Arbuscular Mycorrhizal Network Mediate The Transmission Of Abscisic Acid Signaling Between Tomato Plants

Drought is one of the most important environmental stresses that influence plant growth and development.At the same time,the plants have developed a series of stress resistance mechanisms in the process of long-term evolution.Abscisic acid?ABA?plays an important role in abiotic stresses such as drought.Our previous study showed that arbuscular mycorrhizal networks?AMNs?widely existing in soils can function as a plant-plant underground communication channel where disease/insect defence signals can be transferred between the healthy and its neighbouring disease/insect-attacked tomato plants,which triggers its defence responses.However,whether the drought defense signal can be transferred from ABA-treated donor tomato plants to neighbouring healthy receiver tomato plants by AMNS is unknown.In fact,whether plants can perceive drought stress signal timely and effectively is vital for plant tolerance to drought stress.In this study,a sub-chamber method was used to establish AMNs between tomato plants in the same pot inoculated with mycorrhizal fungus Rhizophagus irregularis,a‘donor'plant was treated with abscisic acid?ABA?,changes in stomatal conductance?g_s?and photosynthetic rate?P_n?,the intercellular CO₂ concentration?C_i?and transpiration rate?T_r?,the dry weight and plant height,the activities of defense resistant enzymes?SOD,POD?,contents of proline and soluble sugar,MDA and gene expression levels were analyzed in the receiver tomato plant with or without connection by AMNs with the drought-challenged plant.The main findings are as follows:?1?Arbuscular mycorrhizal fungus?AMF?R.irregularis was inoculated in the roots of donor tomato plants for 35 days.Using trypan blue staining,WGA-488 fluorescence staining mycorrhizal infection rate was measured in donors and receivers.The structure of arbuscular branches,vesicles and hyphae were detected in tomato roots,and the mycorrhizal infection rate reached up to 38%,indicating that the mycorrhizal mycelium network was successfully established between donor and recipient tomato;?2?After the donor tomato was sprayed with 100?M ABA for 24 h,the stomatal conductance?g_s?,intercellular CO₂ concentration?C_i?and transpiration rate?T_r?of the receiver tomato plants of treatment A?linked to the donor by AMNs?were decreased significantly.And the plant water use efficiency?WUE?increased significantly.This may be due to the AMN-connected receiver tomato can preceive the donor's drought stress signal,and regulating the leaf stomatal closure and reducing water dissipation to improve water use efficiency;It was found that receiver tomato plants of treatment A had significantly higher POD activity than B,C and D treatments at 24 h.There was no significant difference in SOD activity,MDA content and soluble sugar content between the treatment groups,which means that the ABA of donor plants had no obvious direct induction effect on the recipient plants;The ABA synthesis pathway gene S1AO and SlDXR in AMNs-linked receiver plant root of treatment A was significantly induced compared with the B,C,and D treatment groups.?3?After spraying 100?M ABA on donor tomato plant,and meanwhile,the receiver tomato was treated with water-deficient,we found that the drought-resistance ability of the receiver tomato plants of treatment A was significantly enhanced.Compared with other three controls,the whole plant was more erect and grew normally.It was found that the activities of POD and SOD in AMNs-linked receiver plant were increased rapidly and was significantly higher than that of the treatment B,C,and D,while the content of malondialdehyde content?MDA?was significantly lower than that of the B,C,and D treatment groups,indicating that after spraying ABA in donor plant,the AMNs-linked receiver plant perceived the drought signal in advance,and the receiver plants will be in“primed stated”.Primed plants display faster and/or stronger activation of various cellular defense responses following attack by abiotic stress;This study reveals the natural mystery that plants can transmit drought signals via a AMNs.This network is widely found in soils and plant communities,which enables plants to perceive the drought stress signals of the surrounding environment in advance,and prepare for drought resistance in advance,thereby improve the drought resistance of plants.This study is very important to reveal underground plant-plant communications,ecological functions of AMNs and mechanism of plant environmental stress perception and signal transfer between plants.