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Angiosperms that morphologically and physiologically attach to other flowering plants by means of a haustorium have evolved 12 times independently resulting in 292 genera and ca. 4750 species. Although hemiparasites predominate, holoparasitism has evolved in all but two clades, Cassytha (Lauraceae) and Krameria (Krameriaceae). Santalales contains the largest number of genera (179) and species (2428) among the 12 parasitic plant lineageswhereas Orobanchaceae is the largest single family with 102 genera and over 2100 species. This review presents the current state of knowledge on the molecular phylogenetic relationships among all clades of parasitic angiosperms. These methods have been particularly important in revealing the closest non-parasitic relatives of holoparasites, plants that exhibit reduced morphologies, increased substitution rates, and frequent horizontal gene transfers, all of which confound phylogenetics. Although comprehensive molecular phylogenies are still lacking for many of the large genera, nearly complete generic level sampling exists, thus allowing unprecedented understanding of the evolutionary relationships within and among these fascinating plants.

An understanding of the cell biology underlying the burgeoning molecular genetic and genomic knowledge of oomycete pathogenicity is essential to gain the full context of how these pathogens cause disease on plants. An intense research focus on secreted Phytophthora effector proteins, especially those containing a conserved N-terminal RXLR motif, has meant that most cell biological studies into Phytophthora diseases have focussed on the effectors and their host target proteins. While these effector studies have provided novel insights into effector secretion and host defence mechanisms, there remain many unanswered questions about fundamental processes involved in spore biology, host penetration and haustorium formation and function.

This article is a Commentary on Praz et al., 213: 1301–1314.

A parasitic plant produces microRNAs that target host messenger RNAs, causing them to be processed into small interfering RNAs. miRNAs in plant parasitism Dodders are parasitic plants that obtain water and nutrients from the stems of their host plants, and exchange other material with their hosts, through structures called haustoria. Michael Axtell and colleagues report how haustoria mediate dodders' parasitism. Dodders accumulate many microRNAs (miRNAs) in their haustoria while infesting a host plant. These miRNAs seem to then transfer to the host, where they silence target messenger RNAs (mRNAs) through the production of secondary small interfering RNAs and mRNA cleavage. The authors also identify host proteins that are targeted by dodder miRNAs and provide evidence that such regulation of host gene expression through inter-species transfer of miRNAs is not limited to one host. Dodders ( Cuscuta spp.) are obligate parasitic plants that obtain water and nutrients from the stems of host plants via specialized feeding structures called haustoria. Dodder haustoria facilitate bidirectional movement of viruses, proteins and mRNAs between host and parasite 1 , but the functional effects of these movements are not known. Here we show that Cuscuta campestris haustoria accumulate high levels of many novel microRNAs (miRNAs) while parasitizing Arabidopsis thaliana . Many of these miRNAs are 22 nucleotides in length. Plant miRNAs of this length are uncommon, and are associated with amplification of target silencing through secondary short interfering RNA (siRNA) production 2 . Several A. thaliana mRNAs are targeted by 22-nucleotide C. campestris miRNAs during parasitism, resulting in mRNA cleavage, secondary siRNA production, and decreased mRNA accumulation. Hosts with mutations in two of the loci that encode target mRNAs supported significantly higher growth of C. campestris . The same miRNAs that are expressed and active when C. campestris parasitizes A. thaliana are also expressed and active when it infects Nicotiana benthamiana . Homologues of target mRNAs from many other plant species also contain the predicted target sites for the induced C. campestris miRNAs. These data show that C. campestris miRNAs act as trans-species regulators of host-gene expression, and suggest that they may act as virulence factors during parasitism.

