ABA-regulation of root hydraulic conductivity and aquaporin gene- expression is crucial to the plant shoot rise caused by rhizosphere humic acids

Abstract

The physiological and metabolic mechanisms behind the humic acid-mediated plant growth enhancement are discussed in detail. Experiments using cucumber (Cucumis sativus) plants show that the shoot growth enhancement caused by a structurally well-characterized humic acid with sedimentary origin is functionally associated with significant increases in abscisic acid (ABA) root concentration and root hydraulic conductivity. Complementary experiments involving a blocking agent of cell wall pores and water root transport (polyethylenglycol) show that increases in root hydraulic conductivity are essential in the shoot growth-promoting action of the model humic acid. Further experiments involving an inhibitor of ABA biosynthesis in root and shoot (fluridone) show that the humic acid-mediated enhancement of both root hydraulic conductivity and shoot growth depended on ABA signaling pathways. These experiments also show that a significant increase in the gene expression of the main root plasma membrane aquaporins is associated with the increase of root hydraulic conductivity caused by the model humic acid. Finally, experimental data suggest that all of these actions of model humic acid on root functionality, which are linked to its beneficial action on plant shoot growth, are likely related to the conformational structure of humic acid in solution and its interaction with the cell wall at the root surface.

Root ABA and H+‐ATPase are key players in the root and shoot growth‐promoting action of humic acids

“…This action results from the HS capacity to improve plant nutrient availability in soil, mainly through the formation of stable complexes with metals (Chen et al, ; Erro et al, ; Garcia‐Mina, ; Gerke, ; Urrutia et al, ). Besides this effect on improving nutrient bioavailability, HS also promotes plant growth through action on plant metabolism and physiology, which is derived from the interaction of HS with plant roots (Canellas, Olivares, Okorokova‐Facanha, & Facanha, ; Muscolo, Sidari, Francioso, Tugnoli, & Nardi, ; Olaetxea et al, , ; Quaggiotti et al, ). However, knowledge about the mechanisms responsible for this direct effect of HS on the promotion of plant growth is scarce and fragmentary, regarding the effects on both root development and shoot development (García et al, ; Mora, Bacaicoa, Baigorri, Zamarreño, & García‐Mina, ; Olaetxea et al, ).…”

“…Moreover, studies using the DR5::GUS construct (a genetic construct where the synthetic auxin‐responsive promoter DR5 controls the expression of bacterial β‐ d ‐glucuronidase) (Bierfreund, Reski, & Decker, ) in several plant species showed that the application of HA to the roots induced the upregulation of genes controlled by auxin‐dependent signaling pathways (Trevisan, Pizzeghello, et al, ). These effects were also linked to NO‐ and ethylene‐dependent signaling pathways (Mora, Baigorri, Bacaicoa, Zamarreño, & García‐Mina, ; Zandonadi et al, ), as well as to significant increases in root plasma membrane (PM) H + ‐ATPase activity and root ABA concentration (Canellas et al, ; Mora, Bacaicoa, et al, ; Mora et al, ; Olaetxea et al, ; Zandonadi et al, ). Some authors have proposed the hypothesis that the HA‐mediated increase in root development may result from a coordinated effect on proton and Ca 2+ effluxes associated with root PM H + ‐ATPase activity under the “acid‐growth theory” (Ramos et al, ).…”

“…With regard to the shoot‐promoting action of HA, recent studies indicated that it is regulated by the action of HA on IAA‐NO signaling pathways in the root (Mora, Bacaicoa, et al, ). Further studies also showed that an increase in root ABA concentration, which is linked to increases in root plasma membrane aquaporin activity and root hydraulic conductivity (Lp r ), was crucial for the enhancement in shoot growth caused by HA (Olaetxea et al, ). A previous study proposed the hypothesis that the HA capacity to enhance shoot growth involves an increase in cytokinin activity in the shoot which is in turn linked to the HA‐mediated increase in the root PM H + ‐ATPase activity and uptake of nitrate by root (Mora et al, ).…”

“…HA with a sedimentary origin and extracted from leonardite (SHA), which has been extensively characterized in previous studies (Aguirre et al, ; Mora, Bacaicoa, et al, ; Mora et al, , ; Olaetxea et al, ), was employed for the experiments. As in previous studies, the plant species used in the experiment was cucumber (Aguirre et al, ; Mora, Bacaicoa, et al, ; Mora et al, , ; Olaetxea et al, ).…”

MtABCG20 is an ABA exporter influencing root morphology and seed germination of Medicago truncatula

“…Abscisic acid (ABA) regulates plant water status, and promotes seed maturation and dormancy. Under drought stress conditions, ABA inter alia triggers stomatal closure, thus minimizing the loss of water through transpiration (Merilo et al ., ), and changes hydraulic conductivity in a dose‐dependent manner (Dodd, ; Olaetxea et al ., ) as well as modulates root system architecture to improve water uptake (Harris, ). ABA is also well‐recognized as a repressor of seed germination.…”