Root-To-Shoot Signalling: Assessing The Roles of ‘Up’ In the Up and Down World of Long-Distance Signalling In Planta

Abstract

An important mediator of shoot physiological processes can be the supply of signal molecules (other than water and nutrients) from the root system. Root-to-shoot signalling is often considered to be important in regulating shoot growth and water use when soil conditions change without any demonstrable change in shoot water or nutrient status. Changes in xylem sap composition are often thought to be synonymous with changes in root-to-shoot signalling, even though there is considerable re-cycling of compounds between xylem and phloem. Techniques used to collect xylem sap are reviewed. Elucidating the roles of putative root signal molecules in planta has usually taken priority over identifying the sources of signal molecules in xylem sap. The roles of several signal molecules are considered. This choice is selective, and the failure of known signals to account for observed physiological changes in some systems has lead to the conclusions that other novel signals can be important. The efficacy of a given signal molecule can depend on the shoot water and nutrient status, as demonstrated by variation in stomatal responses to abscisic acid. If such variation is widespread in crop species, this may have implications for the increasing intentional use of rootto-shoot signals to modify crop water use and shoot architecture. Research into root-to-shoot signalling may become increasingly reductionist, in trying to evaluate the contribution of root signals versus local processes to observed physiological changes. However, future challenges are to successfully integrate this basic research into improved crop production systems.

Expression of ABA synthesis and metabolism genes under different irrigation strategies and atmospheric VPDs is associated with stomatal conductance in grapevine (Vitis vinifera L. cv Cabernet Sauvignon)

“…Arguments for and against the role of ABA as a root-to-shoot chemical signal have previously been discussed (Introduction and Dodd, 2005; Li et al ., 2011) and it is now commonly accepted that a combination of chemical and hydraulic messages are associated with stomatal regulation. The correlations between [ABA] and g s (Fig.…”

“…It has been well established that reductions in soil water availability increase levels of ABA in the xylem and apoplastic sap which, in turn, are associated with decreasing stomatal conductance (reviewed in Dodd, 2005). However, ABA may not be the only factor involved in stomatal regulation and there are good arguments proposing hydraulic and/or chemical signals other than ABA in the same role (Christmann et al ., 2007; Wilkinson et al ., 2007; Neumann, 2008; Rodrigues et al ., 2008; Schachtman and Goodger, 2008).…”

Understanding Differential Responses of Grapevine (Vitis vinifera L.) Leaf and Fruit to Water Stress and Recovery Following Re-Watering

“…Concomitant with these mo- difications, biochemical changes ensued in the form of ABA (abscisic acid) accumulation (Figure 2) occurring most probably in the petiole xylem and leaf [21,22]. Such response is a highly accepted testament to plants experiencing water stress as this phytohormone serves as the predominant chemical message in plants experiencing water stress [23]. It is well known that whenever ABA accumulates, stomatal conductance de-clines as evidenced by correlations between stomatal conductance and the ABA from both xylem and leaf tissues [24].…”

Partial rootzone drying increases water-use efficiency of lemon Fino 49 trees independently of root-to-shoot ABA signalling

“…Typically, the wet and dry sides are periodically alternated. This technique aims to supply sufficient water to prevent or attenuate any leaf water deficit (Stoll et al 2000a;Sobeih et al 2004), while exposing some roots to drying soil to stimulate production of chemical signals (such as the plant hormone ABA) and their transmission to the shoots to restrict water use and increase crop WUE (Dodd 2005).…”

“…Many authors have emphasised the role of ABA as a root-toshoot signal regulating stomatal behaviour as the soil dries (Davies and Zhang 1991;Dodd 2005). The rationale for PRD is based on an understanding of the action of long-distance chemical signalling in plants, but there are concerns that this form of signalling is not universal (Rodrigues et al 2008).…”