Background The development of CRISPR/Cas9 technology has facilitated targeted mutagenesis in an efficient and precise way. Previously, RNAi silencing of the susceptibility ( S ) gene P owdery M ildew R esistance 4 ( PMR4 ) in tomato has been shown to enhance resistance against the powdery mildew pathogen Oidium neolycopersici ( On ). Results To study whether full knock-out of the tomato PMR4 gene would result in a higher level of resistance than in the RNAi-silenced transgenic plants we generated tomato PMR4 CRISPR mutants. We used a CRISPR/Cas9 construct containing four single-guide RNAs (sgRNAs) targeting the tomato PMR4 gene to increase the possibility of large deletions in the mutants. After PCR-based selection and sequencing of transformants, we identified five different mutation events, including deletions from 4 to 900-bp, a 1-bp insertion and a 892-bp inversion. These mutants all showed reduced susceptibility to On based on visual scoring of disease symptoms and quantification of relative fungal biomass. Histological observations revealed a significantly higher occurrence of hypersensitive response-like cell death at sites of fungal infection in the pmr4 mutants compared to wild-type plants. Both haustorial formation and hyphal growth were diminished but not completely inhibited in the mutants. Conclusion CRISPR/Cas-9 targeted mutagenesis of the tomato PMR4 gene resulted in mutants with reduced but not complete loss of susceptibility to the PM pathogen On. Our study demonstrates the efficiency and versatility of the CRISPR/Cas9 system as a powerful tool to study and characterize S -genes by generating different types of mutations.

Dodders ( Cuscuta spp., Convolvulaceae) are root- and leafless parasitic plants. The physiology, ecology, and evolution of these obligate parasites are poorly understood. A high-quality reference genome of Cuscuta australis was assembled. Our analyses reveal that Cuscuta experienced accelerated molecular evolution, and Cuscuta and the convolvulaceous morning glory ( Ipomoea ) shared a common whole-genome triplication event before their divergence. C. australis genome harbors 19,671 protein-coding genes, and importantly, 11.7% of the conserved orthologs in autotrophic plants are lost in C. australis . Many of these gene loss events likely result from its parasitic lifestyle and the massive changes of its body plan. Moreover, comparison of the gene expression patterns in Cuscuta prehaustoria/haustoria and various tissues of closely related autotrophic plants suggests that Cuscuta haustorium formation requires mostly genes normally involved in root development. The C. australis genome provides important resources for studying the evolution of parasitism, regressive evolution, and evo-devo in plant parasites. Dodders ( Cuscuta spp., Convolvulaceae) are root- and leafless parasitic plants. Here, the authors sequence the genome of Cuscuta australis and find remarkable gene loss associated with parasitic lifestyle and large changes in body plan.

Quinones are produced and sensed in all kingdoms of life 1 – 4 . Plants are primary producers of quinone 1 , 2 , but the role of quinone as a signalling agent in plants remains largely unknown. One well-documented role of quinone is in the induction of haustoria (specialized feeding structures) in plants that parasitize roots, which occurs in the presence of the host-derived quinone compound 2,6-dimethoxy-1,4-benzoquinone (DMBQ) 5 . However, how parasitic plants sense DMBQ remains unclear, as is whether nonparasitic plants are capable of sensing quinones. Here we use Arabidopsis thaliana and DMBQ as a model plant and quinone to show that DMBQ signalling occurs in Arabidopsis via elevation of cytosolic Ca 2 + concentration. We performed a forward genetic screen in Arabidopsis that isolated DMBQ-unresponsive mutants, which we named cannot respond to DMBQ 1 ( card1 ). The CANNOT RESPOND TO DMBQ 1 ( CARD1 ; At5g49760 , also known as HPCA1 ) gene encodes a leucine-rich-repeat receptor-like kinase that is highly conserved in land plants. In Arabidopsis , DMBQ triggers defence-related gene expression, and card1 mutants show impaired immunity against bacterial pathogens. In Phtheirospermum japonicum (a plant that parasitizes roots), DMBQ initiates Ca 2+ signalling in the root and is important for the development of the haustorium. Furthermore, CARD1 homologues from this parasitic plant complement DMBQ-induced elevation of cytosolic Ca 2+ concentration in the card1 mutant. Our results demonstrate that plants—unlike animals and bacteria—use leucine-rich-repeat receptor-like kinases for quinone signalling. This work provides insights into the role of quinone signalling and CARD1 functions in plants that help us to better understand the signalling pathways used during the formation of the haustorium in parasitic plants and in plant immunity in nonparasitic plants. Dimethoxy-1,4-benzoquinone signalling occurs in Arabidopsis and in the root parasite Phtheirospermum japonicum via increases in cytosolic Ca 2+ concentration mediated by the leucine-rich-repeat receptor-like kinase CARD1 in Arabidopsis , or by its homologues in P. japonicum .

