A Feed-Forward Subnetwork Emerging from Integrated TOR- and cAMP/PKA-Signaling Architecture Reinforces Magnaporthe oryzae Appressorium Morphogenesis

Abstract

Appressoria are important mediators of plant–microbe interactions. In the devastating rice blast pathogen Magnaporthe oryzae, appressorial morphogenesis from germ tube tips requires activated cAMP/PKA signaling and inactivated TOR signaling (TORoff). TORoff temporarily arrests G2 at a metabolic checkpoint during the single round of mitosis that occurs following germination. G2 arrest induces autophagy and appressorium formation concomitantly, allowing reprogression of the cell cycle to G1/G0 quiescence and a single appressorial nucleus. Inappropriate TOR activation abrogates G2 arrest and inhibits cAMP/PKA signaling downstream of cPKA. This results in multiple rounds of germ tube mitosis and the loss of autophagy and appressoria formation. How cAMP/PKA signaling connects to cell cycle progression and autophagy is not known. To address this, we interrogated TOR and cAMP/PKA pathways using signaling mutants, different surface properties, and specific cell cycle inhibitors and discovered a feed-forward subnetwork arising from TOR- and cAMP/PKA-signaling integration. This adenylate cyclase-cAMP-TOR-adenylate cyclase subnetwork reinforces cAMP/PKA-dependent appressorium formation under favorable environmental conditions. Under unfavorable conditions, the subnetwork collapses, resulting in reversible cell cycle-mediated germ tube growth regardless of external nutrient status. Collectively, this work provides new molecular insights on germ tube morphogenetic decision-making in response to static and dynamic environmental conditions.

Magnaporthe oryzae nucleoside diphosphate kinase is required for metabolic homeostasis and redox‐mediated host innate immunity suppression

“…This is similar to the responses of the adenylate cyclase mutant Δ mac1 , whereby cAMP but not rapamycin induces Δ mac1 appressorium formation on hydrophobic surfaces due to the role of cAMP in integrating the cAMP/PKA‐ and TOR‐signaling pathways during appressorium formation (Sun et al ., 2019). The loss of MAC1 also abolished conidial autophagy and resulted in multiple rounds of mitosis in germ tubes due to the abrogation of TOR‐mediated G2 arrest (Sun et al ., 2019). To ascertain whether the loss of NDK1 affected MAC1 function, we asked if, like Δ mac1 , the Δ ndk1 mutant strain was also abolished for conidial autophagy and cell cycle arrest following spore germination on coverslips.…”

“…To understand the seemingly disparate roles of NDK1 in infection and infection‐related development, we first sought to determine why the loss of NDK1 affected appressorium development on plastic coverslips but not on rice leaf surfaces. On non‐inductive surfaces, appressoria can be induced in WT by treatment with cAMP or the Tor kinase‐specific inhibitor rapamycin (Marroquin‐Guzman and Wilson, 2015; Marroquin‐Guzman et al ., 2017a; Sun et al ., 2019). Treatment with cAMP increased the rate of appressorium formation by germinating Δ ndk1 spores on hydrophobic coverslips (Figure 2a), suggesting that the cAMP/PKA signaling pathway was inactivated in Δ ndk1 on artificial hydrophobic surfaces.…”

“…Appressoria form on nutrient‐free, hydrophobic surfaces, and appressorial development from spore germ tube tips requires cAMP/PKA‐ and Pmk1‐signaling, a single round of mitosis and autophagic cell death of the spore contents (Wilson and Talbot, 2009). Transient TOR‐dependent G2 arrest, conditioned by the absence of glucose and glutamine and requiring the adenylate cyclase Mac1, induces appressorium formation and autophagy concomitantly (Marroquin‐Guzman et al ., 2017a; Sun et al ., 2019). Following cuticle puncturing, the appressorial penetration peg expands in the underlying apoplastic space into a thin primary hypha that then elaborates bulbous, branching invasive hyphae (IH).…”

Targeting intrinsic cell death pathways to control fungal pathogens

“…[107]. Thus, autophagy is required for the fungus to develop its appressorium structure needed to traverse the plant cell wall [108,109]. In contrast, deletion of genes predicted to be involved in pexophagy or mitophagy (forms of selective autophagy) did not affect development of the appressorium or subsequent infection [170].…”

“…How is autophagy related to conidial cell death? The differentiation process is stimulated by nutrient deprivation, as conidia incubated in high nutrients do not proceed to form an appressorium, but instead form undifferentiated hyphae and continue to undergo mitosis [108,109]. Nutrient depletion in fungi and mammals is known to downregulate cell growth by inhibiting the Tor1 kinase complex (TORC1) to derepress autophagy [110].…”