5 the Sebacinoid Fungus Piriformospora Indica: an Orchid Mycorrhiza Which May Increase Host Plant Reproduction and Fitness
Total Page：16
File Type：pdf, Size：1020Kb
5 The Sebacinoid Fungus Piriformospora indica: an Orchid Mycorrhiza Which May Increase Host Plant Reproduction and Fitness PATRICK SCHÄFER1, KARL-HEINZ KOGEL1 CONTENTS Prokaryotic or eukaryotic organisms with the I. Introduction . 99 capability of colonising plants are generally called II. The Mycorrhizal Order Sebacinales . 100 endophytes. An endophytic lifestyle was reported III. Piriformospora indica – an Orchid among fungi, bacteria, algae, plants and even insects Mycorrhizal Fungus? . 101 (Schulz and Boyle 2005). This broad defintion of IV. Benefits of P. indica Symbiosis endophytism was later specified to more strongly for Host Plants. 103 V. Cell Death Makes a Difference . 104 emphasise infection strategies or the physiological VI. Parasitic Associations of Plants with P. indica . 105 character of interaction types. However, due to the VII. Factors Involved in Plant Colonisation broad spectrum of endophytes and their flexibility by P. indica . 106 (phenotypic plasticity) in host colonisation, along VIII. Impact of Various Plant Mutations with their ability to adapt to environmental factors on P. indica-Induced Resistance . 108 IX. Bacterial Endosymbiotic Associations and the host’s physiological status, a more restric- Within Sebacinales. 108 tive general definition does not exist. Focusing X. Conclusions . 110 on fungal microbes, endophytes were defined as References. 110 organisms that grow in living plant tissue during their entire life cycle (or a significant part of it) without causing disease symptoms (Petrini 1991; I. Introduction Saikkonen et al. 1998; Brundrett 2004). Schulz and Boyle (2005) broadened this definition by Plants are potential targets (hosts) for a broad describing endophytes as plant inhabitants that spectrum of microbial organisms. The outcome of have not yet triggered disease symptoms in plants these associations can be roughly categorised into at the time of detection. This definition excludes mutualistic, commensalistic or pathogenic rela- the impact of endophytes on host fitness at later tionships. Interactions with certain mutualistic interaction stages; depending on their lifestyle in fungal microbes can benefit plants, resulting for plants or impact on host fitness such fungal endo- example in an improved plant development even phytes range under this definition from mutual- under unfavourable environmental conditions istic to pathogenic microbes (Redman et al. 2001; (Chap. 15). Simultaneously, the microbial part- Schulz and Boyle 2005). In order to simplify this ners acquire nutrients from the host and can be heterogeneity, we follow a rather restricted defini- protected from environmental stress or competi- tion of endophytes encompassing microbes with tors (Schulz and Boyle 2005). In other cases it is an asymptomatic lifestyle throughout their inter- the microbes that primarily profit from the asso- action with plants. The intention of this definition ciation, with the host fitness being either appar- is to address those fungi whose association and ently unaffected (commensalism) or thoroughly reproduction in plants cause neutral or beneficial impaired (pathogenesis; Redman et al. 2001). rather than detrimental effects in their hosts. Described in a broad sense, mycorrhizas are highly specialised beneficial associations between plant roots and fungi based on the bilateral 1 Interdisciplinary Research Centre for BioSystems, exchange of nutrients, defence against pathogens Land Use and Nutrition. Institute of Phytopathology and Applied and abiotic stress or an improved water balance. Zoology, Justus Liebig University, Heinrich-Buff-Ring 26–32, 35392 Giessen, Germany. Variations in the benefits for each symbiotic part- e-mail: [email protected], ner gave rise to the terms balanced and exploitive e-mail: [email protected] mycorrhizas. Whereas in the former both partners Plant Relationships, 2nd Edition The Mycota V H. Deising (Ed.) © Springer-Verlag Berlin Heidelberg 2009 100 Patrick Schäfer and Karl-Heinz Kogel benefit equally from each other, the latter type of fungal endophyte rather than a representative of interaction favours the plant partner. Due to their the mycorrhizal fungi. In this chapter we discuss beneficial potential for plants, mycorrhizal fungi current results showing beneficial associations of are among the best-characterised fungal symbionts P. indica with plants, especially emphasising its life (Chaps. 