Earliest colonization events of Rhizophagus irregularis in rice roots occur preferentially in previously uncolonized cells.

Arbuscular mycorrhizal (AM) fungi form a symbiotic association with several plant species. An arbuscule, a finely branched structure of AM fungi, is formed in root cells and plays essential roles in resource exchange. Because arbuscules are ephemeral, host cells containing collapsed arbuscules can be recolonized, and a wide region of roots can be continuously colonized by AM fungi, suggesting that repetitive recolonization in root cells is required for continuous mycorrhization. However, recolonization frequency has not been quantified because of the lack of appropriate markers for visualization of the cellular processes after arbuscule collapse; therefore, the nature of the colonization sequence remains uncertain. Here we observed that a green fluorescent protein (GFP)-tagged secretory carrier membrane protein (SCAMP) of rice was expressed even in cells with collapsed arbuscules, allowing live imaging coupled with GFP-SCAMP to evaluate the colonization and recolonization sequences. The average lifetime of intact arbuscules was 1-2 d. Cells with collapsed arbuscules were rarely recolonized and formed a new arbuscule during the observation period of 5 d, whereas de novo colonization occurred even in close proximity to cells containing collapsed arbuscules and contributed to the expansion of the colonized region. Colonization spread into an uncolonized region of roots but sparsely into a previously colonized region having no metabolically active arbuscule but several intercellular hyphae. Therefore, we propose that a previously colonized region tends to be intolerant to new colonization in rice roots. Our observations highlight the overlooked negative impact of the degeneration stage of arbuscules in the colonization sequence.

Kobae Y ，Fujiwara T 《-》

Dynamics of periarbuscular membranes visualized with a fluorescent phosphate transporter in arbuscular mycorrhizal roots of rice.

Kobae Y ，Hata S 《-》

Lipid droplets of arbuscular mycorrhizal fungi emerge in concert with arbuscule collapse.

Plants share photosynthetically fixed carbon with arbuscular mycorrhizal (AM) fungi to maintain their growth and nutrition. 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 supporting growth of extraradical mycelia and propagation; however, 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 arbuscular branches, indicating that arbuscule collapse and the emergence of lipid droplets may be associated processes. This phenomenon was observed in the model AM fungus Rhizophagus irregularis and the ancestral member of AM fungi Paraglomus occultum. Because the collapsing arbuscules were metabolically inactive, the emerged lipid droplets are probably derived from preformed lipids but not de novo synthesized. Our observations highlight a novel mode of lipid release by AM fungi.

Kobae Y ，Gutjahr C ，Paszkowski U ，Kojima T ，Fujiwara T ，Hata S ... -  《-》

Transcriptome responses in wheat roots to colonization by the arbuscular mycorrhizal fungus Rhizophagus irregularis.

Li M ，Wang R ，Tian H ，Gao Y ... -  《-》

RiPEIP1, a gene from the arbuscular mycorrhizal fungus Rhizophagus irregularis, is preferentially expressed in planta and may be involved in root colonization.

Fiorilli V ，Belmondo S ，Khouja HR ，Abbà S ，Faccio A ，Daghino S ，Lanfranco L ... -  《-》