Nitric oxide (NO) and hydrogen sulphide (H2S) are gaseous molecules that are produced in the cells of organisms from every lineage of life. Though highly toxic at even moderate concentrations (LC50 in humans exposed for 1 hour: 174 ppm and 50 ppm in NO and H2S, respectively), these gases play vital roles as signalling compounds in a number of physiological processes in plants, such as development (seed germination, plant growth, fruit ripening, and leaf senescence) and in communicating and coordinating stress responses to environmental challenges. Their suitability as transient and localised signalling molecules stems from their high reactivity (limiting their lifetime to just a few seconds) and ability to readily diffuse across cell membranes. Additionally, they have important (and duplicitous) roles in modulating oxidative stress; at low concentrations – interacting with powerful antioxidants such as glutathione and thioredoxin to restore redox balance, while at higher concentrations – acting as direct prooxidants, dealing out oxidative damage themselves.
For the month of February, Physiologia Plantarum is releasing a Special Issue on the topic of “Hydrogen sulfide and nitric oxide signal integration and plant development under stressed/non-stressed conditions”. To mark this release, we have selected four articles that comprise a “molecular veggie patch”, investigating the cross-talk between NO and H2S signals during development and in response to stress in a variety of common vegetables. We will be releasing these articles over the next two weeks – watch this space for more details!
1- Integrative roles of nitric oxide and hydrogen sulfide in melatonin‐induced tolerance of pepper (Capsicum annuum L.) plants to iron deficiency and salt stress alone or in combination
For those living in the far northern latitudes, this time of year can be particularly challenging, especially for those striving to maintain productivity in the face of short daylight hours and flagging energy levels. One of the culprits of this feeling of malaise (commonly known as winter-onset seasonal affected disorder or SAD) stems from an over-production in our body’s levels of the hormone melatonin, brought on by a lack of sunlight. In humans, melatonin is synthesised in the pineal gland of the brain and plays an important role in regulating our sleep-wake cycles. It was only relatively recently discovered in plants, and has since been identified in every plant species and tissue type investigated. Presently, the precise role of melatonin in plants remains enigmatic but early evidence suggests it plays an important function as a growth regulator and as a scavenger of harmful free radicals, which, if left unchecked can rapidly damage DNA, proteins, and cell membranes. This article by Kaya et al. (2019) explores the integrative role NO, H2S, and melatonin play in alleviating stress caused by combined iron deficiency and high salinity conditions in pepper (Capsicum annum L). Read on for more!
2- Glutathione and hydrogen sulfide are required for sulfur‐mediated mitigation of Cr(VI) toxicity in tomato, pea and brinjal seedlings
The content of reactive oxygen species (ROS) in a cell is governed by the careful interplay between (1) ROS produced as a by-product of a variety of biochemical pathways, and (2) the ROS-scavenging ability of antioxidants, which can rapidly detoxify these harmful compounds. One of the most potent antioxidants known is glutathione, a seemingly simple tripeptide made-up of a cysteine residue sandwiched between a glutamate and a glycine residue. In the cell, this molecule exists in two primary forms: a reduced state capable of neutralising harmful ROS, a process that results in glutathione converting to its secondary, oxidative state. So crucial is this molecule to the health of the cell that the ratio between glutathione’s reduced and oxidative states is commonly used as a diagnostic of cellular oxidative stress. This article by Kushwaha and Singh (2019), investigates the interrelated roles played by H2S signalling and glutathione in minimising stress caused by chromium toxicity in a range of important vegetable species. Read on for more information!
3- Biochemical and physiological impacts of zinc sulphate, potassium phosphite and hydrogen sulphide in mitigating stress conditions in soybean
Soybeans are thought to have been domesticated in China as early as 7000 BC, which, to put this in context, predates the first recorded form of writing by the Sumerians of Mesopotamia by almost four thousand years. From these early beginnings, soybeans became a staple food product for much of East Asia, being consumed directly in the form of bean curd and soy milk, or being fermented into a variety of food products, which improved their digestibility, preservation, and added an assortment of new textures, flavours, and aromas. Today, soybean is grown worldwide, with up to 80% of global production occurring in the USA, Brazil, and Argentina. Compared to many other crops, soybean is especially sensitive to drought stress, thus, a great deal of research has been directed at finding ways to alleviate this hardship, especially in the face of increasing drought incidence linked to climate change. This article by Batista et al. (2019) investigates the physiological and biochemical changes that occur in soybean during drought stress, when treated with zinc sulphate, potassium phosphite, or hydrogen sulphide. Read on for more!
4- Sodium hydrosulfide priming improves the response of photosynthesis to overnight frost and day high light in avocado (Persea americana Mill, cv. ‘Hass’)
Plants rarely experience stress in isolation and often have to respond to a confluence of different challenges simultaneously. Some of these are self-evident, such as the combined burden of drought and heat stress, while others are far less obvious. For example, plants that experience night-time frost conditions followed by an abundance of sunshine the following day can suffer from a condition known as ‘radiation frost’. What makes this one-two punch of environmental stressors so harmful is that cold stress effectively cripples the plants ability to carry out photosynthesis and the subsequent exposure to high light triggers a rapid and dangerous accumulation of reactive oxygen species (ROS). One effective way to enhance a plant’s inherent ability to resist challenges such as these is to expose it to a low dosage of a known stressor, in a process known as ‘priming’. This article by Joshi et al. (2019) investigates the beneficial effects that priming with a hydrogen sulphide donor has on avocado trees in the face of radiation frost conditions. Read on for more.