A Transcriptome Analysis Revealing the New Insight of Green Light on Tomato Plant Growth and Drought Stress Tolerance

Abstract

Light plays a pivotal role in plant growth, development, and stress responses. Green light has been reported to enhance plant drought tolerance via stomatal regulation. However, the mechanisms of green light-induced drought tolerance in plants remain elusive. To uncover those mechanisms, we investigated the molecular responses of tomato plants under monochromatic red, blue, and green light spectrum with drought and well-water conditions using a comparative transcriptomic approach. The results showed that compared with monochromatic red and blue light treated plants, green light alleviated the drought-induced inhibition of plant growth and photosynthetic capacity, and induced lower stomatal aperture and higher ABA accumulation in tomato leaves after 9 days of drought stress. A total of 3,850 differentially expressed genes (DEGs) was identified in tomato leaves through pairwise comparisons. Functional annotations revealed that those DEGs responses to green light under drought stress were enriched in plant hormone signal transduction, phototransduction, and calcium signaling pathway. The DEGs involved in ABA synthesis and ABA signal transduction both participated in the green light-induced drought tolerance of tomato plants. Compared with ABA signal transduction, more DEGs related to ABA synthesis were detected under different light spectral treatments. The bZIP transcription factor- HY5 was found to play a vital role in green light-induced drought responses. Furthermore, other transcription factors, including WRKY46 and WRKY81 might participate in the regulation of stomatal aperture and ABA accumulation under green light. Taken together, the results of this study might expand our understanding of green light-modulated tomato drought tolerance via regulating ABA accumulation and stomatal aperture.

Keywords: ABA; drought stress; green light; stomatal aperture; tomato; transcriptome.

Figure 1

The photosynthetic response of tomato seedlings under different water and spectral conditions. (A,B), the changes of net photosynthetic rate (A_net) to increased light intensity; (C,D), the changes of photosystem II (PSII) quantum efficiency (ϕ_PSII). RW, BW, and GW: well-water combined with red, blue, and green LED light, respectively; RD, BD, and GD: drought stress combined with red, blue, and green LED light, respectively. The photosynthetic photon flux density (PPFD) for all the treatments was 200 μmol m⁻² s⁻¹.

Figure 2

The stomatal response, water use efficiency (WUE), and abscisic acid (ABA) content in leaves of plants under different water and light spectra condition. (A,B), The responses of stomatal conductance (g_s) and transpiration rate (Tr) of plant leaves; (C), The instantaneous WUE; (D), stomatal aperture; (E), ABA content in plant leaves after 9 days of treatment. RW, BW, and GW: well-water combined with red, blue, and green LED light, respectively; RD, BD, and GD: drought stress combined with red, blue, and green LED light, respectively. The PPFD for all the treatments was 200 μmol m⁻² s⁻¹.

Figure 3

The electrolyte leakage (A), relative water content [RWC, (B)], and phenotype (C,D) of plants were treated with different water and light spectral condition for 9 days. RW, BW, and GW: well-water combined with red, blue, and green LED light, respectively; RD, BD, and GD: drought stress combined with red, blue, and green LED light, respectively. The PPFD for all the treatments was 200 μmol m⁻² s⁻¹.

Figure 4

The changes in gene expression profiles of tomato treated with different light spectra and water conditions. (A), The number of DEGs between treatments; (B,C), Venn diagram presenting up-regulated and down-regulated genes among plants exposed to different light spectra under drought conditions; (D), Hierarchical clustering of DEGs, fragments per kilobase of transcript per million fragments mapped (FPKM), and relative expression of DEGs between different light and water treatments were calculated as log₂ (FPKM) of differentially expressed genes, log₂ (FPKM) of differentially expressed genes were calculated. A scale indicating the color assigned to log₂ (FPKM) is shown to the right of the cluster.

Figure 5

The GO enrichment analysis of DEGs in the comparisons of GD vs. BD (A), GD vs RD (B), and RD vs. BD (C) in three main categories. BP, biological process; CC, cellular component; MF, molecular function.

Figure 6

KEGG pathway enrichment analysis of the annotated DEGs in the comparisons of GD vs BD (A), GD vs. RD (B), and RD vs. BD (C). The Y-axis indicates the KEGG pathway, while the X-axis presents the enrichment factor, which is the ratio of DEGs enriched to specific KEGG pathways to DEGs enriched to all KEGG pathways. The dot size and the dot color indicate the number of DEGs of the pathway and q value, respectively.

Figure 7

Protein-protein interaction (PPI) network analysis based on the DEDs in the comparisons of “GD vs. BD” (A) and “RD vs. BD” (B). Nodes depict proteins encoded by related genes and PPI are represented by edges in the network; the top five hub DEDs encoded proteins are represented by various colors (red: high rank; Yellow: low rank) in the PPI networks.

Figure 8

The heatmap diagram of relative expression profiles of DEGs involved in ABA synthesis and ABA signaling transduction in response to red, blue, and green light under drought conditions (A) and well-water conditions (B). Gene expression is shown in a heatmap with color scale representing log₂ (FPKM) (blue: low expression level; red: high expression level). A scale indicating the color assigned to log₂ (FPKM) is shown to the right of the heatmap.

Figure 9

A proposed model of green light enhances tomato drought tolerance.