"1. Selenium: A Powerful Element
The element selenium (Se) has a narrow range between essentiality and toxicity for living
organisms (one order of magnitude). The perception of this element changed over time. It was known
as toxic to organisms until its essentiality was discovered [1]. Se is chemically similar to sulphur (S),
which leads to nonspecific binding of Se instead of S. This disrupts cell metabolism [2,3] and changes
the structure of proteins [4] resulting in toxicity.
The element is found in the lithosphere, soil, water, and air, but the distribution varies around the
world. Deficit of Se can be found on the Russian Plane (0.01 mg kg−1
) while toxic concentration of
1200 mg kg−1
can be found in Meath, Ireland [5]. There are also some places which have Se pollution
problems, the source of which can be natural or anthropogenic. The essentiality of Se as a nutrient was proven for humans and animals only, while for higher plants
it is a beneficial element. However, it is required for optimal growth of green algae Chlamydomonas
reinhardtii" P.A. Dang. [6].
2. Forms of Selenium and Their Effect
Se has six known isotopes: 74Se, 76Se, 77Se, 78Se, 80Se, and 82Se. In inorganic and organic compounds,
the element exists in various oxidative states: elemental Se (0), selenide (−2), selenite (+4), and
selenate (+6). In the environment, it can be found in various organic forms; gaseous (dimethylselenide,
dimethyldiselenide) and nongaseous (SeCys2, SeMet, SeMeSeCys) [7]. The availability and distribution
of Se depend on the pH and content of organic matter in soil, redox conditions, competing ionic
species such as S, mineralogy, microbial activity, soil texture, temperature, and moisture [8]. Selenate
predominates in aerobic soils with neutral pH, whereas selenite prevails at lower pH and redox potential.
Selenide predominates under strongly reduced soil conditions [9].
The bioavailability and also the toxicity of Se is determined by its forms [10]. In lower concentrations
Se has a positive effect on human health; inhibits some forms of cancer [11], has neuroprotective [12] and
immunological effects (selenoproteins regulate immune cell functions) [13] and inhibit the promotion
of HIV [14]. Insufficient concentrations of Se in humans can cause hypothyroidism, heart disease, and
weaken the immune system [15].
Plants2019, 8, 262; doi:10.3390/plants8080262 www.mdpi.com/journal/plants Selenium as a Protective Agent against Pests:A Review Špela Mechora. Agency for Radwaste Management, Celovška cesta 182, 1000 Ljubljana, Slovenia;
Received: 24 May 2019; Accepted: 30 July 2019; Published: 1 August 2019
