Panicoid crop plants from the C4 type, such as maize, sorghum, and foxtail millet are an important part of staple food worldwide. The high rate of productivity observed in grasses is in part due to their extensive root system. Recently Setaria viridis has emerged as a new genetic model system for panicoid grasses. Natural accessions of S. viridis originating from different parts of the world, with differential leaf physiological behaviour, have been identified.
This work focused on understanding the physiological and molecular mechanisms controlling root hydraulic conductivity and root-to-shoot gas-exchange signaling in S. viridis. We identified two accessions, SHA and ZHA, with contrasting behaviour at the leaf (photosynthesis, transpiration, water potential), root (cell water permeability, hydraulics, transcription pattern), and whole-plant (transpiration) level. Our results indicated a significant role for root aquaporin plasma-membrane intrinsic proteins (AQP PIPs) in the differential behaviour of SHA and ZHA. Moreover, a different response to low levels of abscisic acid (ABA) between SHA and ZHA was observed, which was associated with root AQP and root hydraulics. Using cell imaging, biochemical and reverse-genetic approaches, we identified PIP1;6 as a possible PIP1 candidate that regulates radial root hydraulics and root-to-shoot signalling of gas exchange in S. viridis. In heterologous systems, PIP1;6 was located to the endoplasmic reticulum (ER), and upon interaction with PIP2s, re-localization to the plasma membrane (PM) was observed. PIP1;6 is predominantly expressed at the root endodermis. Generation of two CRISPR/Cas9 knockout PIP1;6 plants (KO-PIP1;6) in S. viridis showed lower root hydraulic conductivity, lower gas exchange, and alteration of root transcriptional pattern associated with cell wall barriers. Our results indicated that PIPs plays an essential role in root water movement and in the regulation of whole-plant water homeostasis in S. viridis. Furthermore, our results suggest that these traits might be regulated by ABA signals.