## 1 Introduction  
Lettuce (_Lactuca sativa_ L.) is one of the most important vegetable crops sown in the United States, ranking third in total acreage (106,027 hectares) and first in economic value (USD 3.493 billion) in 2019 [1]. Over 97% of domestic production is concentrated in California and Arizona where environmental conditions allow multiple crops per season and year-round production [1, 2]. Several impediments to the intensification of field-grown lettuce production have arisen in recent years, including limited acreage of ideally suited farmland, high nitrogen (N) requirements for optimal production, risk of fecal bacterial contamination when manures are used as an N source for organic lettuce production, and significant irrigation demands. These water demands are increasingly placing a burden on lettuce production, with chronic drought and increased salinity hindering yields [2, 3]. Exogenous N applications exacerbate production concerns as leaching and run-off impairs water quality, reduces yields, and increases costs. Large-scale operations in these states have also been impacted by food safety recalls over contaminated produce, leading to millions of dollars of losses and erosion of public trust [1]. The sustainability of such geographically constrained production can be questioned, especially when coupled with the negative environmental impacts of cross-country transportation. Unfortunately, environmental conditions outside of these growing areas are unsuitable for year-round production. Controlled environmental agriculture in the form of hydroponic cultivation has arisen to localize and intensify lettuce production in areas otherwise unsuitable for continual, field-based sowing. In hydroponic production, light, temperature, and nutrient fertilization are all monitored and adjusted to maximize the crop output and can be implemented regardless of climate or latitude. Turnaround time from planting to harvest can be reduced by half under these conditions and yields can increase 11-fold per unit area, increasing the yields per annum and helping to offset the cost of hydroponic production [4; 5]. Additionally, hydroponic systems can be located near centers of consumption to minimize transportation costs, environmental impacts, and loss of nutritive composition during shipping and transport. Hydroonic production also tightly controls water salinity and can recycle and/or reuse irrigation water, thereby, improving their water use efficiency. One costly consideration of hydroponic lettuce production is the need to provide 100% of the fertilization requirements in the form of nutrient salts. Nitrogen salts are often provided in the form of synthetically sourced fertilizers whose production has significant energy and environmental costs. If microbe-assisted crop production strategies [6] can be incorporated into the lettuce production system to promote increased yield and/or fix atmospheric N, it may help alleviate some of the need for exogenous fertilizer additions. _Glucancetobacter diazotrophic_ (_G. diaz._) is an obligate endophyte that was originally identified in sugarcane [7; 8]. The significance of this discovery is that, in sugarcane, _G. diaz_ can fix up to 200 kg N ha\({}^{-1}\) year\({}^{-1}\) meeting nearly half of the crop's N requirement [9; 10]. _G. diaz_ was subsequently found to be naturally associated with coffee [11], finger millet [12], banana, pineapple [13; 14], tea, mango [8; 15], and wetland rice [16]. Research has shown that inoculation with _G. diaz._ can result in non-pathogenically infected crop species across numerous plant families [17], including sweet potato, sugarcane, maize [18; 19], tomato [20], sweet sorghum [21; 22], carrot, beet [23; 24], snap bean [25], and cassava [26]. Unlike most N-fixing bacteria, _G. diaz's_ nitrogenase is not wholly suppressed by agricultural levels of environmental N and can tolerate especially high nitrate levels as it lacks nitrate reductase genes [9; 27]. However, the colonization of sugarcane by _G.