Hormones and light-regulated seedling development

Abstract

Light is one of the most important environmental signals, which has a profound impact on plant growth and development. Once a seed germinates, the proper emergence and establishment of the seedling into an autotrophic organism is extremely crucial for its survival. Various steps in the seedling establishment process are remarkably influenced by external light conditions. These steps include the dark-adapted growth of seedlings to emerge out of the soil cover; the opening and expansion of their embryonic leaves once they hit the light above the ground; the synthesis and accumulation of chloroplasts in the green parts to initiate photosynthesis; the accumulation of photoprotective compounds to safeguard the seedling from stress induced by sudden and excessive light exposure; the bending of the seedlings in response to the direction of light; the adjustment of root architecture in response to the direction and availability of light; and the morphogenic changes they undergo in a crowded population to avoid shade and reach for the sunlight. Plants possess specialized photoreceptors such as the Phytochromes, Cryptochromes, Phototropins and UVR8 to perceive different wavelengths of the light spectra. Downstream to these photoreceptors, central molecular players such as ELONGATED HYPOCOTYL 5 (HY5) and PHYTOCHROME INTERACTING FACTORS (PIFs) together with numerous other regulatory proteins establish a highly sophisticated signaling network. The light signaling network often culminates in the regulation of different phytohormones to induce major developmental changes in the plant. This chapter will discuss the finely regulated interaction between light and hormonal pathways in determining the developmental plasticity of plants during seedling development.