Bacillus thuringiensis (Bt)-based products are the most successful microbial insecticides
to date. This entomopathogenic bacterium produces different kinds of proteins whose
specific toxicity has been shown against a wide range of insect orders, nematodes,
mites, protozoa, and human cancer cells. Some of these proteins are accumulated in
parasporal crystals during the sporulation phase (Cry and Cyt proteins), whereas other
proteins are secreted in the vegetative phase of growth (Vip and Sip toxins). Currently,
insecticidal proteins belonging to different groups (Cry and Vip3 proteins) are widely
used to control insect pests and vectors both in formulated sprays and in transgenic
crops (the so-called Bt crops). Despite the extensive use of these proteins in insect
pest control, especially Cry and Vip3, their mode of action is not completely understood.
The aim of this Special Issue was to gather information that could summarize (in the
form of review papers) or expand (research papers) the knowledge of the structure
and function of Bt proteins, as well as shed light on their mode of action, especially
regarding the insect receptors. This subject has generated great interest, and this
interest has been materialized into the 18 papers published in this issue.
This Special Issue, “Bacillus thuringiensis Toxins: Functional Characterization and
Mechanism of Action”, includes five review papers and 13 research papers. The review
papers bring up to date important aspects of Bt pathogenicity, such as its interaction
with the intestinal microbiota and the immune system of the insect [1]. The current
knowledge about Vip proteins has also been reviewed [2], as has the contribution that
the use of toxin mutants has made to the knowledge of the mode of action of the three-domain
Cry proteins [3]. On the other hand, two more review papers recapitulate the information
on the cytocidal activity of Bt proteins [4] or the insecticidal activity of Bt proteins
against coleopteran pests [5]. All these review papers are of high value, allowing
readers to stay updated on the different aspects of the Bt field described here.
The Special Issue also gathers information that could expand the knowledge of the
structure and function of Bt proteins and sheds light on their mode of action, especially
regarding the insect receptors. Publishing papers focusing on the steps that remain
blurred within the mode of action of all Bt insecticidal proteins, including the three-domain
Cry proteins, was one of the main goals. The role of receptors such as cadherin, ABCC2,
and ABCA2 on the toxicity of Bt proteins in different lepidopterans has been investigated
in three different papers [6,7,8]. In addition, other steps in the mode of action
(that comprises protein solubilization, activation, binding, oligomerization, and
pore formation) have also been addressed. Examples of these steps include the involvement
of a novel trypsin protein for toxin activation in Plutella xylostella, discovered
after studying a Cry1Ac resistant strain [9], and the promotion of oligomerization
of the activated Cry1Ia with insect brush border midgut vesicles, in vitro [10]. The
toxicity-promoting effect of a Bt chitin-binding protein that binds to the insect
peritrophic matrix has also been studied [11]. Moreover, the Special Issue includes
a paper highlighting the synergistic mosquitocidal activity of the parasporal Cry
and Cyt proteins present in B. thuringiensis ser. israelensis [12], and it also includes
a manuscript focused on deciphering the amino acid residues important for the interaction
of Cyt2A protein with membrane lipids, a binding step necessary to exert its cytolytic
action [13].
The vegetative insecticidal proteins (Vip3) secreted by Bacillus thuringiensis are
nowadays considered as the new generation of insecticidal Bt toxins because of their
different structural and molecular properties regarding the classical Bt 3-D Cry proteins.
Vip3 toxins have been already introduced in Bt-crops to control lepidopteran pests.
However, little is known about their mode of action. In the Special Issue, five papers
analyze different aspects of its biology. They cover aspects ranging from its crystal
structure [14] and structural–functional domain analyses [15] to different aspects
in the mode of action, such as a study of a possible receptor (the alkaline phosphatase)
in a resistant strain [16], the role of oligomerization in toxicity [17], and the
study of intracellular events promoted by Vip3A intoxication in Spodoptera frugiperda
Sf9 cells [18].
In summary, the Special Issue brings together papers of important scientific value
in the field of Bt. The review and research papers included will help keep readers
up to date on the topic and, at the same time, will contribute to increasing the vast
knowledge of Bt and its insecticidal proteins. These studies will help to provide
useful information for the development of new strategies to fight against pest insects,
in the least aggressive and harmful but better environmental scenario.