INTRODUCTION
Environmental stresses frequently affect plant growth and development. Among the multitudinous adverse factors, drought is a severe environmental factor constraining plant growth and development (Zhu, 2016; Zhu, 2002). Plants have evolved complex mechanisms to cope with drought, including morphological and physiological mechanisms (Zhu, 2002; Bohnert et al. , 2006). For example, plants can improve drought resistance by closing their stomata to decrease water loss from leaves under drought stress (Zhu, 2002). The movement (opening and closure) of the stomatal aperture, which is induced by many factors including abscisic acid (ABA), salicylic acid (SA), CO2, reactive oxygen species (ROS), and water status, controls both the influx of CO2 and water loss through transpiration to the atmosphere (Zhao et al ., 2015; Ullah et al ., 2018). Under drought conditions, ROS accumulate rapidly and act as an important second messenger in stomatal movement (Munemasa et al. , 2007), leading to rapid stomatal closure (Maija et al ., 2016; Singhaet al ., 2017) to reduce water loss. Salicylic acid is widely considered to enhance plant defense responses against pathogens (Vermaet al. , 2016; Lee et al. , 2010). It also plays roles in the plant response to abiotic stresses, such as chilling, heat, drought, salt, ultraviolet radiation, and heavy metals (Janda et al., 1999; Senaratna et al. , 2000; Munné-Bosch and Peñuelas, 2003; Chiniet al. , 2004; Hayata et al. , 2010). Previous studies have shown that treatment with SA increases tolerance to drought stress (Kanget al. , 2012). Under drought conditions, the concentration of SA increases five-fold in the evergreen shrub Phillyrea angustifolia(Munné-Bosch and Peñuelas, 2003) and two-fold in barley roots (Bandurska and Stroiński, 2005). In addition, under drought stress, the expression of SA-responsive genes, such as PATHOGENESIS-RELATED (PR ) genes, are also induced (Miura r et al. , 2013; Liu et al. , 2013). Salicylic acid induces ROS production in extracellular spaces and subsequently ROS accumulate in guard cells by diffusion (Khokon et al. , 2011; Mori et al. , 2001). However, the molecular mechanisms related to SA in Populus are not fully understood.
The WRKY transcription factor (TF) family is among the most widely studied TFs. Many WRKY TFs are involved in a variety of biotic and abiotic stress responses (Rushton et al. , 2010; Parinita et al. , 2011; Chen et al. , 2012; Jiang et al. , 2014).ABO3 /WRKY63 , TaWRKY2 , TaWRKY19 , andWRKY57 participate in the response to drought stress inArabidopsis (Ren et al ., 2010; Niu et al ., 2012; Jiang et al ., 2012). OsWRKY45 in rice and AtWRKY46in Arabidopsis respond to drought and salt stress by regulating stomatal movement (Qiu, 2008; Ding et al ., 2014). Previous research has confirmed that ZmWRKY 40 is a positive regulator that improves drought tolerance by regulating stress-related genes and the ROS content in maize (Wang et al ., 2018). AtWRKY75functions as a positive regulator of flowering as a component of the GA-mediated signaling pathway in Arabidopsis (Zhang et al ., 2017) and induces leaf senescence by interacting with SA and ROS (Guo et al ., 2017). In Populus, PtrWRKY18 andPtrWRKY35 promote tolerance the leaf rust pathogenMelampsora (Jiang et al ., 2017), and PtrWRKY73 andPtrWRKY89 play important roles in disease resistance mediated by SA and ROS, as demonstrated by experiments on Arabidopsis and transgenic poplar (Duanet al ., 2015; Jiang et al ., 2014). However, little is known about the role of WRKY TFs in the drought tolerance ofPopulus .
Poplars are among the most adaptable trees and are grown worldwide. They grow rapidly and their wood is used for diverse purposes. Thus, poplars provide substantial economic, ecological, and social benefits. However, poplars have a strong demand for water resources (Monclus et al. , 2006; Han et al. , 2013). North China is predominantly arid and semi-arid, and drought may negatively affect poplar growth (Tschaplinski et al. , 1994). For growth of poplars in such areas, it is important to enhance their drought tolerance. Previous studies have shown that PtrWRKY75 responds to SA (Jiang et al ., 2014), and that an increase in SA content can improve drought tolerance (Kang et al. , 2012). To assess the potential biological functions of PtrWRKY75 in tolerance to water stress,PtrWRKY75- overexpressing poplars were generated. Our results demonstrate that PtrWRKY75 responds to exogenous SA treatment by inducing PAL1 expression. This leads to ROS accumulation in the leaves, which subsequently induces stomatal closure and reduced transpiration, thereby increasing water-use efficiency (WUE) and drought tolerance.