Exogenous strigolactones alleviate low-temperature stress in peppers seedlings by reducing the degree of photoinhibition

Abstract

Background: The growth and yield of pepper, a typical temperature-loving vegetable, are limited by low-temperature environments. Using low-temperature sensitive 'Hangjiao No. 4' (Capsicum annuum L.) as experimental material, this study analyzed the changes in plant growth and photosynthesis under different treatments: normal control (NT), low-temperature stress alone (LT), low-temperature stress in strigolactone pretreated plants (SL_LT), and low-temperature stress in strigolactone biosynthesis inhibitor pretreated plants (Tis_LT).

Results: SL pretreatment increased the net photosynthetic rate (Pn) and PSII actual photochemical efficiency (φPSII), reducing the inhibition of LT on the growth of pepper by 17.44% (dry weight of shoot). Due to promoting the accumulation of carotenoids, such as lutein, and the de-epoxidation of the xanthophyll cycle [(Z + A)/(Z + A + V)] by strigolactone after long-term low-temperature stress (120 h), non-photochemical quenching (NPQ) of pepper was increased to reduce the excess excitation energy [(1-qP)/NPQ] and the photoinhibition degree (Fv/Fm) of pepper seedlings under long-term low-temperature stress was alleviated. Twelve cDNA libraries were constructed from pepper leaves by transcriptome sequencing. There were 8776 differentially expressed genes (DEGs), including 4473 (51.0%) upregulated and 4303 (49.0%) downregulated genes. Gene ontology pathway annotation showed that based on LT, the DEGs of SL_LT and Tis_LT were significantly enriched in the cellular component, which is mainly related to the photosystem and thylakoids. Further analysis of the porphyrin and chlorophyll biosynthesis, carotenoid biosynthesis, photosynthesis-antenna protein, and photosynthetic metabolic pathways and the Calvin cycle under low-temperature stress highlighted 18, 15, 21, 29, and 31 DEGs for further study, which were almost all highly expressed under SL_LT treatment and moderately expressed under LT treatment, whereas Tis_LT showed low expression.

Conclusion: The positive regulatory effect of SLs on the low-temperature tolerance of pepper seedlings was confirmed. This study provided new insights for the development of temperature-tolerant pepper lines through breeding programs.

Keywords: Chilling stress; Pepper; Photosystem; RNA-seq; Strigolactones; Xanthophyll cycle.

Fig. 1

Growth and photosynthesis pigments analysis of different treated pepper plants. (A) seedlings phenotype in different time points and (B) dry weight, (C) fresh weight, (D) chlorophyll a, (E) chlorophyll b, (F) total chlorophyll, (G) carotenoid content of seedlings after low-temperature stress 120 h. Data are means ± SD (n = 5), and different lowercase letters represent significant differences (P ≤ 0.05) among treatments

Fig. 2

Net photosynthetic rate and Calvin cycle enzymes analysis of different treated pepper plants. (A) Net photosynthetic rate, Pn. (B) Ketosaccharide-1, 5-diphosphate carboxylase, Rubisco. (C) glyceraldehyde-3-phosphate dehydrogenase, GAPDH. (D) Fructose-1, 6-diphosphate aldolase, FBA. (E) Fructose-1,6-bisphosphatase, FBPase. (F) Transketolase, TK. Data are mean ± SD (n = 5), and different lowercase letters indicate significant differences among treatments (P ≤ 0.05). * indicate significant differences at P ≤ 0.05 relative to LT pepper plants

Fig. 3

Chlorophyll fluorescence parameters and light response curve of different treated pepper plants. (A) Images of Fv/Fm. False colors represent values of the parameter ranging from 0 (black) to 1.0 (purple). (B) Maximum photochemical efficiencies of PSΠ, Fv/Fm. (C) Actual photochemical efficiency of PSII, φPSII. (D) Nonphotochemical quenching, NPQ. (E) Photochemical quenching, qP. (F) Excess excitation energy, (1-qP)/NPQ. (G) Photosynthetic electron transport rate, ETR. (H-L) Light response curves of pepper leaves under chilling stress for 0, 24, 48, 72, and 120 h. Data are mean ± SD (n = 3). * indicate significant differences at P ≤ 0.05 relative to LT pepper plants

Fig. 4

Photoprotective carotenoid content and the activity of xanthophyll cycle enzymes of different treated pepper plants. (A) lutein content, (B) zeaxanthin content, (C) antheraxanthin content, (D) vioxanthin content, (E) de-epoxidation degree of xanthophyll cycle, (F) zeaxanthin epoxidase (ZEP), and (G) violaxanthin de-epoxidase (VDE). Data are mean ± SD (n = 3). Different lowercase letters indicate significant differences among treatments (P ≤ 0.05). * indicate significant differences at P ≤ 0.05 relative to LT pepper plants

Fig. 5

ATP, endogenous strigolactone, and counts of differentially expressed genes. (A) ATP content, (B) Ca²⁺-ATPase activity, (C) Mg²⁺-ATPase activity, (D) endogenous strigolactone content, and (E) counts of differentially expressed genes under different treatments. Data are mean ± SD (n = 3). Different lowercase letters indicate significant differences among treatments (P ≤ 0.05). * indicate significant differences at P ≤ 0.05 relative to LT pepper plants

Fig. 6

GO enrichment analysis of DEGs among different treatment combinations under chilling stress

Fig. 7

KEGG enrichment analysis of DEGs among different treatment combinations under chilling stress. (A) SL_LT vs. LT, (B) Tis _LT vs. LT, and (C) SL_LT vs. Tis_LT

Fig. 8

Analysis of DGEs in (A) porphyrin and chlorophyll metabolism and (B) carotenoid biosynthesis of pepper. Colors indicate the expression values of the genes, which are presented as FPKM-normalized log₂ transformed counts

Fig. 9

Analysis of DGEs in (A) photosynthesis - antenna proteins, photosynthesis pathways and (B) Calvin cycle of pepper. Colors indicate the expression values of the genes, which are presented as FPKM-normalized log₂ transformed counts

Fig. 10

qRT-PCR validation of transcriptome data. The gray bar represents the qRT-PCR results and the different colored blocks represent the transcriptome gene expression results shown as Log₂(FPKM)