Chorismate mutase and isochorismatase, two potential effectors of the migratory nematode Hirschmanniella oryzae, increase host susceptibility by manipulating secondary metabolite content of rice

Abstract

Hirschmanniella oryzae is one of the most devastating pests in rice fields worldwide. Because H. oryzae is well adapted to flooded conditions, it is a major problem in flooded rice ecosystems (Babatola, 1981). This migratory nematode penetrates the root and migrates through the aerenchyma, completing its life cycle in about 33 days (Karakas, 2004). Plant-parasitic nematodes are equipped with an arsenal of cell wall-modifying proteins that are secreted through the stylet into the host tissue to hydrolyse cell walls (Davis et al., 2011). Sedentary nematodes establish a feeding site, whereby it is important to remain undetected by the plant to prevent an adequate defence response.

Pathogens pulling the strings: Effectors manipulating salicylic acid and phenylpropanoid biosynthesis in plants

“…It should be mentioned that SA measurements were performed on unchallenged plants in the latter study, which could explain the discrepancy in results with the two former studies (Bauters et al, 2020;Djamei et al, 2011;Wang et al, 2018). Plants have evolved a way to inhibit the effect of secreted CMs by expressing kiwellins.…”

“…Although rice lines overexpressing this putative effector showed increased susceptibility, no difference in SA content was detected, but it should be noted that data was collected from unchallenged plants (Bauters et al, 2020). It is worth mentioning that the conventional signal peptide that usually guides effector proteins to the secretory pathway is absent for ICMs in fungi and nematodes (Bauters et al, 2020). In fungi, ICM has been shown to be targeted for secretion by an unconventional secretion F I G U R E 1 Schematic representation illustrating pathogen effectors within a plant cell, having an effect on salicylic acid (SA) content in plants.…”

“…More recently, an ICM was also characterized (HoICM) from H. oryzae. Although rice lines overexpressing this putative effector showed increased susceptibility, no difference in SA content was detected, but it should be noted that data was collected from unchallenged plants (Bauters et al, 2020). It is worth mentioning that the conventional signal peptide that usually guides effector proteins to the secretory pathway is absent for ICMs in fungi and nematodes (Bauters et al, 2020).…”

“…These results suggest that Cmu1 increases the virulence of U. maydis by directing the metabolite flow into the phenylpropanoid pathway, reducing SA production. In contrast, it was shown that a secreted CM from the nematode H. oryzae might lower the phenylpropanoid content of the host, thereby making it more vulnerable to infection (Bauters et al, 2020). These seemingly contradictory results illustrate that different pathosystems can respond in another way, and that thorough research is needed to unravel all mechanisms.…”

“…The two best-studied examples of effectors manipulating the SA biosynthesis pathway are chorismate mutase (CM) and isochorismatase (ICM). Both have been identified in several fungi and also in plant-parasitic nematodes (Bauters et al, 2020;Djamei et al, 2011;Liu et al, 2014;Wang et al, 2018). Figure 1 summarizes the action of these and other pathogen effectors affecting SA levels.…”