## 1 Introduction
These members of the plant microbiome might strengthen the host's ability to withstand extreme conditions, such as those found in the Mediterranean Basin [24]. The different plant organs offer many habitats varying in nutrient availability and surface area to adhere and grow, which host microbial communities of variable composition and density [5]. The phyllosphere, comprising the aerial surfaces of plants, constitutes Earth's most extensive biological interface [25]. This ecological niche is a less nutrient and water-rich plant habitat for microorganisms than the rhizosphere [26]. Phyllospheric microorganisms must face ultraviolet radiation and climatic fluctuations [27]. Despite the challenging conditions for epiphytic bacterial survival, this habitat supports a remarkable diversity of microorganisms [28]. Nevertheless, fewer studies have analyzed plant phyllosphere as a source of novel microorganisms than the rhizosphere [29]. Among PGPR, evidence suggests that the leaf surface provides a crucial habitat for establishing free-living diazotrophic bacteria capable of N fixation [30]. Plant species-specific variations in free-living N fixation rates are often correlated to variations in P concentration, indicating that this element and Fe are important in diazotrophic activity [27; 30; 31]. This study examined the influence of inoculation methods (leaf, root, or combined inoculation) on the PGP potential of a SynCom. The SynCom was constructed using bacterial isolates obtained from the phyllosphere of _Pistacia lenticus_ L. growing in marginal areas of Sardinia. _P. lentiiscus_, a widespread evergreen sclerophyllous shrub characteristic of the Mediterranean maquis, exhibits a high degree of drought tolerance and is predicted to increase in abundance under future aridity scenarios [32]. This species demonstratesa rich phyllosphere microbiome, supporting a greater abundance of bacterial cells than co-occurring species [33; 34]. Furthermore, _P. lentiscus_ harbors a diverse and abundant community of free-living N-fixing bacteria on its leaf surface [35], representing a promising reservoir of microorganisms with potential plant growth-promoting (PGP) traits. Leaf samples for this study were collected from diverse vegetation types in Northern Sardinia during late summer following a prolonged period of drought. The collection of samples within the Mediterranean macchia during the peak of summer likely enriched the phyllosphere with microbial communities possessing adaptive traits to high temperatures, water stress, and elevated UV radiation. Such traits may confer enhanced survival for PGPR following foliar or root application under challenging environmental conditions. Bacterial isolates were screened for key PGP traits, including N fixation, phosphate solubilization, and salt tolerance. Recognizing the critical role of nitrogen in plant growth and development and the environmental effects associated with intensive use of synthetic N-fertilizers [36], the SynCom was constructed using free-living diazotrophs with varying salt tolerance and phosphate solubilizing capacities, which can represent a key natural source of N in natural and agricultural ecosystems [37]. The SynCom was applied to _Lactuca sativa_ L. (lettuce) grown in a hydroponic system under nutrient-limiting conditions. This experimental setup allowed for the precise evaluation of SynCom's efficacy in promoting plant growth under controlled conditions. The impact of SynCom on lettuce growth was assessed by analyzing plant biomass, structure, and leaf traits.