Blast disease caused by the fungal pathogen Magnaporthe oryzae is one of the main limiting factors for crop production worldwide. This disease can infect at least 59 species of grasses, including rice, wheat, maize, barley, foxtail millet, and their wild relatives, resulting in serious yield losses and threats to global food security [1–3]. In rice, blast disease can lead to yield losses of up to 100% in infected regions, and a global yield loss corresponding to food for 212–742 million people annually [3, 4]. Thus, it important to gain insight into the mechanism of pathogenesis to develop effective disease control methods.
As a hemibiotrophic pathogen, M. oryzae initiates infection via a biotrophic interaction with the host [5]. During the infection processes, effector proteins secreted by M. oryzae interact with components in the host plant, thus inhibiting their immune responses and altering metabolism [6, 7]. However, intracellular immune receptors in host plants can recognize effectors and trigger immune responses to resist pathogen attack [8]. Understanding the functions of M. oryzae effectors, their host targets, and pathogen avirulent (AVR)-resistance (R) gene interactions are the key factors for understanding the mechanism of M. oryzae infection.
Numerous R genes (Pb1, Pia, Pib, Pid2, Pid3, Pik, Pikh/Pi54, Pikm, Pikp, Pish, Pit, Pita, Pizt, Pi1, Pi2, Pi5, Pi9, pi21, Pi25, Pi36, Pi37, Pi56, Pi63, PiCO39, Pi64, Pigm) and AVR genes (AVR-Pi54, AVR-Pi9, AVR-Pib, AVR-Pia, AVR-Pii, AVR-Pik/km/kp, AVR-Pizt, ACE1, AVR-Pita, AVR1-CO39, PWL1, PWL2) have been cloned [9–21]. Among them, those encoding AVR-Pik effectors have been widely investigated. Maidment et al. demonstrated that AVR-PikD can interact with both the heavy-metal-associated (HMA) domain of OsHIPP19 and the integrated HMA domains encoded by Pik-1 alleles, with the former interaction being tighter [22]. Maqbool et al. reported the crystal structure of the Pikp-HMA/AVR-PikD complex and provided details of the immune recognition event in tobacco (Nicotiana benthamiana) [1]. The crystal structure of the effector proteins APikL2A (host target sHMA25) and APikL2F (host target sHMA94) from foxtail millet has been revealed by X-ray diffraction with a resolution better than 2.3 Å [23].
Changes in temperature, light, and other conditions can significantly affect pathogen infection. Rajput et al. found that the appressoria formation and spore germination of M. oryzae were much slower at a supraoptimal temperature (32°C) than at the optimal temperature (27°C), suggesting that M. oryzae is non-infectious at higher temperatures [24]. Qiu et al. demonstrated that jasmonic acid biosynthesis and signaling genes in rice were effectively induced by M. oryzae at 28°C but not at 22°C [25]. Madhusudhan et al. observed lower expression of Pi54 and WRKY45 at the optimal temperature for pathogen aggressiveness, indicating that temperature may affect defense processes [26]. In addition to temperature, light can also affect fungal infection. Lee et al. observed that both blue and red light suppressed the spore release of M. oryzae, while only blue light suppressed its asexual development [27]. Li et al. observed that ultraviolet radiation weakened the infectivity of M. oryzae [28]. However, the effects of temperature and light on M. oryzae effector proteins are unclear.
Molecular dynamic (MD) simulation is a powerful tool for studying biomolecular structure and dynamics in a manner similar to experiments [29]. To quantify the effects of temperature and light on the dynamics of effector proteins, we performed MD simulations on the complex consisting of APikL2A from M. oryzae and its target host protein, sHMA25 from foxtail millet. The root mean square deviation (RMSD), radius of gyration (Rg), surface area (SA), secondary structures, and binding free energy under different temperature and light conditions were determined. While the structure of APikL2A/sHMA25 remained stable in the range of 290 K – 320 K and under weak oscillating electric fields, it was destroyed by strong electric fields. We observed weak binding affinities between APikL2A and sHMA25 in a temperature window from 300 K to 310 K, and an increase in binding free energy as the electric field intensity increased from 0.002 to 0.008 V/nm.