The role of hormones in the aging of plants - a mini-review.

BACKGROUND In plants, the final stage of organ development is termed senescence. This is a deterioration process that leads to the decay of tissues and organs, and that, in the case of annual, biennial and/or monocarpic plants, leads to the death of the plant itself. The main function of leaf senescence is nutrient recycle and, since this confers an adaptive advantage, it can be considered an evolutionary selected process. Multiple developmental and environmental signals control senescence, and among them plant hormones are understood to play important roles. In particular, the function of cytokinins and ethylene in senescence has been studied for decades, but it is only since Arabidopsis thaliana was established as a model organism for molecular genetic studies that the underlying molecular and biochemical events have begun to be elucidated. METHODS In this review, we summarize the present understanding of the role of hormones in the developmental control of leaf senescence in plants and in particular highlight recent studies which address its molecular control. RESULTS Important findings which connect hormone action to developmental senescence were made in the past few years. For example, it was shown that ethylene activity in natural, age-dependent leaf senescence is conferred by the regulatory function of EIN2, an ethylene-signaling component, in the control of the transcription factor oresara 1 (ORE1), which regulates a large set of senescence-associated genes in their expression. ORE1 mRNA abundance is regulated by the microRNA miR164, which in aging plants is degraded in an EIN2-dependent manner, and it is interesting that another microRNA also governs the hormonal control of senescence. miR319 regulates mRNA abundance of a class of transcription factors which control the expression of LOX2 (lipoxygenase 2), a key enzyme in the JA biosynthetic pathway, and thereby regulates JA homeostasis in senescing leaves. CONCLUSION Reverse and forward genetics have facilitated the elucidation of molecular mechanisms involved in the control of leaf senescence by phytohormones. Studies initiated on the interactions between the different hormonal pathways that control leaf senescence should improve our knowledge in the future.