East Indian sandalwood (Santalum album L.) is one of the precious woods known for sweet fragrant aroma and commercial value in both national and international markets. Besides, its wide adaptability to varied climate, hosts and edaphic conditions have attracted farmers and corporates for commercial venture. Sandalwood being a hemi-root parasite, the successful establishment of sandalwood plantation depends on the understanding of parasitism ecology, especially the relations between host and parasite, their ratio and other silviculture techniques. Hence, the present article aims to understand these issues for commercial upscaling. Sandalwood relies largely on host plants for mineral nutrients and water through haustorial connections which act as a physiological and physical bridge between the parasite and the host. It parasitizes over a vast array of plants from grasses to trees, whereas leguminous associations are superior. Deep-rooted and slow-growing perennial hosts help in sustained growth. For better yield of heartwood and oil sandalwood should be grown over 15 years, whereas optimum rotation age would be 25–30 years. Sandalwood can be planted either through direct dibbling of seeds near the hosts in the same pit or adjacent to the hosts. Better establishment occurs when sandalwood is grown with potted host followed by secondary hosts in the field. Association of sandalwood and host would be strong when the distance is around 2.5–3 m with host to parasitic ratio of 2 : 1 or 3 : 1. However, questions on parasitism ecology still remain.

Oomycetes such as the potato blight pathogen Phytophthora infestans deliver RXLR effectors into plant cells to manipulate host processes and promote disease. Knowledge of where they localize inside host cells is important in understanding their function. Fifty-two P. infestans RXLR effectors (PiRXLRs) up-regulated during early stages of infection were expressed as fluorescent protein (FP) fusions inside cells of the model host Nicotiana benthamiana. FP–PiRXLR fusions were predominantly nucleo-cytoplasmic, nuclear, or plasma membrane-associated. Some also localized to the endoplasmic reticulum, mitochondria, peroxisomes, or microtubules, suggesting diverse sites of subcellular activity. Seven of the 25 PiRXLRs examined during infection accumulated at sites of haustorium penetration, probably due to co-localization with host target processes; Pi16663 (Avr1), for example, localized to Sec5-associated mobile bodies which showed perihaustorial accumulation. Forty-five FP–RXLR fusions enhanced pathogen leaf colonization when expressed in Nicotiana benthamiana, revealing that their presence was beneficial to infection. Co-expression of PiRXLRs that target and suppress different immune pathways resulted in an additive enhancement of colonization, indicating the potential to study effector combinations using transient expression assays. We provide a broad platform of high confidence P. infestans effector candidates from which to investigate the mechanisms, singly and in combination, by which this pathogen causes disease.

Plants are the source of most of our food, whether directly or as feed for the animals we eat. Our dinner table is a trophic level we share with the microbes that also feed on the primary photosynthetic producers. Microbes that enter into close interactions with plants need to evade or suppress detection and host immunity to access nutrients. They do this by deploying molecular tools – effectors – which target host processes. The mode of action of effector proteins in these events is varied and complex. Recent data from diverse systems indicate that RNA-interacting proteins and RNA itself are delivered by eukaryotic microbes, such as fungi and oomycetes, to host plants and contribute to the establishment of successful interactions. This is evidence that pathogenic microbes can interfere with the host software. We are beginning to see that pathogenic microbes are capable of hacking into the plants' immunity programs.