Basal Signaling Regulates Plant Growth and Development

Abstract

The term signal transduction refers to the classical paradigm where an external stimulus is sensed and initiates an increase in second messengers. Each second messenger transmits and amplifies the signal by activating a subset of downstream pathways. This complex network of interwoven downstream events ultimately converges to produce measurable responses. While the paradigms for signal transduction are well known, little consideration has been given to the second messengers in the nonstimulated cell, the basal signal, and yet, basal signals also impact plant growth and development.Basal signals are low levels of oscillating second messengers that are being sensed by the cell. These are difficult to measure and are often considered to be background noise. Not surprisingly, there is a dearth of knowledge about how plants respond to this constant flux of signaling metabolites or about how basal signals define homeostasis or contribute to species diversity. Both genetics and environment will define the basal signal of a cell. To truly understand how plants regulate growth and development, future research must focus on understanding how basal signals regulate fundamental metabolism.Our hypothesis is that in a nonstimulated cell, a low level of basal signal will constitutively repress some downstream events and stimulate others. If this is true, then lowering the basal level of a second messenger should derepress or enhance those downstream events specifically targeted by this second messenger, while events that depend on the basal level of second messenger will not be activated (Fig. 1).Monitoring rapid, transient changes in second messengers within stimulated cells is challenge enough. The problem is amplified when trying to measure rapidly oscillating basal signals. There are methods for monitoring external oscillations of ions and metabolites using vibrating probes (Jaffe and Nuccitelli, 1974; Shabala et al., 1997) and recently developed, self-referencing biosensors (Porterfield, 2007;McLamore et al., 2010), respectively. Similar technical advances are needed for studying fluxes within cells. In vivo fluorescent reporters can indicate changes in stimulus-induced oscillations within single cells (Monshausen et al., 2008(Monshausen et al., , 2009); however, the fluorophores, by necessity, are selected to report signals above the background noise so that the basal signals of a nonstimulated cell are often below the limits of detection. Furthermore, the basal levels of second messengers are usually at or below the limits of detection for metabolomic analyses, and metabolic fluxes are difficult to assess with metabolomics (Fernie et al., 2005).Alternative approaches are needed to assess the impact of basal signals. In principle, if one lowered a basal signal, downstream events regulated by the second messenger in the nonstimulated cell would be revealed. Because basal signals are inherently a part of normal metabolism, it is difficult to lower basal signals; however, there are several examples where expression of gen...

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“…to represent constant basal or constitutive growth (e i ) of X i [44,45] plus the effect of the external stimulus with value s i and rate a i . In this paper, we assume that g i is constant.…”

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“…to represent constant basal or constitutive growth (e i ) of X i [42,43] plus the effect of the external stimulus with concentration s i and rate α i . In this paper, we assume that g i is constant.…”

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