Strategies of seedlings to overcome their sessile nature: auxin in mobility control

Abstract

Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.

Ethylene Is Not Responsible for Phytochrome-Mediated Apical Hook Exaggeration in Tomato

“…These results exclude the possibility that ethylene evolution is controlled by auxin and regulates LIHE in tomato. At the same time this experiment with NPA suggests that LIHE is controlled by localization of auxin, as already shown with the apical hook curvature formed in the dark (Abbas et al, 2013; Mazzella et al, 2014; Willige and Chory, 2015; Žádníková et al, 2015). …”

“…Literature shows that an apical hook is formed by lateral localization of auxin at the apical part of the hypocotyl or epicotyl, and ethylene exerts its hook-exaggerating action through enhancing the localization of auxin (Vandenbussche et al, 2010; Žádníková et al, 2010, 2015; Abbas et al, 2013; Mazzella et al, 2014; Willige and Chory, 2015). It has also been known that auxin induces ethylene evolution in tissues (Kang et al, 1967; Kang and Ray, 1969; Yip et al, 1992; Schwark and Bopp, 1993; Abel et al, 1995; Peck and Kende, 1995; Coenen et al, 2003; Tsuchisaka and Theologis, 2004).…”

“…So far in the past, on the other hand, many works clarified with a range of dicotyledonous species represented by pea, bean, and recently Arabidopsis that the apical hook is formed in the dark and opens in the light, mediated by phytochrome, and that it is enhanced by ambient ethylene, and further, exaggerated at higher concentrations (Withrow et al, 1957; Goeschl et al, 1967; Kang and Ray, 1969; Powell and Morgan, 1980; Britz and Galston, 1982; Liscum and Hangarter, 1993a,b; Li et al, 2004; Abbas et al, 2013; Mazzella et al, 2014; Willige and Chory, 2015; Žádníková et al, 2015). It was also shown that ethylene emission from seedlings is enhanced by mechanical stresses, and ethylene-induced hook exaggeration is suppressed by light through phytochrome mediation (Harpham et al, 1991; Ecker, 1995; Lehman et al, 1996; Raz and Ecker, 1999; Knee et al, 2000; Zhong et al, 2014; Shi et al, 2016).…”

Ethylene and Hormonal Cross Talk in Vegetative Growth and Development

“…The gradual bending of the hook rests predominantly on differential growth mediated by an auxin gradient, regulated by a complex hormonal cross talk ( Fig. 1; Abbas et al, 2013;Mazzella et al, 2014;Zádníková et al, 2015). In dark-grown Arabidopsis seedlings, ethylene elicits an exaggeration of the apical hook, the inhibition of both hypocotyl and root elongation, and radial swelling of the seedling stem.…”

“…1). The auxin gradient is established by polar auxin transport regulated by a complex hormonal network (Abbas et al, 2013;Mazzella et al, 2014;Zádníková et al, 2015).…”