Rice pyramided line IRBB67 (Xa4/Xa7) homeostasis under combined stress of high temperature and bacterial blight

Abstract

Rice bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) implies substantial yield loss to rice. In times of climate change, increasing temperatures are observed and further acceleration is expected worldwide. Increasing temperature often turns into inhibition of host plant defense to pathogens. Recently, a reduced resistance in rice IRBB4 carrying Xa4, but an increase in resistance in IRBB7 carrying Xa7 resistance by increasing temperature has been reported. Influence of high temperature on both R genes (Xa4+Xa7) combined in IRBB67 was analyzed under growth chamber conditions and transcriptomic analysis performed. The pyramided line IRBB67 showed no differences in lesion length between both temperature regimes, demonstrating that non-effectiveness of Xa4 at high temperature did not affect IRBB67 resistance. Moreover, Xa4 complements Xa7 resistance with no Xoo spread in planta beyond the symptomatic area under both temperature regimes in IRBB67. Time course transcriptomic analysis revealed that temperature enhanced IRBB67 resistance to combined heat and Xoo. Our findings highlight altered cellular compartments and point at a role of the cell wall involved in Xoo resistance and heat stress tolerance in both susceptible (IR24) and the resistant (IRBB67) NILs. Interestingly, up-regulation of trehalose-6-phosphatase gene and low affinity cation transporter in IRBB67 suggest that IRBB67 maintained a certain homeostasis under high temperature which may have enhanced its resistance. The interplay of both heat stress and Xoo responses as determined by up-regulated and down-regulated genes demonstrates how resistant plants cope with combined biotic and abiotic stresses.

Figure 1

(A) Plant height under low and high temperature regimes conditions. Plant height was recorded at 32 days old plants by measuring the length from soil surface to the tip of the most expanded leaf. Plants under high temperature conditions were significantly higher compared to low temperature. The letters a and b represent the significant differences between low and high temperature on each NIL as determined by least significant differences means (LSD means) at α < 0.05. (B) Picture of rice NIL IR24 under both temperature regimes. Plants under high temperature conditions were significantly higher compared to low temperature. (C) Bacterial blight lesion length under low and high temperature regimes conditions. Bacterial blight lesion length was recorded at 32 days old (11dpi). The letters a and b represent the significant differences between low and high temperature on each NIL as determined by least significant differences means (LSD means) at α < 0.05. (D) PXO145 spread in planta in four rice NILs under two temperature regimes. Data were collected from leaf clipping inoculated leaves of 21 days old plants and collected 11 days after inoculation. A, B and C represent leaf segments of 5 cm length beyond bacterial blight symptomatic area. The letters a and b represent the significant differences between low and high temperature on each NIL as determined by least significant differences means (LSD means) at α < 0.05.

Figure 2

(A) Rice NILs IR24, IRBB4, IRBB7 and IRBB67 showing bacterial blight lesion length progression under low temperature regime. (B) Rice NILs IR24, IRBB4, IRBB7 and IRBB67 showing bacterial blight lesion length progression under high temperature regime.

Figure 3

(A) Venn diagram showing the distribution of DEGs between Xoo and mock inoculated samples under both temperature regimes at 3 hpi. The Venn diagrams were created using jvenny. (B) Venn diagram showing the distribution of DEGs between Xoo and mock inoculated samples under both temperature regimes at 72 hpi. The Venn diagrams were created using jvenny. (C) Venn diagram showing the distribution of DEGs between Xoo and mock inoculated samples under both temperature regimes at 120 hpi. The Venn diagrams were created using jvenny.

Figure 4

GO enrichment in DEGs from high vs low temperature conditions after mock and Xoo inoculation. The enriched GO terms across the three time points and from both NILs (IR24 and IRBB67) as predicted using AgriGO tools Parametric Analysis of Gene Set Enrichment (PAGE) at p-value 0.05.

Figure 5

GO enrichment analysis of 188 DEGs expressed between IRBB67 and IR24 under high temperature after Xoo inoculation as predicted using AgriGO tools Parametric Analysis of Gene Set Enrichment (PAGE) at p-value 0.05.

Figure 6

Validation of RNA-Seq data by qRT–PCR.