Global climate change has recently become increasingly dramatic, with extreme erratic weather events commonly occurring (Field and Barros 2014; Ummenhofer and Meehl 2017). Salt marsh wetlands at the interface of the sea and inland are among the most fragile ecosystems due to their unique coastal location (Gabler et al. 2017; Sarika and Zikos 2020; Wu et al. 2020). The rise in sea level due to climate warming, and increased drought conditions in inland areas cause immense directional differences in soil moisture contents of the salt marsh wetland from the coastal to interior areas (Voesenek and Bailey-Serres 2015; Maxwell et al. 2019). The underground plant root system is the first organ to sense environmental stresses such as alteration of soil moisture content, and their growth and development are a consequence of the combined effects of genetic mechanisms and external biotic and/or abiotic factors (Sengupta et al. 2011; Opitz et al. 2016).
In the Soil-Plant-Atmosphere Continuum system, plant roots form the largest source of resistance to liquid water flow. The root hydraulic resistance consists of radial and axial resistance components, and the former is the absorption resistance of water transported in the radial direction from the root-soil interface to the root xylem in the soil-plant system, which is crucial for root-drought resistance (Lynch 2022; Lynch et al. 2022). Axial resistance is generated by the axial transport of water in the roots along the xylem pipeline, which is mainly composed of mature tracheids and vessels. Of the two resistance components, the effect of axial resistance is very small and generally insignificant (Cruz et al. 1992). Regardless of radial or longitudinal water transfer, flow resistance often depends on the root anatomy (Knipfer and Fricke 2011).
The root hydraulic structure is composed of root growth and water transport properties, which tandemly affects plant water acquisition capacity under changing or non-uniform soil conditions (Tang et al. 2018; Maurel and Nacry 2020; Cai et al. 2022). Hydraulic conductivity (Lpr) is the ratio of water flux through the root to water potential difference between root xylem and root surface soil. It is an important hydraulic parameter for characterizing the capacity of plant water absorption and can be expressed at the whole root, single root, and cell levels (Mu et al. 2006; Bramley et al. 2009). Hydraulic conductivity of the root system is influenced by multiple root system architecture traits(Strock et al. 2021), such as root hairs (Cai and Ahmed 2022), root diameter (Liao et al. 2022), and root length / root length density (Kato and Okami 2011),.and root anatomical properties (Cornelis and Hazak 2022).
Numerous studies have shown that abscisic acid (ABA) is a vital regulator of plant drought resistance (Parent et al. 2009; Wilkinson et al. 2012; Zhang et al. 2022). ABA is produced in the roots, released into the xylem and distantly translocated to the leaves to regulate the stomata and alter the hydraulic conditions of the whole plant (Jiang and Hartung 2007). Several enzymes are involved in ABA synthesis, including 9-cis-epoxide dioxygenase (NCED), cytoplasmic short chain dehydrogenase/reductase, and aldehyde oxidase (AAO) (Ren et al. 2021). The hydraulic action of ABA is mediated mainly by the tissue-specific regulation of aquaporins (AQPs) (Rosales et al. 2019). However, the effects of ABA on hydraulic conductivity under water deficit remain controversial (Parent et al. 2009; Raghavendra et al. 2010; Golldack et al. 2014).
AQPs are essential transmembrane transporters are distributed in the plant membrane system, which regulate hydrodynamic conductance at the cellular level (Törnroth-Horsefield et al. 2006; Dayer et al. 2020). AQPs consists of five subclasses, including PIPs, tonoplast intrinsic proteins (TIPs), small basic intrinsic proteins (SIPs), nodulin 26-like intrinsic proteins (NIPs), and X intrinsic proteins (Maurel et al. 2008; Wang et al. 2014). AQPs are plasma membrane protein, and have been implicated in the process of plant tolerance to water deficit stress (Aroca et al. 2007; Domec et al. 2021). For example, blocking aquaporin activity by different inhibitors significantly reduced root hydraulic conductivity in Arabidopsis (Sutka et al. 2011). Changes in radial transport in barley (Hordeum vulgare), which in turn altered root hydraulic conductivity mainly caused by AQPs (Knipfer and Fricke 2011; Maurel and Nacry 2020). Overexpression of PIP2;9 in transgenic soybean plants significantly improved drought resistance and increased yield relative to wild-type plants (Lu et al. 2018). However, AQPs have mainly been investigated in food crops, with few studies reported on their roles in wetland plants.
Growth and formation of plant roots under drought conditions is currently a trending research area in plants, such as Arabidopsis thaliana (Rasheed et al. 2016), maize (Zea Mays) (Wang et al. 2016), wheat (Triticum aestivum) (Hu et al. 2018a) and sorghum (Sorghum bicolor) (Assefa et al. 2010), while such studies remain limited in wetland plants. Common reed (Phragmites australis) is an important commercial and a widely distributed perennial grass species in salt marsh wetlands all over the world (Köbbing et al. 2013). Currently, common reed is primarily studied under flooding conditions, with most studies focusing on the changes in aboveground physiological and ecological characteristics. However, research gaps on common reed root responses to soil water conditions still exist (Hellings and Gallagher 1992; Pagter et al. 2005).
This study aimed to investigate alteration in water absorption performance, root morphology, and anatomical structure of common reed root system under different soil moisture contents, and explore the responding mechanism of wetland plant roots to water stress based on the activities of ABA and AQPs as revealed by transcriptome sequencing.