzae by reprogramming pathogen attacked epidermal cells to undergo a rapid HR-like response, thereby providing a possible functional interface between rhizobacteria-mediated ISR and avirulent pathogen-induced ETI. Such mechanistic similarities between ISR and ETI are compatible with the idea that defense signals from multiple ‘entry points’ can converge and target overlapping sets of defense effectors. Particularly relevant in this regard is the substantial overlap between gene expression changes and alterations in SA content induced during an avirulent pathogen-triggered ETI response, and those induced by treatment with flg22, an 22-amino-acid epitope of the archetypal MAMP elicitor flagellin. Although unequivocal evidence is still lacking, the striking homologies with the sensitive perception mechanisms for pathogen-derived MAMPs that function in PTI suggest that ISR-triggering rhizobacteria are recognized in a similar manner. In this perspective, it is not inconceivable that the mechanistic parallels between IC1270-mediated ISR and ETI can be traced to converging MAMP- and R-protein-induced defense responses. Furthermore, consistent with the view of ETI as an accelerated and amplified PTI response, such MAMP-orchestrated ISR elicitation may also explain the partial nature of the IC1270-induced resistance against M. oryzae. Apart from S. plymuthica IC1270, several other biological and chemical agents have been shown to be capable of inducing resistance to M. oryzae, among which the SA analog BTH and the redox-active pigment pyocyanin, key determinant of ISR induced by P. aeruginosa 7NSK2. Interestingly, both these resistance inducers appear to mimic IC1270 in that they produce a similar resistance phenotype, characterized by hypersensitively dying cells in the vicinity of fungal hyphae. Although it does not follow that the signaling conduit governing IC1270-mediated ISR is necessarily the same as that leading to pyocyanin- or BTH-inducible blast resistance, such commonalities apparent at the level of defense mobilization suggest that these elicitors may feed into related, if not identical, resistance pathways. Further supporting this hypothesis is the overlap manifest at the level of resistance to attackers, with IC1270, BTH and pyocyanin all being ineffective or even increasing vulnerability to C. miyabeanus and R. solani. Intriguingly, induction of ISR by the PGPR strain P. fluorescens WCS374r appears to rely on a different resistance strategy and was found to be associated with priming for a diverse set of HR-independent cellular defenses, the prompt elaboration of invading hyphaeembedding tubules being a prominent component. Considering this apparent plasticity in the molecular processes leading to induced resistance against M. oryzae, it is tempting to speculate that rice is endowed with multiple blast-effective induced resistance pathways.
The rapid production of ROS during the so-called oxidative burst is a hallmark of the plant’s defense response. Although ROS are generally viewed as initiating agents in the disease resistance network ,grow table accumulating evidence indicates that ROS formation can cascade either to the detriment or benefit of the plant depending on the lifestyle and parasitic habits of the invading pathogen. Hence, ROS can play a dual role in pathogen defense, acting as key players in resistance to biotrophic pathogens on the one hand, while weakening necrotroph resistance by assisting pathogen-induced host cell death on the other. Taking these facts into account, we propose that priming for enhanced ROS generation may likewise function in IC1270-mediated ISR, thereby accounting for the differential effectiveness of this resistance against hemibiotrophic and necrotrophic pathogen assault. Critical to the formation of a hypothesis of primed ROS generation as a key event in ISR by IC1270 was the observation that artificially increased H2O2 levels, either resulting from infiltration of ROS-generating mixtures, inhibition of endogenous catalase activity or hydroponic feeding of pro-oxidative pyocyanin, faithfully mimicked IC1270 in conditioning resistance to M. oryzae but susceptibility to C. miyabeanus and R. solani. Although we are aware that final proof for primed ROS generation as the causal resistance mechanism underpinning IC1270-mediated ISR requires the use of inhibitor compounds able to abrogate the oxidative burst , such scavenger experiments could not be performed since detached leaves, needed for effective infiltration of chemicals in rice, somehow failed to develop ISR. Therefore, we can not rule out the possibility that the altered pathogen response of IC1270-induced plants may result in part from ROS-independent processes. Nonetheless, the involvement of boosted ROS generation in the establishment of IC1270-mediated ISR is apparent. In accordance with previous studies, continuous generation of H2O2 in situ by infiltration of G/GO or 3-AT did not induce any detectable cell death per se, indicating that additional pathogen-induced signals are needed for expression of HR-like cell death. Indeed, current concepts suggest that death of host cells during the HR requires the poised production of nitric oxideand ROS, coupled to simultaneous suppression of the plant’s antioxidant machinery. In view of these data, it could be reasoned that IC1270-mediated priming for potentiated ROS generation might lower the threshold for activation of programmed cell death, thereby blocking the hemibiotroph M. oryzae in its initial biotrophic phase. In line with this concept, there is ample evidence demonstrating that early-produced H2O2 is a central signal leading to the elicitation of a wide range of blast-effective defenses, among which programmed cell death. Most tellingly, Kachroo and associates [84] reported a fungal glucose oxidase gene to sequentially induce H2O2 generation, rapid HR-like cell death and enhanced resistance against M. oryzae when ectopically expressed in young rice plants. On the other hand, it is not inconceivable that IC1270-mediated priming for H2O2 may tilt the ROS-controlled cellular life-or death balance toward death, thereby facilitating subsequent tissue colonization by the necrotrophs R. solani and C. miyabeanus.
This notion is corroborated by recent observations demonstrating that IC1270 pretreatment has no marked impact on the early infection events in C. miyabeanus– or R. solani-challenged plants except for a substantial increase in the number of dying cells preceding the fungal growth front . However, given the myriad defense related plant responses modulated by ROS, other yet unidentified mechanisms also may play a role.Parasitic plants directly invade and rob nutrients from host plants. The consequences can be devastating to the host plant and some of the world’s most pernicious agricultural pests are parasitic weeds. The number of parasitic angiosperms is surprisingly large with over four thousand parasitic species identified in nineteen different plant families. Parasitic plants have a wide diversity of growth habits ranging from the tiny flowered mistletoes that live in the tops of trees to the enormously flowered and rootless Rafflesia whose entire vegetative body is endophytic. The degree to which parasites rely on hostresources also varies. Some obligate parasites, like Rafflesia, have lost photosynthetic capabilities and are fully heterotrophic. Others, like Triphysaria, are facultative parasites that can mature without a host plant but will parasitize neighboring plants when available. The single feature shared by all parasitic plants is the ability to invade host tissues via a haustorium. Haustoria of parasitic plants fulfill multiple functions including host attachment, penetration, and translocation of resources from host to parasite. Interestingly, the competence to develop haustoria has originated in autotrophic ancestors multiple times during the evolution of angiosperms. There are two general hypotheses for the evolutionary origins of haustoria. One hypothesis suggests that the genes encoding haustorium development are derived from nonplant organisms, such as bacteria or fungi, that are endophytic or which have transferred a set of genes required for haustorium formation into the parasite genome. The second is that genes encoding haustorium developmentare derived from those present in autotrophic angiosperms where they fulfill functions unrelated to parasitism. The identification of genes associated with haustorium development will provide insights into the evolutionary origins of plant parasitism. These genes will also elucidate the degree to which haustoria in different parasitic families are encoded by convergent or homologous genetic pathways. Parasitic plants in the Orobanchaceae develop haustoria on their roots in response to contact with host roots. Several molecules, typically products of the phenylpropanoid pathway, have been identified that induce haustorium development when applied to Orobanchaceae roots in vitro.