Common Mechanisms of Developmental Reprogramming in Plants-Lessons From Regeneration, Symbiosis, and Parasitism

Switching perspectives: The roles of plant cellular reprogramming during nematode parasitism

We propose exploring plant biotrophic parasitism from both a pathogen‐centred and a plant‐centred perspective. This can generate novel research questions and reveal common plant mitigation strategies in response to biotrophic pathogens.

Developing for nutrient uptake: Induced organogenesis in parasitic plants and root nodule symbiosis

Plants have evolved diverse strategies to meet their nutritional needs. Parasitic plants employ haustoria, specialized structures that facilitate invasion of host plants and nutrient acquisition. Legumes have adapted to nitrogen-limited conditions by developing nodules that accommodate nitrogen-fixing rhizobia. The formation of both haustoria and nodules is induced by signals originating from the interacting organisms, namely host plants and rhizobial bacteria, respectively. Emerging studies showed that both organogenesis crucially involves plant hormones such as auxin, cytokinins, and ethylene and also integrate nutrient availability, particularly nitrogen. In this review, we discuss recent advances on hormonal and environmental control of haustoria and nodules development with side-by-side comparison. These underscore the remarkable plasticity of plant organogenesis.

Strigolactones in Rhizosphere Communication: Multiple Molecules With Diverse Functions

Strigolactones (SLs) are root-secreted small molecules that influence organisms living in the rhizosphere. While SLs are known as germination stimulants for root parasitic plants and as hyphal branching factors for arbuscular mycorrhizal fungi, recent studies have also identified them as chemoattractants for parasitic plants, sensors of neighboring plants and key players in shaping the microbiome community. Furthermore, the discovery of structurally diverged SLs, including so-called canonical and non-canonical SLs in various plant species, raises the question of whether the same SLs are responsible for their diverse functions 'in planta' and the rhizosphere or whether different molecules play different roles. Emerging evidence supports the latter, with each SL exhibiting different activities as rhizosphere signals and plant hormones. The evolution of D14/KAI2 receptors has enabled the perception of various SLs or SL-like compounds to control downstream signaling, highlighting the complex interplay between plants and their rhizosphere environment. This review summarizes the recent advances in our understanding of the diverse functions of SLs in the rhizosphere.

Development specifies, diversifies and empowers root immunity

Roots are a highly organised plant tissue consisting of different cell types with distinct developmental functions defined by cell identity networks. Roots are the target of some of the most devastating diseases and possess a highly effective immune system. The recognition of microbe- or plant-derived molecules released in response to microbial attack is highly important in the activation of complex immunity gene networks. Development and immunity are intertwined, and immunity activation can result in growth inhibition. In turn, by connecting immunity and cell identity regulators, cell types are able to launch a cell type-specific immunity based on the developmental function of each cell type. By this strategy, fundamental developmental processes of each cell type contribute their most basic functions to drive cost-effective but highly diverse and, thus, efficient immune responses. This review highlights the interdependence of root development and immunity and how the developmental age of root cells contributes to positive and negative outcomes of development-immunity cross-talk.

Characterization of Transcription Regulatory Domains of OsMADS29: Identification of Proximal Auxin-Responsive Domains and a Strong Distal Negative Element

OsMADS29 (M29) is a seed-specific MADS-box transcription factor involved in programmed cell death of nucellar tissue and maintaining auxin:cytokinin homeostasis. It affects embryo and endosperm development and starch filling during seed development in rice. Its expression seems to be tightly regulated by developmental, spatial, and temporal cues; however, cis- and trans-regulatory factors that affect its expression are largely unknown. In silico analysis of the 1.7 kb upstream regulatory region (URR) consisting of 1,290 bp promoter and 425 bp 5'-UTR regions revealed several auxin-responsive and seed-specific cis-regulatory elements distributed across the URR. In this study, the analysis of four URR deletions fused to a downstream β-glucuronidase (GUS) reporter in transgenic rice has revealed the presence of several proximal positive elements and a strong distal negative element (NE). The promoter regions containing auxin-responsive elements responded positively to the exogenous application of auxins to transgenic seedlings. The proximal positive elements are capable of driving reporter expression in both vegetative and reproductive tissues. In contrast, the NE strongly suppresses reporter gene expression in both vegetative and reproductive tissues. In a transient onion peel assay system, the NE could reduce the efficacy of a 2x CaMV 35S promoter by ∼90%. Our results indicate the existence of a complex array of positive and negative regulatory regions along with auxin-responsive elements guiding the development-dependent and spatial expression of M29.

