Fast Regulation of Hormone Metabolism Contributes to Salt Tolerance in Rice (Oryzasativa spp. Japonica, L.) by Inducing Specific Morpho-Physiological Responses

Abstract

Clear evidence has highlighted a role for hormones in the plant stress response, including salt stress. Interplay and cross-talk among different hormonal pathways are of vital importance in abiotic stress tolerance. A genome-wide transcriptional analysis was performed on leaves and roots of three-day salt treated and untreated plants of two Italian rice varieties, Baldo and Vialone Nano, which differ in salt sensitivity. Genes correlated with hormonal pathways were identified and analyzed. The contents of abscisic acid, indoleacetic acid, cytokinins, and gibberellins were measured in roots, stems, and leaves of seedlings exposed for one and three days to salt stress. From the transcriptomic analysis, a huge number of genes emerged as being involved in hormone regulation in response to salt stress. The expression profile of genes involved in biosynthesis, signaling, response, catabolism, and conjugation of phytohormones was analyzed and integrated with the measurements of hormones in roots, stems, and leaves of seedlings. Significant changes in the hormone levels, along with differences in morphological responses, emerged between the two varieties. These results support the faster regulation of hormones metabolism in the tolerant variety that allows a prompt growth reprogramming and the setting up of an acclimation program, leading to specific morpho-physiological responses and growth recovery.

Keywords: Oryza sativa; RNA sequencing; phenotypic plasticity; phytohormones; salt stress.

Figure 1

Baldo and Vialone Nano plants grown in hydroponics. (A,B,E,F) Untreated plants (CNT). (C,D,G,H) Plants at stage V2 of growth were treated with a saline solution containing 100 mM NaCl, for (C,G) three or (D,H) six days. Baldo-treated plants showed a resuming of growth at six days (D). (G) VN plants showed the yellowish of second leaves at day three and (H) died after six days of treatment. Bar = 3 cm.

Figure 2

Differentially expressed genes (DEGs) grouped by functional categories. DEGs were re-annotated by using Argot2 tool and grouped in functional categories basing on their biological process ontology terms. (A,B) Up- and down-regulated genes in leaves of three-day treated plants. (C,D) Up- and down-regulated genes in roots of three-day treated plants.

Figure 3

Number of DEGs involved in hormones metabolism in leaves of three-day treated plants. Genes belonging to gene ontology (GO) terms associated with hormonal biosynthesis, signaling, response, catabolism, and conjugation were gathered from RNAseq data [47].

Figure 4

Number of DEGs involved in hormones metabolism in roots of three-day treated plants. Genes belonging to GO terms associated with hormonal biosynthesis, signaling, response, catabolism, and conjugation were gathered from RNAseq data [47].

Figure 5

Hormone contents in plants treated for one day. Plants at the V2 stage of growth were treated with 100 mM NaCl. The content of main hormones in different organs was measured by mass spectrometry after one day. Data are expressed as mean ± SD, n = 6, * p < 0.05, ** p < 0.01, *** p < 0.001.

Figure 6

Hormone contents in plants treated for three days. Plants at the V2 stage of growth were treated with 100 mM NaCl. The content of main hormones in different organs was measured by mass spectrometry after three days. Data are expressed as mean ± SD, n = 6, * p < 0.05, ** p < 0.01, *** p < 0.001.

Figure 7

Stomata aperture and growth parameters measurements. (A) Stomata aperture was measured as the elongation of the stomatal complex. The longer the stomata, the smaller the stomata aperture. (B) Daily growth of the first inter-collar region. (C) Third and fourth leaf lengths at different timepoints under stress conditions. Data are expressed as mean ± SD, n = 6. * p < 0.05; ** p < 0.01. (D) Root total length and (E) topological index of the sensitive (black symbols) and tolerant (empty symbols) cultivars are shown in presence (triangles) and absence (circles) of salt. The tolerant cultivar shows a change in root structure architecture (RSA) in response to stress (E, arrow), while the sensitive plants display an irreversible growth arrest (D). Data are expressed as mean ± SEM, n = 9, p < 0.01.