Metabolomic and Transcriptomic Changes Induced by Potassium Deficiency During Sarocladium oryzae Infection Reveal Insights into Rice Sheath Rot Disease Resistance

Optimizing nutrient transporters to enhance disease resistance in rice

Fertilizers and plant diseases contribute positively and negatively to crop production, respectively. Macro- and micronutrients provided by the soil and fertilizers are transported by various plant nutrient transporters from the soil to the roots and shoots, facilitating growth and development. However, the homeostasis of different nutrients has different effects on plant disease. This review is aimed at providing insights into the interconnected regulation between nutrient homeostasis and immune responses, and it highlights strategies to enhance disease resistance by optimal manipulation of nutrient transporters in rice. First, we highlight the essential roles of six macronutrients (nitrogen, phosphorus, potassium, sulfur, calcium, magnesium) and eight micronutrients (iron, manganese, zinc, copper, boron, molybdenum, silicon, nickel), and summarize the diverse effects of each on rice diseases. We then systematically review the molecular mechanisms of immune responses modulated by nutrient transporters and the genetic regulatory pathways that control the specific nutrient-mediated immune signaling that is regulated by the pathogens and the host plant. Finally, we discuss putative strategies for breeding disease-resistant rice by genetic engineering of nutrient transporters.

Integrated transcriptome and metabolome analysis unveil the response mechanism in wild rice (Zizania latifolia griseb.) against sheath rot infection

Sheath rot disease (SRD) is one of the most devastating diseases of Manchurian wild rice (MWR) (Zizania latifolia Griseb). Pilot experiments in our laboratory have shown that an MWR cultivar "Zhejiao NO.7"exhibits signs of SRD tolerance. To explore the responses of Zhejiao No. 7 to SRD infection, we used a combined transcriptome and metabolome analysis approach. A total of 136 differentially accumulated metabolites (DAMs, 114 up- and 22 down-accumulated in FA compared to CK) were detected. These up-accumulated metabolites were enriched in tryptophan metabolism, amino acid biosynthesis, flavonoids, and phytohormone signaling. Transcriptome sequencing results showed the differential expression of 11,280 genes (DEGs, 5,933 up-, and 5,347 downregulated in FA compared to CK). The genes expressed in tryptophan metabolism, amino acid biosynthesis, phytohormone biosynthesis and signaling, and reactive oxygen species homeostasis confirmed the metabolite results. In addition, genes related to the cell wall, carbohydrate metabolism, and plant-pathogen interaction (especially hypersensitive response) showed changes in expression in response to SRD infection. These results provide a basis for understanding the response mechanisms in MWR to FA attack that can be used for breeding SRD-tolerant MWR.

Inhibition of rice germination by ustiloxin A involves alteration in carbon metabolism and amino acid utilization

Ustiloxins are the main mycotoxin in rice false smut, a devastating disease caused by Ustilaginoidea virens. A typical phytotoxicity of ustiloxins is strong inhibition of seed germination, but the physiological mechanism is not clear. Here, we show that the inhibition of rice germination by ustiloxin A (UA) is dose-dependent. The sugar availability in UA-treated embryo was lower while the starch residue in endosperm was higher. The transcripts and metabolites responsive to typical UA treatment were investigated. The expression of several SWEET genes responsible for sugar transport in embryo was down-regulated by UA. Glycolysis and pentose phosphate processes in embryo were transcriptionally repressed. Most of the amino acids detected in endosperm and embryo were variously decreased. Ribosomal RNAs for growth were inhibited while the secondary metabolite salicylic acid was also decreased under UA. Hence, we propose that the inhibition of seed germination by UA involves the block of sugar transport from endosperm to embryo, leading to altered carbon metabolism and amino acid utilization in rice plants. Our analysis provides a framework for understanding of the molecular mechanisms of ustiloxins on rice growth and in pathogen infection.