LATERAL ORGAN BOUNDARIES DOMAIN 25 functions as a key regulator of haustorium development in dodders

Parasitic plants reduce crop yield worldwide. Dodder (Cuscuta campestris) is a stem parasite that attaches to its host, using haustoria to extract nutrients and water. We analyzed the transcriptome of six C. campestris tissues and identified a key gene, LATERAL ORGAN BOUNDARIES DOMAIN 25 (CcLBD25), as highly expressed in prehaustoria and haustoria. Gene coexpression networks from different tissue types and laser-capture microdissection RNA-sequencing data indicated that CcLBD25 could be essential for regulating cell wall loosening and organogenesis. We employed host-induced gene silencing by generating transgenic tomato (Solanum lycopersicum) hosts that express hairpin RNAs to target and down-regulate CcLBD25 in the parasite. Our results showed that C. campestris growing on CcLBD25 RNAi transgenic tomatoes transited to the flowering stage earlier and had reduced biomass compared with C. campestris growing on wild-type (WT) hosts, suggesting that parasites growing on transgenic plants were stressed due to insufficient nutrient acquisition. We developed an in vitro haustorium system to assay the number of prehaustoria produced on strands from C. campestris. Cuscuta campestris grown on CcLBD25 RNAi tomatoes produced fewer prehaustoria than those grown on WT tomatoes, indicating that down-regulating CcLBD25 may affect haustorium initiation. Cuscuta campestris haustoria growing on CcLBD25 RNAi tomatoes exhibited reduced pectin digestion and lacked searching hyphae, which interfered with haustorium penetration and formation of vascular connections. The results of this study elucidate the role of CcLBD25 in haustorium development and might contribute to developing parasite-resistant crops.

Molecular dissection of haustorium development in Orobanchaceae parasitic plants

Characterizing molecular aspects of haustorium development by parasitic plants in the Orobanchaceae family has identified hormone signaling/transport and specific genes as major players.

Orobanchaceae parasite-host interactions

Parasitic plants in the family Orobanchaceae, such as Striga, Orobanche and Phelipanche, often cause significant damage to agricultural crops. The Orobanchaceae family comprises more than 2000 species in about 100 genera, providing an excellent system for studying the molecular basis of parasitism and its evolution. Notably, the establishment of model Orobanchaceae parasites, such as Triphysaria versicolor and Phtheirospermum japonicum, that can infect the model host Arabidopsis, has greatly facilitated transgenic analyses of genes important for parasitism. In addition, recent genomic and transcriptomic analyses of several Orobanchaceae parasites have revealed fascinating molecular insights into the evolution of parasitism and strategies for adaptation in this family. This review highlights recent progress in understanding how Orobanchaceae parasites attack their hosts and how the hosts mount a defense against the threats.

Plant stem cell research is uncovering the secrets of longevity and persistent growth

Plant stem cells have several extraordinary features: they are generated de novo during development and regeneration, maintain their pluripotency, and produce another stem cell niche in an orderly manner. This enables plants to survive for an extended period and to continuously make new organs, representing a clear difference in their developmental program from animals. To uncover regulatory principles governing plant stem cell characteristics, our research project 'Principles of pluripotent stem cells underlying plant vitality' was launched in 2017, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Japanese government. Through a collaboration involving 28 research groups, we aim to identify key factors that trigger epigenetic reprogramming and global changes in gene networks, and thereby contribute to stem cell generation. Pluripotent stem cells in the shoot apical meristem are controlled by cytokinin and auxin, which also play a crucial role in terminating stem cell activity in the floral meristem; therefore, we are focusing on biosynthesis, metabolism, transport, perception, and signaling of these hormones. Besides, we are uncovering the mechanisms of asymmetric cell division and of stem cell death and replenishment under DNA stress, which will illuminate plant-specific features in preserving stemness. Our technology support groups expand single-cell omics to describe stem cell behavior in a spatiotemporal context, and provide correlative light and electron microscopic technology to enable live imaging of cell and subcellular dynamics at high spatiotemporal resolution. In this perspective, we discuss future directions of our ongoing projects and related research fields.