Plant water transport is governed by dynamic hydraulic conductance mechanisms, tightly regulated across spatial and temporal scales in response to environmental stimuli. In leaves, all non-living xylem vessels are tightly enwrapped by a dense layer of living bundle sheath cells (BSCs), which function as a selective physiological membrane regulating the radial flow of water into the mesophyll. This makes the BSC layer a pivotal determinant of leaf hydraulic conductance (Kleaf).
Recent findings highlight the dual role of BSCs in integrating environmental signals and adjusting water flow. Blue light (BL), perceived by phototropins in BSCs, activates the plasma membrane H⁺-ATPase (AHA2), acidifying the xylem sap and enhancing the water permeability of BSC membranes via aquaporins, thus increasing Kleaf. This response is rapid, Ca²⁺-dependent, and complements stomatal conductance, optimizing transpiration and photosynthesis under high light conditions. In contrast, the stress hormone abscisic acid (ABA) suppresses AHA2 while stimulating Vacuolar H⁺-ATPase (VHA), leading to xylem alkalinization, cytosolic acidification, and a reduction in aquaporin activity, culminating in decreased Kleaf under drought stress. Interestingly, similar hydraulic suppression is observed under biotic stress. In tomato plants infected with Fusarium oxysporum, early reductions in Kleaf and transpiration precede visible symptoms. These are driven by xylem occlusion and fungal-secreted compounds such as fusaric acid and chitin, which impair membrane water permeability. These insights position BSCs as a central, stress-responsive hydraulic valve, modulating leaf water flux in response to light, hormones, and pathogens.
