Which plant hormones are critical in responding to drought conditions? How do plant hormone responses adjust to environmental changes? An article published in the journal Trends in Plant Science redefines and classifies the functions of the 10 groups of plant hormones identified to date in the plant world, molecules with vital roles in plants and with various agricultural applications related to herbicides, bio-stimulants, fruit and vegetable crops, among others.
The study also highlights which groups of phytohormones are essential to respond to changing environmental conditions (water stress, flooding, etc.) and facilitate plant survival in increasingly extreme environments.
The author of this study is Sergi Munné-Bosch, professor at the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona, and head of the Consolidated Research Group on Antioxidants in Agrobiotechnology.
A hierarchy in the world of plant hormones
Today, there is little scientific review and systematization of data on phytohormones and their action mechanisms. Plant hormones are organic molecules present in very low concentrations. They have at least one identified hormone receptor to which they bind to initiate signalling and specific hormone action, and are transported over long distances by vascular tissue (xylem or phloem).
The study examines the most important characteristics and functions of the 10 hormone groups considered so far in plants: auxins, gibberellins, cytokinins, abscisic acid (ABA), ethylene, salicylates, jasmonates, brassinosteroids, peptide hormones and strigolactones.
“Since the discovery of auxins as cell division factors in 1927 by Fritz W. Went, scientific breakthroughs on phytohormones have revolutionized plant biology and agricultural techniques,” says Munné-Bosch, professor at the Department of Evolutionary Biology, Ecology and Environmental Sciences.
Despite the importance of the hormone hierarchy in plants, little experimental progress has been made in this area. Auxins, cytokinins and gibberellins are the most decisive for plant growth and development and are part of level 1 regulation according to the hormone hierarchy proposed by the author.
At a second level, ABA, ethylene, salicylates and jasmonates help modulate the most appropriate plant responses as plants grow in constantly changing environmental conditions, and are key determinants of the stress response.
“In the case of water stress, ethylene and ABA, which is responsible for stomatal closing (small pores in the leaf that regulate gas exchange) and other responses to cope with water deficit and thus prevent desiccation, are particularly important. Some plants are really very efficient in their use of water largely thanks to the regulation by ABA,” says Munné-Bosch.
Brassinosteroids, peptide hormones and strigolactones form a third level of hormones that give plants greater flexibility and optimal responses to a wide range of situations.
Molecules that cannot yet be included in the list of phytohormones
There is also a waiting list of candidate phytohormone molecules that do not yet meet all the requirements. “Melatonin and gamma-aminobutyric acid (GABA) are two good examples. Melatonin meets the requirements, but the identification of the receptor is still at an early stage (only the PMTR1 receptor has been described in the species Arabidopsis thaliana). However, it is possible that in the near future there will be scientific consensus to validate it as a phytohormone.”
“In the case of GABA, no receptor has yet been identified in plants. It acts by modulating channels and, being a known neurotransmitter and an animal hormone, it is curiously not so in plants,” the expert points out.
From basic biology to plant biotechnology
Throughout the evolutionary history of plants, hormone systems emerged at different stages. However, eight of the ten hormone groups were already present before the emergence of vascular plants. “Cytokinins, some peptide hormones and ethylene were already present in algae. And auxins, ABA, salicylates, jasmonates and strigolactones were already present in bryophytes such as mosses,” says Munné-Bosch. Thus, the functional coordination between ancestral and more recent hormones reflects the adaptive strategies of plants in the face of the ecological pressures of the natural environment.
In the future, knowledge about hormone groups in plants will have to be expanded, given their scientific interest not only in basic biology but also in agriculture and plant biotechnology.
“It will be important to study the still poorly known phytohormones, such as strigolactones, brassinosteroids and peptide hormones. We need more research on hormone interaction, an area that is still little explored, and on molecules that are not yet categorised as hormones in the plant kingdom, such as melatonin and GABA,” concludes Sergi Munné-Bosch.