Beneficial Effects of Phosphite in Arabidopsis thaliana Mediated by Activation of ABA, SA, and JA Biosynthesis and Signaling Pathways

Abstract

Phosphite (Phi) has gained attention in agriculture due to its biostimulant effect on crops. This molecule has been found to benefit plant performance by providing protection against pathogens, improving yield and fruit quality as well as nutrient and water use efficiency. It is still unclear how Phi enhances plant growth and protects against multiple stresses. It has been hypothesized that Phi acts by directly affecting the pathogens and interacting with the plant cellular components and molecular machinery to elicit defense responses. This study elucidates the mechanisms underlying Phi's beneficial effects on plants, revealing their complex interplay with fundamental signaling pathways. An RNA-seq study of Arabidopsis seedlings under optimal and limiting phosphate conditions helped us unveil Phi's role in promoting plant growth by activating the expression of the genes involved in the biosynthesis and signaling pathways associated with abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA). The expression of ABA-related genes, known for their involvement in stress response and development regulation, is triggered by Phi treatment, contributing to enhanced resilience and growth. Simultaneously, the activation of the SA pathway, associated with defense responses, suggests Phi's potential in bolstering plant immunity. Moreover, Phi influences JA biosynthesis and signaling, which are crucial for defense against herbivores and pathogens, thereby strengthening plants' defenses. Our findings reveal a multifaceted mechanism through which Phi benefits Arabidopsis development. Understanding its intricate interplay with key signaling pathways opens avenues for leveraging Phi as a strategic tool to enhance plant resilience, immunity, and growth in agricultural and ecological contexts.

Keywords: Arabidopsis thaliana; abscisic acid; defense responses; jasmonic acid; phosphite; phosphorus sensing; salicylic acid; stress conditions.

Figure 1

Effects of phosphite (Phi) on Arabidopsis thaliana on different levels of Phosphate (Pi). (a) Representative photographs of 10 dag Arabidopsis plants under different phosphite and phosphate treatments. (b) Representative photographs of 10 dag Arabidopsis hair roots under different phosphite and phosphate treatments, (c) Shoot fresh weight (n = 24, each data point is the average of ten plants), (d) Root fresh weight (n = 24, each data point is the average of ten plants) (e) Total root length (n = 40), (f) Convex hull area (n = 40), (g) Lateral root count (n = 40), (h) Principal root length (n = 60), (i) Hair root length (n = 10, each data point is the average of 10 roots hairs, each data point was taken from a different plant), (j) Root hair density (n = 10). HSD tests were performed, and different letters mean significant differences. Panels (c–h): half of the data points were taken randomly from data from two different and independent experiments.

Figure 2

Transcriptional landscape modification by phosphite treatments under different Phosphate (Pi) levels (a) Volcano plot of all the differentially expressed genes (DEGs) in High Phosphate and phosphite combined treatment (+Pi + Phi) relative to High Phosphate treatment (+Pi). Upregulated DEGs are presented in orange, in blue are the downregulated DEGs, and the top 10 genes ranked according to fold-change are highlighted. (b) Same as (a) but for the treatment LPi + Phi relative to LPi. (c) Shared and unique downregulated DEGs in both contrasts. (d) Shared and unique upregulated DEGs in both contrasts. (e) Enriched Terms of Gene Ontology of the Up- and Down-regulated genes of (a). Enriched Terms in the Upregulated DEGs are presented in orange, in blue are the enriched terms of the downregulated DEGs. (f) Is the same as (e) but of (b) DEGs.

Figure 6

Effect of phosphite (Phi) on the DEGs of the LPi treatment relative to the HPi. (a) Flow diagram describing how the gene filtering was performed. Expression levels of all genes that changed expression in the LPi + Phi treatment in reference to the LPi treatment relative to HPi were taken. Quasi likelihood F test was performed to determine if the change was significant. (b) Heatmap of the genes filtered in (a). (c) Enriched GO terms of the DEGs that are downregulated by LPi relative to HPi genes that have further diminished expression in the LPi + Phi treatment. (d) Enriched GO terms of the DEGs that are downregulated and activated by LPi relative to HPi genes did not diminish their expression in the LPi + Phi treatment. (e) Enriched GO terms of the DEGs that are activated by LPi relative to HPi genes that have enhanced expression in the LPi + Phi treatment. (f) Enriched GO terms of the DEGs that are activated by LPi relative to HPi that have attenuated expression in the LPi + Phi treatment.

Figure 3

Effect of phosphite treatments on the transcription of different hormones. (a) Heatmap of genes of the ABA biosynthesis pathway. (b) Heatmap of the GO term “positive regulation of abscisic acid-activated signaling pathway”. (c) Heatmap of DEGS related to salicylic acid biosynthesis. (d) Heatmap of the GO term “systemic acquired resistance”. (e) Heatmap of DEGs related to jasmonic acid biosynthesis. (f) Heatmap of the GO Term “Jasmonic acid-mediated signaling pathway”. Genes of the biosynthetic pathways were taken from the Plant Metabolic Network database. For all plots: Blue triangles represent downregulated genes, red triangles represent upregulated genes, and n.s. stands for non-significant. Likelihood ratio tests according to the edgeR pipeline were performed on each pair of genes to determine if there are significant differences between treatments.

Figure 4

Effect of phosphite (Phi) on local and systemic PSRs. (a) Heatmap of the local PSRs genes. (b) Heatmap of the systemic PSRs. Tukey’s multiple comparisons of means test were performed between treatments; different letters mean significant differences between treatments.

Figure 5

Effect of phosphite (Phi) on STOP1 and PHR1/PHL1 targets. (a) Raincloud plot and heatmap of the STOP1 targets. (b) Raincloud plot and heatmap of the PHR1/PHL1 targets. (c) Changes in gene expression of STOP1 signaling pathway related genes. (d) Changes in gene expression of PHR1 signaling pathway related genes. Question marks in (c,d) represent putative or still unknown regulatory components of the pathway. (e) Venn diagram of the genes related to PHR1/PHL1, STOP1, Systemic PSR, and local PSRs, including genes that are not part of these sets. For (a,b): Tukey´s multiple comparisons of means test were performed between treatments; different letters mean significant differences between treatments. In (c,d): blue triangles represent downregulated genes and red triangles represent upregulated genes, n.s. stands for non-significant. Likelihood ratio tests according to the edgeR pipeline were performed on each pair of genes to determine if there are significant differences between treatments.