ABSTRACT
Senescence is a natural, energy-dependent, physiological, developmental
and an ecological process that is controlled by the plant’s own genetic
program, allowing maximum recovery of nutrients from older organs for the
survival of the plant, as such; it is classified as essential component of the
growth and development of plants. In some cases, under one or many
environmental stresses, senescence is triggered in plants. Despite many studies in the area, less
consideration has been given to plant secondary metabolites, especially the
role of VTCs on plant senescence. This review seeks to capture the biosynthesis and signal transduction of
VTCs, the physiology of VTCs in plant development and how that is linked to
some phytohormones to induce senescence. Much progress has been made in the elucidation of
metabolic pathways leading to the biosynthesis of VTCs. In addition to the
classical cytosolic mevalonic acid (MVA) pathway from acetyl-CoA, the
2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, originating from
glyceraldehyde-3-phosphate (GAP) and pyruvate, leads to the biosynthesis of isoprenoid precursors,
isopentenyl diphosphate and dimethyl allyl diphosphate. VTCs synthesis and
emission are believed to be tightly regulated by photosynthetic carbon supply
into MEP pathway. Thus, under abiotic stresses such as drought, high salinity,
high and low temperature, and low CO2 that directly affect stomatal
conductance and ultimately biochemical limitation to photosynthesis, there has been observed
induction of VTC synthesis and emissions, reflecting the elicitation of MEP
pathway. This reveals the possibility of important function(s) of VTCs in plant
defense against stress by mobilizing resources from components of plants and
therefore, senescence. Our
current understanding of the relationship between environmental responses and
senescence mostly comes from the
study of senescence response to phytohormones such as abscisic acid, jasmonic
acid, ethylene and salicylic acid, which are extensively involved in response
to various abiotic and biotic stresses. These stresses affect synthesis and/or
signaling pathways of phytohormones
to eventually trigger expression of stress-responsive genes, which in turn appears
to affect leaf senescence. Comparison of plant response to stresses in relation
to patterns of VTCs and phytohormones biosynthesis indicates a considerable
crosstalk between these metabolic processes and their signal to plant
senescence.