Non-host resistance to penetration and hyphal growth of Magnaporthe oryzae in Arabidopsis

Rice blast caused by Magnaporthe oryzae is a devastating disease of rice. Mechanisms of rice resistance to blast have been studied extensively, and the rice–M. oryzae pathosystem has become a model for plant–microbe interaction studies. However, the mechanisms of non-host resistance (NHR) to rice blast in other plants remain poorly understood. We found that penetration resistance to M. oryzae in multiple mutants, including pen2 NahG pmr5 agb1 and pen2 NahG pmr5 mlo2 plants, was severely compromised and that fungal growth was permitted in penetrated epidermal cells. Furthermore, rice Pi21 enhanced movement of infection hyphae from penetrated Arabidopsis epidermal cells to adjacent mesophyll cells. These results indicate that PEN2, PMR5, AGB1, and MLO2 function in both penetration and post-penetration resistance to M. oryzae in Arabidopsis, and suggest that the absence of rice Pi21 contributed to Arabidopsis NHR to M. oryzae

Structure and evolution of the plant cation diffusion facilitator family of ion transporters

Peer reviewedPublisher PD

A Mutant Strain Arabidopsis thaliana that Lacks Vacuolar Membrane Zinc Transporter MTP1 Revealed the Latent Tolerance to Excessive Zinc

A mutant line of Arabidopsis thaliana that lacks a vacuolar membrane Zn-2/H antiporter MTP1 is sensitive to zinc. We examined the physiological changes in this loss-of-function mutant under high-Zn conditions to gain an understanding of the mechanism of adaptation to Zn stress. When grown in excessive Zn and observed using energy-dispersive X-ray analysis, wild-type roots were found to accumulate Zn in vacuolar-like organelles but mutant roots did not. The Zn content of mutant roots, determined by chemical analysis, was one-third that of wild-type roots grown in high-Zn medium. Severe inhibition of root growth was observed in mtp1-1 seedlings in 500M ZnSO4. Suppression of cell division and elonga-tion by excessive Zn was reversible and the cells resumed growth in normal medium. In mutant roots, a marked formation of reactive oxygen species (ROS) appeared in the meristematic zone, where the MTP1 gene was highly expressed. Zn treatment enhanced the expression of several genes involved in Zn tolerance: namely, the plasma membrane Zn-2-export ATPase, HMA4, and plasma and vacuolar membrane proton pumps. CuZn-superoxide dismutases, involved in the detoxification of ROS, were also induced. The expression of plasma membrane Zn-uptake transporter, ZIP1, was suppressed. The up- or down-regulation of these genes might confer the resistance to Zn toxicity. These results indicate an essential role of MTP1 in detoxification of excessive Zn and provide novel information on the latent adaptation mechanism to Zn stress, which is hidden by MTP1.X115759sciescopu

The use of wideband filters in distinguish green fluorescent protein in roots of arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) fungi form arbuscules in the inner cortical cells of roots. Accumulation of autofluorescent materials within the roots, especially around senescent arbuscules, has hampered analyses of the localization and dynamics of green fluorescent protein (GFP)-fusion proteins in arbusculated cells. In this report, the author proposes an efficient method to distinguish GFP from autofluorescence. To detect GFP fluorescence, colonized cells were observed with wideband filters rather than GFP-specialized filters because the autofluorescence generally has a broad fluorescent spectrum that can easily be distinguished from GFP by color. Moreover, the autofluorescence of arbusculated cells could possibly contain strong green fluorescence that could not be excluded by GFP-specialized filters. The multicolor imaging and in vivo real-time observations suggested that the expression of autofluorescence in arbusculated cells did not overlap with the expression of OsPT11-GFP, a useful marker for active arbuscules, and that autofluorescent materials appeared after the initiation of senescence in infection units

PLANT AND CELL PHYSIOLOGY

Plants share photosynthetically fixed carbon with arbuscular mycorrhizal (AM) fungi to maintain their growth and nutri-tion. AM fungi are oleogenic fungi that contain numerous lipid droplets in their syncytial mycelia during most of their life cycle. These lipid droplets are probably used for support-ing growth of extraradical mycelia and propagation; how-ever, when and where the lipid droplets are produced remains unclear. To address these issues, we investigated the correlation between intracellular colonization stages and the appearance of fungal lipid droplets in roots by a combination of vital staining of fungal structures, selective staining of lipids and live imaging. We discovered that a surge of lipid droplets coincided with the collapse of arbus-cular branches, indicating that arbuscule collapse and the emergence of lipid droplets may be associated processes

Up-regulation of genes involved in N-acetylglucosamine uptake and metabolism suggests a recycling mode of chitin in intraradical mycelium of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi colonize roots and form two kinds of mycelium, intraradical mycelium (IRM) and extraradical mycelium (ERM). Arbuscules are characteristic IRM structures that highly branch within host cells in order to mediate resource exchange between the symbionts. They are ephemeral structures and at the end of their life span, arbuscular branches collapse from the tip, fungal cytoplasm withdraws, and the whole arbuscule shrinks into fungal clumps. The exoskeleton of an arbuscule contains structured chitin, which is a polymer of N-acetylglucosamine (GlcNAc), whereas a collapsed arbuscule does not. The molecular mechanisms underlying the turnover of chitin in AM fungi remain unknown. Here, a GlcNAc transporter, RiNGT, was identified from the AM fungus Rhizophagus irregularis. Yeast mutants defective in endogenous GlcNAc uptake and expressing RiNGT took up 14C-GlcNAc, and the optimum uptake was at acidic pH values (pH 4.0–4.5). The transcript levels of RiNGT in IRM in mycorrhizal Lotus japonicus roots were over 1000 times higher than those in ERM. GlcNAc-6-phosphate deacetylase (DAC1) and glucosamine-6-phosphate isomerase (NAG1) genes, which are related to the GlcNAc catabolism pathway, were also induced in IRM. Altogether, data suggest the existence of an enhanced recycling mode of GlcNAc in IRM of AM fungi