Two C6 transcription factors act as host immune terminator (HitA) and regulator of insect pathogenicity (Rip4) in Beuaveria bassiana.

Fungal C6 transcription factors (TFs) are generally underexplored. Here, we report the first host immune terminator (HitA) and the fourth regulator of insect pathogenicity (Rip4) identified from the C6 TFs of Beauveria bassiana, a wide-spectrum mycoinsecticide. The deletion mutant of hitA lost all insecticidal activity via normal cuticle or cuticle-bypassing (injection) infection but reserved a capability of hyphal penetration through host cuticle. Its conidia injected into host hemocoel were internalized by cellular encapsulation within 8 h, followed by aggregation of small melanotic capsules into irregular nodulations surrounded persistently by hemocytes. The injected wild-type conidia were also encapsulated by aggregated host hemocytes but started germination at 12-16 h, followed by a burst of germ tubes and hyphae extending from cracked melanotic capsules at 24-36 h, then hyphal growth and transition to unicellular hyphal bodies for rapid yeast-like budding proliferation at 48-72 h but no sign of nodulation. An essentiality of HitA for prompt host immune termination was further validated by changes of phenoloxidase activities and transcriptomic profiles exhibiting time-course host immune responses to fungal (mutant/wildtype) intrusion. The rip4 deletion led to blocked cuticular penetration and attenuated virulence via the normal infection. HitA and Rip4 also played important roles in mediating aerial conidiation and multiple stress responses, respectively. These findings provide the first insight into greater importance of prompt host immune termination than that of cuticular penetration in the insecticidal process of B. bassiana and add knowledge to fungal C6 TFs.