Nitric Oxide, A Multifaceted Signaling Molecule in Plants
Authors:
Lekshmy Sathee, Scientist (Plant Physiology) at ICAR-IARI, New Delhi, Plant Physiologist working in the area of nitrogen use efficiency of crop plants.
Hari Singh Meena, Technical assistant, (Plant Physiology) at ICAR-IARI, New Delhi.
Sandeep Adavi B, Ph.D Scholar (Plant Physiology) at ICAR-IARI, New Delhi
Shailendra K Jha, Scientist (Plant Breeding) at ICAR-IARI, New Delhi, Plant Breeder
Niharika Mallick, Scientist (Plant Breeding) at ICAR-IARI, New Delhi, Plant Breeder
Kumar Durgesh, Scientist (Plant Breeding) at ICAR-IARI, New Delhi, Plant Breeder

Nitric oxide (NO) is a free radical, that functions as a diffusible messenger molecule that was first recognized in animals, where in it plays variable functions ranging from neural transmission and blood circulation to immune system responses and fertilization. NO is a member of ROS family of molecules, more specifically, falls in the subgroup of molecules that contain nitrogen: also known as reactive nitrogen species (RNS).

It is well established that NO plays essential role in animal fertilization and embryogenesis. In plants, NO has proven roles in various plant processes ranging from seed germination to reproduction. NO mediated protein modification plays important role in seed germination. The close interactions of NO, ROS and free calcium ions control stomatal aperture in plants. Another major role of NO is in pathogen defense, NO accumulates at fungal infection sites, and resistance phenotype correlates with NO production, temporally and volumetrically. The reaction of NO and ROS results in the formation of peroxinitrite. The cross talk of ROS and NO have extensive roles in defense, hypersensitive response and programmed cell death (PCD), pollen tube growth and polarity, pollen tube rupture, root growth and self-incompatibility responses.

Being major cellular antioxidant and precursor for nitrosoglutathione (GSNO) a storage form of NO, glutathione is a connecting node between NO and ROS. NO exerts its function largely by way of S-nitrosylation of target proteins, leading to alteration in enzyme kinetics and transcription of genes. Other ways of NO signal transduction includes protein tyrosine nitration and through extracellular nucleotides. The research facet of NO production and its influences in plants has progressed extensively since the first description in 1998 (Delledonne et al., 1998; Durner et al., 1998). However, the research is far away in deciphering NO biosynthesis by plants cells and how NO signaling mediates umpteen aspects of plant development and responses.

NO production in land plants classically involves two main routes. First, a reductive pathway involving both enzymatic and non-enzymatic reduction of nitrite into NO. Second, an oxidative pathway requiring a putative nitric oxide synthase (NOS)-like enzyme. Under hypoxia/anoxia condition NO ₂^- acts as an alternative electron acceptor in mitochondrial electron transport chain allowing mitochondria to oxidize NADH/NADPH and generate limited amount of ATP together with NO as shown in root mitochondria of several species.

The first suggestion that plants do possess mammalian NOS like enzyme was published in the middle of the nineties. However, the possibility that such enzyme could catalyze NO synthesis in plants has also been a main controversial issue. First, several studies provided evidences that protein extracts from plant tissues, cultured cells or purified organelles display a NOS-like activity. Second, many reports pointed out the efficiency of animal NOS inhibitors in reducing NO production in plants. When screening for sequences showing homologies to the human neuronal NOS gene was done with transcriptome sequence data of over 1000 species. It was shown that no typical mammalian NOS-like sequences were found, even in species in which NOS activities and effects of mammalian NOS inhibitors have been reported. Although the past two decades have seen significant advances towards the understanding of the function of NO in plants, many questions related to the processes underlying its synthesis remain to be answered. Clearly, land plants do not possess typical mammalian NOS-like proteins.

References:


1. Delledonne M, Xia Y, Dixon RA, Lamb C (1998) Nitric oxide functions as a signaling plant disease resistance. Nature 394: 585-588.

2. Durner J, Wendehenne D, Klessig F (1998) Defense gene induction in tobacco by nitric oxide cyclic GMP and cyclic ADP ribose. Proc Natl Acad Sci USA 95: 10328-10333.


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