Mechanisms of resistance and virulence in parasitic plant-host interactions

Abstract

Parasitic plants pose a major biotic threat to plant growth and development and lead to losses in crop productivity of billions of USD annually. By comparison with "normal" autotrophic plants, parasitic plants live a heterotrophic lifestyle and rely on water, solutes and to a greater (holoparasitic plants) or lesser extent (hemiparasitic plants) on sugars from other host plants. Most hosts are unable to detect an infestation by plant parasites or unable to fend off these parasitic invaders. However, a few hosts have evolved defense strategies to avoid infestation or protect themselves actively post-attack often leading to full or partial resistance. Here, we review the current state of our understanding of the defense strategies to plant parasitism used by host plants with emphasis on the active molecular resistance mechanisms. Furthermore, we outline the perspectives and the potential of future studies that will be indispensable to develop and breed resistant crops.

Figure 1

Resistance symptoms of hosts against parasitic plants. (A–D) Response of resistant and susceptible legume hosts to parasitism by S. gesnerioides. Shown in (A) and (B) is the response of cowpea cultivar B301 to attempted parasitism by S. gesnerioides race SG3 at 6 and 13-d post attachment, respectively, and in (C) and (D) the corresponding response of susceptible cultivar Blackeye. Note the HR response (A) and parasite death (B) in the response on the resistant cultivar and the growth of the Striga tubercle after successful vascular connection (C, D) on the susceptible cultivar (photographs courtesy of Karolina Lis). (E) Cuscuta reflexa on S. lycopersicum ∼5-d post attachment, starting HR response is slightly visible (F) same plants 10 d post parasite attachment; HR is clearly visible at the attempted haustoria penetration sites and C. reflexa starts to die off (photographs of C. reflexa on S. lycopersicum by Max Körner).

Figure 2

Host plant immunity against parasitic plants on cellular level. PPAMPs, like the C. reflexa GRP (Hegenauer et al., 2020) or other yet unknown PPAMPs are recognized at the host cell surface by membrane-bound pattern-recognition receptors, such as HaOr7 (Duriez et al., 2019) or CuRe1 (Hegenauer et al., 2016), and induce PTI in host plant cells. The parasite transmits effector proteins (Su et al., 2020) or miRNAs (Shahid et al., 2018) through a cell–cell connection (interspecific plasmodesmatum) and suppresses host plant PTI in the manner of an ETS. Some host plants possess resistance proteins such as CC-NBS–LRR proteins that may recognize effectors intracellularly and induce ETI, often accompanied by cell wall lignification or a HR—finally leading to resistance.