Hormonal Regulation in Shade Avoidance

Abstract

At high vegetation density, shade-intolerant plants sense a reduction in the red (660 nm) to far-red (730 nm) light ratio (R/FR) in addition to a general reduction in light intensity. These light signals trigger a spectrum of morphological changes manifested by growth of stem-like tissue (hypocotyl, petiole, etc.) instead of harvestable organs (leaves, fruits, seeds, etc.)—namely, shade avoidance syndrome (SAS). Common phenotypical changes related to SAS are changes in leaf hyponasty, an increase in hypocotyl and internode elongation and extended petioles. Prolonged shade exposure leads to early flowering, less branching, increased susceptibility to insect herbivory, and decreased seed yield. Thus, shade avoidance significantly impacts on agronomic traits. Many genetic and molecular studies have revealed that phytochromes, cryptochromes and UVR8 (UV-B photoreceptor protein) monitor the changes in light intensity under shade and regulate the stability or activity of phytochrome-interacting factors (PIFs). PIF-governed modulation of the expression of auxin biosynthesis, transporter and signaling genes is the major driver for shade-induced hypocotyl elongation. Besides auxin, gibberellins, brassinosteroids, and ethylene are also required for shade-induced hypocotyl or petiole elongation growth. In leaves, accumulated auxin stimulates cytokinin oxidase expression to break down cytokinins and inhibit leaf growth. In the young buds, shade light promotes the accumulation of abscisic acid to repress branching. Shade light also represses jasmonate- and salicylic acid-induced defense responses to balance resource allocation between growth and defense. Here we will summarize recent findings relating to such hormonal regulation in SAS in Arabidopsis thaliana, Brassica rapa, and certain crops.

Progress of Research on the Regulatory Pathway of the Plant Shade-Avoidance Syndrome

“…Shade-avoidance responses involve a cascade reaction of the light signal system, plant hormone signaling pathways, and growth regulation (Smith and Whitelam, 1997;Yang and Li, 2017). Current progress in this field of study has mainly focused on the model plant Arabidopsis and limited research has been conducted on other plants, especially crops.…”

The Arabidopsis geneRGOmediates cytokinin responses and increases seed yield

“…There was however, no indication the report was peer reviewed and that whether the knockouts of AtRGO were genotyped to assure homozygosity as well as absence of AtRGO gene transcripts. Nevertheless, it is known that shade avoidance is in part hormonally regulated and involves auxin and cytokinin levels mediated by leaf CKXs (Wu et al, 2017; Yang and Li, 2017). It is conceivable therefore that AtRGO could play a role in shade avoidance and should offer opportunities for future studies.…”

Bioinformatics analysis of endophytic bacteria related to berberine in the Chinese medicinal plant Coptis teeta Wall

“…The development of microbiology and related technologies will provide strong technical support for solving major problems in the sustainable development of agriculture, such as the abuse of pesticides and fertilisers, environmental pollution, and the rise of crop diseases. Therefore, it is urgent for us to investigate crop microbiomes to ensuring the safety of medicinal and food plants[23].…”