Guard cell and whole plant expression of AtTOR improves performance under drought and enhances water use efficiency

Water use efficiency is an important target for breeding of improved drought resistance. Minimizing leaf transpirational water loss plays a key role in drought resistance. But this reduces CO₂ levels in leaves, which often reduces photosynthetic efficiency and yield. Signaling pathways play important roles in stress responses, and identifying the molecular, biochemical, and physiological determinants underlying drought signaling may offer new drought mitigating strategies. To explore these possibilities, and because of the importance of stomata in drought response and photosynthesis, we employed guard cell (GC)-targeted and constitutive overexpression of the Target of Rapamycin (TOR) kinase, a master regulator of signaling networks, in transgenic Arabidopsis. To investigate the impact of these AtTOR transgenes in drought, we conducted physiological and molecular investigations into drought responses, including leaf water loss, photosynthetic CO₂ assimilation, stomatal H₂O/CO₂ conductance, leaf chlorophyll content, and global gene expression in response to drought in wild-type and AtTOR-expressing Arabidopsis. Links between both guard cell-localized and whole plant AtTOR overexpression were identified, revealing TOR is involved in conservation of water and sustained photosynthetic performance, along with identification of genes associated with drought response in WT versus AtTOR-expressing transgenic lines. These findings suggest that targeted guard cell AtTOR expression should help achieve a balance between plant water conservation during drought, and maintaining plant performance, by minimizing reductions in photosynthesis. Manipulation of guard cell AtTOR expression could be an effective avenue for developing crops with enhanced drought resistance and increased yield under drought stress, resulting in enhanced water use efficiency.