Photoperiod Following Inoculation of Arabidopsis with Pyricularia oryzae (syn. Magnaporthe oryzae) Influences on the Plant-Pathogen Interaction.

In plant-pathogen interactions, a proper light environment affects the establishment of defense responses in plants. In our previous experiments, we found that nonhost resistance (NHR) to Pyricularia oryzae Cav. in Arabidopsis thaliana (L.) Heynh. (Arabidopsis), in diurnal conditions, varies with the inoculation time. Moreover, we indicated that the circadian clock plays an important role in regulating time-of-day differences in NHR to P. oryzae in Arabidopsis. However, the involvement of photoperiod in regulating NHR was still not understood. To determine the photoperiod role, we performed the experiments in continuous light and darkness during the early Arabidopsis-P. oryzae interaction. We found that the light period after the inoculation in the evening enhanced the resistance to penetration. However, the dark period after the inoculation in the morning suppressed the penetration resistance. Furthermore, the genetic analysis indicated that jasmonic acid, reactive oxygen species, and tryptophan-derived metabolite(s) contribute to the photoperiod regulation of NHR in Arabidopsis. The present results denote that photoperiod plays an important role in regulating time-of-day differences in NHR to P. oryzae in Arabidopsis.

CCA1 and LHY contribute to nonhost resistance to Pyricularia oryzae (syn. Magnaporthe oryzae) in Arabidopsis thaliana.

The circadian clock enables plants to adapt to their environment and control numerous physiological processes, including plant-pathogen interactions. However, it is unknown if the circadian clock controls nonhost resistance (NHR) in plants. To find out, we analyzed microarray data with the web-based tool DIURNAL to reveal that NHR-related genes show rhythmic expression patterns in the absence of a pathogen challenge. Our clock mutant analyses found that cca1-1 lhy-11 double mutant showed compromised NHR to Pyricularia oryzae, suggesting that two components of the circadian clock, CCA1 and LHY, are involved in regulating penetration resistance in Arabidopsis thaliana. By analyzing pen2 double mutants, we revealed that CCA1 contributes to time-of-day-dependent penetration resistance as a positive regulator and that LHY regulates post-penetration resistance as a positive regulator. Taken together, our results suggest that the circadian clock regulates the time-of-day-dependent NHR to P. oryzae and thus enables A. thaliana to counteract pathogen attacks.Abbreviations: EE: evening element; ETI: effector-triggered immunity; NHR: nonhost resistance; PAMP: pathogen-associated molecular pattern; PTI: PAMP-triggered immunity; SAR: systemic acquired resistance.

Luminal Na+ homeostasis has an important role in intestinal peptide absorption in vivo.

Intestinal cell line studies indicated luminal Na+ homeostasis is essential for proton-coupled peptide absorption, because the driving force of PepT1 activity is supported by the apical Na+/H+ exchanger NHE3. However, there is no direct evidence demonstrating the importance of in vivo luminal Na+ for peptide absorption in animal experiments. To investigate the relationship between luminal Na+ homeostasis and peptide absorption, we took advantage of claudin 15-deficient (cldn15-/-) mice, whereby Na+ homeostasis is disrupted. We quantitatively assessed the intestinal segment responsible for peptide absorption using radiolabeled nonhydrolyzable dipeptide (glycylsarcosine, Gly-Sar) and nonabsorbable fluid phase marker polyethylene glycol (PEG) 4000 in vivo. In wild-type (WT) mice, the concentration ratio of Gly-Sar to PEG 4000 decreased in the upper jejunum, suggesting the upper jejunum is responsible for peptide absorption. Gly-Sar absorption was decreased in the jejunum of cldn15-/- mice. To elucidate the mechanism underlining these impairments, a Gly-Sar-induced short-circuit ( Isc) current was measured. In WT mice, increments of Gly-Sar-induced Isc were inhibited by the luminal application of a NHE3-specific inhibitor S3226 in a dose-dependent fashion. In contrast to in vivo experiments, robust Gly-Sar-induced Isc increments were observed in the jejunal mucosa of cldn15-/- mice. Gly-Sar-induced Isc was inhibited by S3226 or a reduction of luminal Na+ concentration, which mimics low luminal Na+ concentrations in vivo . Our study demonstrates that luminal Na+ homeostasis is important for peptide absorption in native epithelia and that there is a cooperative functional relationship between PepT1 and NHE3. NEW & NOTEWORTHY Our study is the first to demonstrate that luminal Na+ homeostasis is important for proton-coupled peptide absorption in in vivo animal experiments.

Leaf age and time of inoculation contribute to nonhost resistance to Pyricularia oryzae in Arabidopsis thaliana.

The outcome of plant-pathogen interactions varies both with developmental stage and time of day. Rice blast caused by Pyricularia oryzae (syn. Magnaporthe oryzae) is a devastating disease of rice. The mechanisms of resistance to P. oryzae have been extensively studied and the rice-P. oryzae pathosystem has become a model system in plant-microbe interaction studies. However, the mechanisms of resistance to P. oryzae in nonhost remain poorly understood. To determine whether leaf age and time of inoculation would affect nonhost resistance (NHR) to P. oryzae in Arabidopsis thaliana, Columbia-0 (Col-0) and penetration2-1 (pen2-1) plants were inoculated with P. oryzae. The rate of entry of P. oryzae into Arabidopsis pen2-1 old leaves was significantly higher than that into young leaves after inoculation at dusk. However, there was no difference in the rates after inoculation at dawn. These results suggest that leaf age and time of inoculation are involved in nonhost resistance to P. oryzae in Arabidopsis.