BackgroundAuxin plays important roles in hormone crosstalk and the plant’s stress response. The auxin-responsive Gretchen Hagen3 (GH3) gene family maintains hormonal homeostasis by conjugating excess indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonic acids (JAs) to amino acids during hormone- and stress-related signaling pathways. With the sequencing of the apple (Malus × domestica) genome completed, it is possible to carry out genomic studies on GH3 genes to indentify candidates with roles in abiotic/biotic stress responses.ResultsMalus sieversii Roem., an apple rootstock with strong drought tolerance and the ancestral species of cultivated apple species, was used as the experimental material. Following genome-wide computational and experimental identification of MdGH3 genes, we showed that MdGH3s were differentially expressed in the leaves and roots of M. sieversii and that some of these genes were significantly induced after various phytohormone and abiotic stress treatments. Given the role of GH3 in the negative feedback regulation of free IAA concentration, we examined whether phytohormones and abiotic stresses could alter the endogenous auxin level. By analyzing the GUS activity of DR5::GUS-transformed Arabidopsis seedlings, we showed that ABA, SA, salt, and cold treatments suppressed the auxin response. These findings suggest that other phytohormones and abiotic stress factors might alter endogenous auxin levels.ConclusionPrevious studies showed that GH3 genes regulate hormonal homeostasis. Our study indicated that some GH3 genes were significantly induced in M. sieversii after various phytohormone and abiotic stress treatments, and that ABA, SA, salt, and cold treatments reduce the endogenous level of axuin. Taken together, this study provides evidence that GH3 genes play important roles in the crosstalk between auxin, other phytohormones, and the abiotic stress response by maintaining auxin homeostasis.
Sorbitol, a major end-product of photosynthesis in many species of the Rosaceae family, accumulates in response to abiotic stressors. However, the relationship that arises between the expression of sorbitol transporters and sorbitol accumulation under abiotic stress remains unclear. In this study, micropropagated 'Fuji' apple plants (Malus domestica Borkh. 'Fuji') were exposed to two varying degrees of osmotic stress and compared relative to an unstressed control. The osmotic stress was generated by adding PEG 6000 into full-strength Hoagland solution and adjusted the osmotic potential to either −0.75 MPa (mild drought stress [MIS]) or −1.5 MPa (severe drought stress [SES]). Analysis of sorbitol levels via high performance liquid chromatography (HPLC) showed that the sorbitol concentration was elevated in roots, phloem tissues and leaves in both the MIS and SES treatments compared to controls for the entire duration of the experiment. Three cDNA sequences, encoding sorbitol transporters (MdSOT3, MdSOT4 and MdSOT5), were isolated from leaves. Realtime quantitative PCR (RT-qPCR) data suggests that the expression levels of MdSOT3 and MdSOT5 were higher under MIS and SES in roots, phloem tissues and leaves compared to unstressed controls. The average mRNA levels of MdSOT4 in phloem tissues declined under both drought treatments (with the exception being at 2 h of SES). In roots and leaves under SES, mRNA production was increased. These results indicate that the up-regulation of MdSOT3 and MdSOT5 expression is consistent with the accumulation of sorbitol under conditions of osmotic stress in apple plants. They enhanced drought tolerance in vegetative tissues. Increased MdSOT4 mRNA enhanced drought tolerance under SES.
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