Interacció entre l'homeòstasi de fosfat i la resposta immune en plantes

Resumen

In nature, plants are constantly confronted to abiotic and biotic stresses. Research carried out to understand the plant response to biotic and abiotic stress has mainly concentrated on individual stresses. Notoriously, the plant defense response to a particular stress might have a positive or negative effects in the response to other type of stress. Regarding biotic stresses, evidences support that plant immune responses can be posttranscriptionally regulated by the activity of microRNAs (miRNAs) but limited information is available on miRNAs participating in the crosstalk between pathogen-induced signaling pathways and nutrient signaling in plants. Phosphorus is an essential nutrient for plant growth and development, which is acquire from the soil in the form of inorganic phosphate (Pi). The low bioavailability of Pi in agricultural soils represents a limiting factor for plant growth. As a consequence, Pi fertilizers are routinely used in modern agriculture to optimize crop yields, leading to a scenario of Pi excess in many ecosystems. At the molecular level, a large effort has been made to understand how plants adapt to Pi limiting conditions through the so called Phosphate Starvation Response (PSR). The main topic in this Ph.D. Thesis is the investigation of interconnected regulations between Pi homeostasis and immune responses in Arabidopsis (Chapter I and Chapter II) and rice (Chapter III). Chapter I describes the function of miR399 and its target gene PHO2 (PHOSPHATE2) in the regulation of phosphate homeostasis and disease resistance in Arabidopsis. Growing plants grown under high Pi supply, as well as miR399 overexpression and loss-of-function of PHO2 plants, leads to an increase in Pi content, these plants also exhibiting resistance to infection by P. cucumerina and C. higginsianum. Disease resistance is accompanied by increased ROS production. Furthermore, up-regulation of genes involved in salicylic acid (SA) and jasmonic acid (JA) signaling pathways occurs in Arabidopsis plants overaccumulating Pi, which is accompanied by a higher level of SA and JA in these plants. An opposite regulation of the two branches in the JA signaling pathway, the ERF1/PDF1.2 and MYC2/VSP2 branches, is observed in high Pi plants. Together, these findings support that miR399 plays a positive role in regulating hormone levels and immune responses in Arabidopsis. Chapter II describes the impact of alterations in the expression of MIR827 and its target gene NLA (NITROGEN LIMITATION ADAPTATION) in the response of Arabidopsis plants to infection by pathogenic fungi. Both miR827 overexpressing and nla mutant lines overaccumulated phosphate and were more resistant to P. cucumerina infection. The nla plants and plants grown under high Pi conditions show higher callose deposition and ROS production. Besides, in the absence of pathogen infection, genes involved in the SA- and JA-signaling pathways are up-regulated in nla plants compared to wild type plants. Accumulation of SA and JA also occurs in nla mutant plants. Additionally, loss-of-function of nla results in accumulation of the phytoalexin camalexin. These results support that NLA negative regulates defense responses in Arabidopsis. Chapter III describes the contribution of miR827 in the regulation of Pi homeostasis and resistance to pathogen infection in rice. CRISPR/Cas9-edited miR827 plants lines together with miR827 overexpressor plants were assayed for disease resistance against Magnaporthe oryzae. Compared with wild-type plants, the CRISPR-miiR827 plants and miR827 overexpressor rice plants showed lower and higher Pi content, respectively, and therefore, enhanced and decreased disease resistance to M. oryzae. Using 31P-HR-MAS, differences in the vacuolar-to-cytosolic distribution of Pi were detected between loss-of-function and gain-of-funtion miR827 plants. These findings support that, not only total Pi content, but also disturbance in the subcellular distribution of Pi might have an impact on resistance to infection by M. oryzae in rice.