Three-dimensional reconstructions of the internal structures of haustoria in parasitic Orobanchaceae

Abstract

One-sentence summary 31Three-dimensional image reconstruction was used to visualize the spatial organization of cell types in 32 the haustoria of parasitic plants with special reference to their interaction with host roots. 33 34 Author contributions 35 SC and SY conceived the idea of this study, designed the experiment, analyzed the data and wrote the 36 manuscript. NM was responsible for color coding, FE-SEM and 3-D reconstruction. YSu and YSa 37 2 developed methods to align section images, automated the color coding process, and provided crucial 38 technical assistance for 3-D reconstruction. YI, AS and KK assisted with image handling. KT, MW and 39 MS prepared serial thin sections and performed FE-SEM. KF analyzed the AtPEAR promoter. KS 40 provided critical comments on the manuscript. 41 42 Abstract 51 Parasitic plants infect other plants by forming haustoria, specialized multicellular organs consisting of 52 several cell types each of which has unique morphological features and physiological roles associated 53 with parasitism. Understanding the spatial organization of cell types is, therefore, of great importance 54 in elucidating the functions of haustoria. Here, we report a three-dimensional (3-D) reconstruction of 55 haustoria from two Orobanchaceae species, the obligate parasite Striga hermonthica infecting rice and 56 the facultative parasite Phtheirospermum japonicum infecting Arabidopsis. Our images reveal the 57 spatial arrangements of multiple cell types inside haustoria and their interaction with host roots. The 3-58 D internal structures of haustoria highlight differences between the two parasites, particularly at the 59 xylem connection site with the host. Our study provides structural insights into how organs interact 60 between hosts and parasitic plants. 61 62 65 Approximately 4,500 species in 28 families, representing 1% of all angiosperm species, are proposed 66 to be parasitic plants (Westwood et al., 2010; Heide-Jørgensen, 2013). The common feature of parasitic 67 plants is the ability to form a specialized invasive organ called a haustorium. Unlike the unicellular 68 haustoria found in plant-infecting pathogenic fungi, haustoria in parasitic plants are multicellular organs 69 that function in host attachment, host tissue invasion and establishing a vascular connection between 70 the parasite and host for material transfer (Yoshida et al., 2016). Haustoria facilitate water and nutrient 71 acquisition as well as translocation of RNA molecules, peptides and plant hormones between the host 72 and parasite (Kim et al., 2014; Spallek et al., 2017; Shahid et al., 2018; Liu et al., 2019; Yoshida et al., 73 3 2019). Thus, haustoria act as efficient biological channels for interspecies material transfer, but the 74 internal structures and physiological functions of haustoria are largely unexplored. 75 The Orobanchaceae family contains the largest number of root parasitic species and present various 76 degrees of host dependency. An exception among Orobanchaceae family members is in the genus 77 L...

CcLBD25functions as a key regulator of haustorium development inCuscuta campestris

“…Intriguingly, many genes that are involved in haustoria development also play important roles in flower and pollen development (Yang et al, 2015; Yoshida et al, 2019). Recent research on haustoria 3D structure also indicates that the growth pattern of intrusive cells is similar to the rapid polar growth of pollen tubes (Masumoto et al, 2020). Taken together with our results in this study and previous findings in other organisms, we suggest that the genes that are regulating pollen development or pollen tube growth, like LBD25 , might be adopted by parasitic plants for haustorium intrusive cell and searching hyphae development.…”

Orobanchaceae parasite–host interactions

“…As pathogenicity and genetic diversity in the parasitic weed population are less understood, understanding plant-plant interactions at the molecular level should go hand-in-hand with assessments of the genetic diversity of parasite populations. The recent availability of genomic information for S. asiatica (Yoshida et al, 2019) and P. japonicum (Cui et al, 2020), as well as transcriptome information on other Orobanchaceae plants (Yoshida et al, 2010;Wickett et al, 2011;Yang et al, 2015;Kado & Innan, 2018;Lopez et al, 2019) will allow comparative and ecological analyses that should deepen our understanding.…”

“…Phloem connections are established in holoparasites, such as Orobanche spp. but not in hemiparasites such as S. hermonthica and P. japonicum (Masumoto et al, 2020).…”

“…Prolonged auxin-responsive marker expression was observed in the actively dividing haustorial apex cells in the mutants, indicating that ethylene-auxin dependent regulation is utilized for intrusive cell differentiation. Interestingly, host ethylene-defective mutants also perturb P. japonicum invasion (Cui et al, 2020). The gaseous phytohormone ethylene may act as a signal for parasitic plants to precisely locate hosts in order to transiently repress cell division and promote differentiation ( Fig.…”

“…Interestingly, a XB is not found in haustoria induced by HIF treatment only, indicating that other signal(s) are required for the initiation of XB development (Ishida et al ., 2016; Cui et al ., 2018). Phloem connections are established in holoparasites, such as Orobanche spp., but not in hemiparasites such as S. hermonthica and P. japonicum (Masumoto et al ., 20202021). By acquiring the ability to form phloem connections, holoparasites may have been able to afford the loss of photosynthetic ability.…”