Root growth dynamics of Nicotiana attenuata seedlings are affected by simulated herbivore attack

Abstract

Many studies demonstrate resource-based trade-offs between growth and defence on a large timescale. Yet, the short-term dynamics of this growth reaction are still completely unclear, making it difficult to explain growthdefence trade-offs mechanistically. In this study, imagebased non-destructive methods were used to quantify root growth reactions happening within hours following simulated herbivore attack. The induction of wound reactions in Nicotiana attenuata in the seedling stage led to transiently decreased root growth rates. Application of the oral secretion of the specialist herbivore Manduca sexta to the leaves led to a transient decrease in root growth that was more pronounced than if a mere mechanical wounding was imposed. Root growth reduction was more pronounced than leaf growth reduction. When fatty acid-amino acid conjugates (FACs) were applied to wounds, root growth reduction occurred in the same intensity as when oral secretion was applied. Timing of the transient growth reduction coincided with endogenous bursts of jasmonate (JA) and ethylene emissions reported in literature. Simulation of a wound response by applying methyl jasmonate (MeJA) led to more prolonged negative effects on root growth. Increased nicotine concentrations, trichome lengths and densities were observed within 72 h in seedlings that were treated with MeJA or that were mechanically wounded. Overall, these reactions indicate that even in a very early developmental stage, the diversion of plant metabolism from primary (growth-sustaining) to secondary (defencerelated) metabolism can cause profound alterations of plant growth performance.

The underestimated role of roots in defense against leaf attackers

“…This suggests that many metabolic processes are downregulated belowground when defense mechanisms are induced in leaves. In accordance with this, a rapid and transient reduction of root growth after shoot herbivore attack occurs in N. attenuata [87]. It remains to be investigated if this reduced investment into growth is a part of the strategy of plants to ''turn down its spatial expansion and strengthen its function as a safe retreat'' [87] or if it helps to liberate additional resources to boost leaf defenses.…”

“…Known shoot-root signals such as auxins [89] and other phloem-borne molecules including proteins [90] and small RNAs [91] are candidates for such investigations. Furthermore, it has been suggested that the reduction of root growth after leaf wounding could be mediated by a hydraulic signal [87]. Interestingly, sucrose is not only a simple assimilate that is moved to roots for tolerance purposes upon insect attack but also by a potential shoot-root signal [92].…”

Developmentally equivalent tissue sampling based on growth kinematic profiling of Arabidopsis inflorescence stems

“…Primarily for this reason, assessment of REGRs from two-dimensional (2D) observation of growing roots (single-point perspective) has largely been conducted on restrictive 2D surfaces such as glass plates (Erickson & Sax, 1956;Basu et al, 2007) or Petri dishes (Hummel et al, 2007;Mullen et al, 1998;Nagel et al, 2006). These data are recognized as being informative despite ongoing * ** r e t f A e r o f e B Fig.…”

Why Does Herbivore Attack Reconfigure Primary Metabolism?

“…The increased flux of C to the roots in response to herbivory would be expected to increase the rate of root growth, but in young seedlings of N. attenuata, for example, sometimes just the opposite occurs (Hummel et al, 2007). Unlike the FAC-elicited C flux, this rapid inhibition of root growth requires an intact JA-signaling cascade (G.M.…”