Sufficient potassium improves inorganic phosphate-limited photosynthesis in Brassica napus by enhancing metabolic phosphorus fractions and Rubisco activity

Crop photosynthesis (A) and productivity are often limited by a combination of nutrient stresses, such that changes in the availability of one nutrient may affect the availability of another nutrient, in turn influencing A. In this study, we examined the synergistic effects of phosphorus (P) and potassium (K) on leaf A in a nutrient amendment experiment, in which P and K were added individually or in combination to Brassica napus grown under P and K co-limitation. The data revealed that the addition of P gradually removed the dominant limiting factor (i.e. the limited availability of P) and improved leaf A. Strikingly, the addition of K synergistically improved the overall uptake of P, mainly by boosting plant growth, and compensated for the physiological demand for P by prioritizing investment in metabolic pools of P (P-containing metabolites and inorganic phosphate, Pi). The enlarged pool of metabolically active P was partially associated with the upregulation of Pi regeneration through release from triose phosphates rather than replacement of P-containing lipids. This process mitigated P restrictions on A by maintaining the ATP/NADPH and NADPH/NADP⁺ ratios and increasing the content and activity of Rubisco. Our findings demonstrate that sufficient K increased Pi-limited A by enhancing metabolic P fractions and Rubisco activity. Thus, ionic synergism may be exploited to mitigate nutrient-limiting factors to improve crop productivity.

Potassium Deficiency in Rice Aggravates Sarocladium oryzae Infection and Ultimately Leads to Alterations in Endophyte Communities and Suppression of Nutrient Uptake

Sheath rot disease is an emerging fungal disease in rice, whose infection causes severe yield loss. Sarocladium oryzae (S. oryzae) is the major causal agent. Previous study has demonstrated that rice deficiency in potassium (K) aggravates S. oryzae infection. However, the effects of S. oryzae infection on the nutrient-uptake process, endophyte communities, and hormone level of host plant under K-deficiency condition remain unclear, the mechanism of K mediated S. oryzae infection needs to be further study. The present study analyzed alterations in the endophytic community and nutrient-uptake process of host plants through an exogenous inoculation of S. oryzae in pot and hydroponics experiments. S. oryzae infection sharply increased the relative abundance of Ascomycota and decreased the Shannon and Simpson index of the endophytic community. Compared with the K-sufficient rice infected with S. oryzae, K-starved rice infected with S. oryzae (-K + I) increased the relative abundance of Ascomycota in leaf sheaths by 52.3%. Likewise, the -K + I treatment significantly decreased the Shannon and Simpson indexes by 27.7 and 25.0%, respectively. Sufficient K supply increased the relative abundance of Pseudomonas spp. in the host plant. S. oryzae infection profoundly inhibited the nutrient uptake of the host plant. The accumulation of oleic acid and linoleic acid in diseased rice decreased the biosynthesis of jasmonic acid (JA), and the content of JA was lowest in the -K + I treatment, which suppressed K⁺ uptake. These results emphasize the importance of K in resistance to S. oryzae infection by modulating endophyte community diversity and enhancing the nutrient-uptake capacity of the host plant.

Unlocking the Changes of Phyllosphere Fungal Communities of Fishscale Bamboo (Phyllachora heterocladae) under Rhombic-Spot Disease Stressed Conditions

As an important nonwood bioresource, fishscale bamboo (Phyllachora heterocladae Oliver) is widely distributed in the subtropical region of China. Rhombic-spot disease, caused by Neostagonosporella sichuanensis, is one of the most serious diseases that threatens fishscale bamboo health. However, there is limited knowledge about how rhombic-spot disease influences the diversity and structures of phyllosphere fungal communities. In this study, we investigated the phyllosphere fungal communities from stems, branches, and leaves of fishscale bamboo during a rhombic-spot disease outbreak using 18S rRNA sequencing. We found that only the phyllosphere fungal community from stems was significantly affected by pathogen invasion in terms of community richness, diversity, and structure. FUNGuild analysis revealed that the major classifications of phyllosphere fungi based on trophic modes in stems, branches, and leaves changed from symbiotroph-pathotroph, no obvious dominant trophic mode, and symbiotroph to saprotroph, saprotroph–pathotroph–symbiotroph, and saprotroph–symbiotroph, respectively, after pathogen invasion. The fungal community composition of the three tissues displayed significant differences at the genus level between healthy and diseased plants. The associations among fungal species in diseased samples showed more complex co-occurrence network structures than those of healthy samples. Taken together, our results highlight the importance of plant pathological conditions for the assembly of phyllosphere fungal communities in different tissues.