A leucine-rich repeat-receptor-like kinase gene SbER2-1 from sorghum (Sorghum bicolor L.) confers drought tolerance in maize

Abstract

Background: ERECTA (ER) is a leucine-rich repeat-receptor-like kinase gene (LRR-RLK) encoding a protein isolated from Arabidopsis. Although the regulatory functions of ER genes have been widely explored in plant development and disease resistance, their roles in drought stress responses remain to be clarified.

Results: In this study, we cloned and characterized two ER genes, SbER1-1 and SbER2-1, from the drought-tolerant model plant sorghum (Sorghum bicolor L.). Under drought stress, the two genes were expressed in the leaves and stems but not in the roots, and SbER2-1 transcript accumulation in the stem was increased. SbER2-1 was localized both on the plasma membrane and in the chloroplast. Moreover, SbER2-1 expression in Arabidopsis and maize conferred increased drought tolerance, especially in regard to water-use efficiency, increasing the net photosynthetic rate in maize under drought stress. Based on RNA-Seq analysis together with the physiological data, we conclude that the transgenic maize plants have upregulated phenylpropanoid metabolism and increased lignin accumulation under drought stress.

Conclusions: Our results demonstrate that SbER2-1 plays an important role in response to drought stress. Furthermore, photosynthetic systems and phenylpropanoid metabolism are implicated in SbER2-1-mediated drought stress tolerance mechanisms. The use of genetic engineering to regulate SbER2-1 expression in plants and to breed new varieties tolerant to drought is a research field full of potential.

Keywords: Drought; Lignin accumulation; Maize; SbER2–1; Water-use efficiency.

Fig. 1

Sequence characteristics and phylogenetic analyses of sorghum ER family genes. a The exon-intron structure of sorghum ER family genes. b Phylogenetic tree of ER family genes in Arabidopsis (At), maize (Zm), rice (Os), and sorghum (Sb)

Fig. 2

Seedling phenotypes of six sorghum varieties under drought stress. Each variety was subjected to three conditions: from left to right in each photograph, well-watered (WW), moderate drought stress (MS), and severe drought stress (SS)

Fig. 3

Expression analyses of SbER genes. a Expression of SbER genes in the shoots and roots. zSSIIb is an endogenous reference gene in maize. b Expression of SbER1and SbER2 in well-watered (WW) and drought-treated (MS and SS) seedlings of sorghum. c Percentage of up- and downregulated SbER1 genes in sorghum seedlings under the same three conditions. d Percentage of up- and downregulated SbER2 gene in sorghum seedlings under the same three conditions

Fig. 4

Subcellular localization of SbER2–1 in Arabidopsis. Mesophyll protoplasts from Arabidopsis transfected with 35S::eGFP or 35S::SbER2–1-eGFP were examined by confocal fluorescence microscopy. Scale bars = 5 μm. Representative images are shown

Fig. 5

Phenotypes, molecular identification and SbER2–1 expression level in transgenic plants. a Phenotype identification of T₃ Arabidopsis plant transformed with SbER2–1. NT, non-transgenic (wild type); OE-SbER2–1, transgenic plants; CG, control group with normal irrigation; EG, experimental group treated with PEG. b Plant aboveground phenotypes of transgenic maize lines VE2–1 and VE2–2, as well as NT ZPM9 (CK), under WW (left), MS (middle), and SS (right) conditions. c PCR detection of transgenic lines by agarose gel electrophoresis. Transgenic plants (1–5 lane) and positive control (+) show amplified 506-bp fragment; negative control (−) and blank control (b) had no amplified fragment. d SbER2–1 expression level of transgenic maize plants under WW, MS, and SS conditions. e, f Drought-tolerance index (DTI) of shoot biomass of VE2–1, VE2–2, and ZPM9 maize under MS (e) and SS (f) conditions, respectively

Fig. 6

Analysis of differentially expressed genes (DEGs) between the non-transgenic maize line ZPM9 and the SbER2–1-overexpressing lines. a DEGs in leaves (left) and stems (right). b Wynn map analysis of DEGs in leaves (left) and stems (right)

Fig. 7

Functional enrichment analysis of DEGs between the non-transgenic ZPM9 and SbER2–1 overexpression maize lines. a GO Functional enrichment analysis of DEGs in leaves (left) and stems (right). b KEGG pathway enrichment analysis of the pathways of DEGs in leaves (left) and stems (right)

Fig. 8

Phenotypic, physiological and biochemical characteristics of the SbER2–1 overexpression maize lines VE2–1, VE2–2 and the non-transgenic receptor line ZPM9 under drought stress. a Leaf relative water content. b, c, d, e Physiological parameters related to photosynthesis and transpiration: stomatal conductance, transpiration rate, net photosynthetic rate and WUE. f, g, h Soluble sugar, MDA and proline contents. i, j Lignin contents in stems and leaves, respectively