Silencing MdGH3-2/12 in apple reduces drought resistance by regulating AM colonization.

Drought leads to reductions in plant growth and crop yields. Arbuscular mycorrhizal fungi (AMF), which form symbioses with the roots of the most important crop species, alleviate drought stress in plants. In the present work, we identified 14 GH3 genes in apple (Malus domestica) and provided evidence that MdGH3-2 and MdGH3-12 play important roles during AM symbiosis. The expression of both MdGH3-2 and MdGH3-12 was upregulated during mycorrhization, and the silencing of MdGH3-2/12 had a negative impact on AM colonization. MdGH3-2/12 silencing resulted in the downregulation of five genes involved in strigolactone synthesis, and there was a corresponding change in root strigolactone content. Furthermore, we observed lower root dry weights in RNAi lines under AM inoculation conditions. Mycorrhizal transgenic plants showed greater sensitivity to drought stress than WT, as indicated by their higher relative electrolytic leakage and lower relative water contents, osmotic adjustment ability, ROS scavenging ability, photosynthetic capacity, chlorophyll fluorescence values, and abscisic acid contents. Taken together, these data demonstrate that MdGH3-2/12 plays an important role in AM symbiosis and drought stress tolerance in apple.

Silencing MdGH3-2/12 in apple reduces cadmium resistance via the regulation of AM colonization.

Arbuscular mycorrhizal fungi (AMF) can form a symbiotic relationship with most terrestrial plant roots, promote plant growth, and heavy metal (HM) tolerance and thus plays a crucial role in phytoremediation. However, research on the relationship between colonization level and HM tolerance is limited. In this study, apple (Malus domestica) Gretchen Hagen3 genes MdGH3-2/12 silencing plants were treated with four AMF and Cd combination treatments to determine AMF colonization levels, biomass, Cd accumulation, photosynthesis, fluorescence, reactive oxygen species (ROS) and antioxidant substance accumulation, and Cd uptake, transport and detoxification gene expression levels. Results indicate the greater sensitivity of transgenic plants under AMF inoculation and Cd treatment compared with wild type (WT) via lower AMF colonization levels, biomass accumulation, photosynthetic parameters, and the accumulation and clearance homeostasis of ROS, as well as lower detoxification expression levels and higher Cd uptake and transport expression levels. Our study essentially demonstrates that MdGH3-2/12 plays an important role in Cd stress tolerance by regulating AM colonization in apple.

Overexpression of MdIAA24 improves apple drought resistance by positively regulating strigolactone biosynthesis and mycorrhization.

Most land plant species have the ability to establish a symbiosis with arbuscular mycorrhizal (AM) fungi. These fungi penetrate into root cortical cells and form branched structures (known as arbuscules) for nutrient exchange. We cloned the MdIAA24 from apple (Malus domestica) following its up-regulation during AM symbiosis. Results demonstrate the positive impact of the overexpression (OE) of MdIAA24 in apple on AM colonization. We observed the strigolactone (SL) synthesis genes, including MdD27, MdCCD7, MdCCD8a, MdCCD8b and MdMAXa, to be up-regulated in the OE lines. Thus, the OE lines exhibited both a higher SL content and colonization rate. Furthermore, we observed that the OE lines were able to maintain better growth parameters under AM inoculation conditions. Under drought stress with the AM inoculation, the OE lines were less damaged, which was demonstrated by a higher relative water content, a lower relative electrolytic leakage, a greater osmotic adjustment, a higher reactive oxygen species scavenging ability, an improved gas exchange capacity and an increased chlorophyll fluorescence performance. Our findings demonstrate that the OE of MdIAA24 in apple positively regulates the synthesis of SL and the formation of arbuscules as a drought stress coping mechanism.

Arbuscular mycorrhizal fungi enhanced drought resistance in apple by regulating genes in the MAPK pathway.

Arbuscular mycorrhizal fungi (AMF) can form a symbiotic relationships with most terrestrial plants and play an important role in plant growth and adaptation to various stresses. To study the role of AMF in regulating drought resistance in apple, the effects of drought stress on Malus hupehensis inoculated with AMF were investigated. Inoculation of AMF enhanced apple plants growth. Mycorrhizal plants had higher total chlorophyll concentrations but lower relative electrolyte leakage under drought stress. Mycorrhizal plants increased net photosynthetic rate, stomatal conductance, and transpiration rate under drought stress, however, they showed lower inhibition in the quantum yield of PSII photochemistry. Mycorrhizal plants also had higher superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzyme activities under drought conditions. Thus, mycorrhizal plants had lower accumulated MDA, H2O2, and O2- than non-mycorrhizal seedlings. Total sugar and proline concentrations also significantly increased, helping maintain the osmotic balance. Furthermore, mitogen-activated protein kinase (MAPK) cascades, which participate in the regulation of responses of plants and microorganisms to biotic and abiotic stress, were up-regulated in apple plants and AMF during drought. We saw that there were at least two motifs that were identical in MAPK proteins and many elements that responded to hormones and stress from these MAPK genes. In summary, our results showed that mycorrhizal colonization enhanced apple drought tolerance by improving gas exchange capacity, increasing chlorophyll fluorescence parameters, creating a greater osmotic adjustment capacity, increasing scavenging of reactive oxygen species (ROS), and using MAPK signals for interactions between AMF and their apple plant hosts.

Overexpression of MdIAA9 confers high tolerance to osmotic stress in transgenic tobacco.

Auxin is a plant hormone that takes part in a series of developmental and physiological processes. There are three major gene families that play a role in the early response of auxin and auxin/indole-3-acetic acid (Aux/IAA) is one of these. Although the genomic organization and function of Aux/IAA genes have been recognized in reference plants there have only been a few focused studies conducted with non-model crop plants, especially in the woody perennial species. We conducted a genomic census and expression analysis of Aux/IAA genes in the cultivated apple (Malus × domestica Borkh.). The Aux/IAA gene family of the apple genome was identified and analyzed in this study. Phylogenetic analysis showed that MdIAAs could be categorized into nine subfamilies and that these MdIAA proteins contained four whole or partially conserved domains of the MdIAA family. The spatio-specific expression profiles showed that most of the MdIAAs were preferentially expressed in specific tissues. Some of these genes were significantly induced by treatments with one or more abiotic stresses. The overexpression of MdIAA9 in tobacco (Nicotiana tabacum L.) plants significantly increased their tolerance to osmotic stresses. Our cumulative data supports the interactions between abiotic stresses and plant hormones and provides a theoretical basis for the mechanism of Aux/IAA and drought resistance in apples.