13, 14). According to the above definition, strategies in host plants. Intriguingly, it has been mycorrhizal fungi would be considered as endo- shown that root colonisation by P. indica and its phytes displaying mutualistic interactions with lifestyle in planta may vary depending on envi- plants. However, in order to distinguish mycor- ronmental factors, the genetic predisposition and rhizas from endophytes, a more precise definition the developmental stage of host plants and plant was conceived: Endophytic plant–microbe asso- organs, respectively. These findings are discussed ciations lack a synchronised plant–fungus devel- in the context of the phylogenetic classification opment, specialised microbial structures serving of P. indica within the newly defined mycorrhizal as localised plant–microbe interfaces and nutrient order Sebacinales. transfer to the plant (Brundrett 2004). Irrespective of these characteristics and as mentioned above, host plants are well known to benefit from non- mycorrhizal endophytes to their hosts. A common II. The Mycorrhizal Order Sebacinales example is the release of toxic or antimicrobial compounds distracting herbivoric and microbial Based on morphological and ultrastructural char- competitors (Schulz et al. 2002; Chap. 15). In other acteristics, members of the order Sebacinales were cases plant fitness is enhanced by improved water originally classified as wood-decaying basidi- use efficiency, drought tolerance and enhanced omycetes of the order Auriculariales (Bandoni germination rates (Saikkonen et al. 1998; Brun- 1984; Weiss et al. 2004). However, recent phyloge- drett 2004). In addition, several endophytes netic studies using the nuclear DNA sequence of promote plant growth and confer local and the large ribosomal subunit resulted in the defi- systemic induced resistance to plant pathogens nition of the fungal order Sebacinales, occupying (Varma et al. 1999; Schulz and Boyle 2005; Waller a central position within the Hymenomycetidae. et al. 2005). The order Sebacinales exclusively harbours ben- The fungal basidiomycete Piriformospora indica eficial fungi; however these show an extraordinary has drawn attention since its discovery in India diversity, encompassing ectomycorrhizas, orchid during the final decade of the past century – not mycorrhizas, ericoid mycorrhizas, cavendishioid least due to its versatile beneficial effects con- mycorrhizas and jungermannioid mycorrhizas in ferred to a broad variety of host plant species, e.g. liverworts (McKendrick et al. 2002; Selosse et al. barley, maize, parsley, poplar, tobacco and wheat 2002, 2007; Kottke et al. 2003; Urban et al. 2003; (Sahay and Varma 1999; Varma et al. 1999; Waller Weiss et al. 2004; Setaro et al. 2006). Hence, the et al. 2005; Serfling et al. 2007). This broad host Sebacinales might possess remarkable significance range, combined with its easy handling, makes in natural ecosystems (Weiss et al. 2004). the fungus a potential agent for protecting plants Phylogenetic analysis divided the Sebacinales against abiotic and biotic stresses under green- into two subgroups. Subgroup A harbours ecto- house or field conditions. Hence, P. indica could mycorrhizas and orchid mycorrhizas that usually support sustainability in horticulture and agricul- form hyphal sheaths and occasionally intracellu- ture. Because of the reported beneficial effects, it lar hyphae. Fungi of this group are associated with was rather unexpected that colonisation of barley achlorophyllous or rather heterotrophic orchids roots was found to be associated with cell death (Weiss et al. 2004). Recently, ectendomycorrhizal (Deshmukh et al. 2006). In agreement with other sebacinoids were isolated from Ericaceae. In addi- endophytic plant–fungus interactions, colonised tion to hyphal sheaths, colonised roots showed plants were observed to lack visible disease symp- intercellular networks as well as intracellular struc- toms (e.g. stunted root and shoot development, tures (Selosse et al. 2007). Since some members of or root necrosis). Due to its colonising behaviour, subgroup A are thought to form tripartite symbi- the lack of distinctive colonisation structures and oses connecting trees with orchids, it is speculated the as yet missing evidence for nutrient transfer that most of these fungi are able to form both ecto- to its host plants, P. indica was suggested to be a and orchid mycorrhizal interactions. Subgroup B The Sebacinoid Fungus Piriformospora indica 101 is more heterogenic with respect to the types of nated as Rhizoctonia sp., due to its morphological mycorrhizal associations. It mainly consists of traits, recent phylogenetic studies clearly identi- Sebacina vermifera isolates from autotrophic fied this fungus as a member of the Sebacinales. orchids, ericoid mycorrhizas associated with Hence, this isolate is not closely related with the Gaultheria shallon, cavendishioid mycorrhizas pathogenic Rhizoctonia solani spp. (teleomorphs and liverwort-associated jungermannioid mycor- = Thanatephorus) and binucleate Rhizoctonia rhizas (Weiss et al. 2004; Selosse et al.