Leaf PIP2.1 and PIP2.5 are down-regulated in ‘Mandarin’ lime under drought

Aquaporins are water channel proteins that facilitate water transport through the transcellular pathway, a route with high resistance due to lipid bilayer membranes. Aquaporins occur as multiple isoforms and plasma membrane aquaporins (PIPs) are known to modulate plant responses to abiotic stress such as drought. Previous studies have reported changes in PIPs expression in roots of Citrus plants under abiotic stresses; however, such changes are limited to grafted citrus trees with plant responses to environmental stresses being driven by two species. Therefore, the ability of a given Citrus species—i.e., non-grafted Citrus saplings—in modulating aquaporin expression under water deficit remains unknown. This study aimed to evaluate how water deficit changes aquaporin expression (PIP1; PIP2.1; and PIP2.5) and leaf gas exchange of saplings of three Citrus species: ‘Mandarin’ lime, ‘Swingle’ citrumelo and ‘Valencia’ sweet orange. Root aquaporin expression did not vary under water deficit or among species, but the predawn leaf water potential of ‘Valencia’ sweet orange was reduced due to water deficit. Decreases in stomatal conductance were found in all species under water deficit, which were associated with reduced PIP2.1 expression in leaves. However, ‘Mandarin’ lime exhibited the largest reduction in stomatal conductance and significant gene repression of not only PIP2.1 but also PIP2.5 in leaves under water deficit. ‘Mandarin’ lime is known as a drought tolerant rootstock widely used in citriculture and our data suggest that this rootstock presents a fast-response mechanism against drought based on stomatal closure associated with downregulation of PIP2.1 and PIP2.5 in leaves.