Introduction
Inducing and activating plant immune response is one of the most
effective ways to combat plant virus diseases [1].
Plant virus that has a variety of infection modes, a wide range of host
and a long infection cycle causes a common systemic infection[2]. Control of plant virus disease largely
depends on the application of the virus passivation agents. However,
these agents are not able to provide complete control of the plant viral
diseases owing to their ability to partially reduce the virus infection
and inhibiting the replication and transfer of the virus in the plant[3]. Inducers with the ability to activate plant
resistance are becoming an increasingly important way to control plant
virus disease. Previous studies showed that they can
significantly induce the expression of resistance-related genes and
enhance the immune response of plants to biotic stresses[4]. However, the application of the existing
inducers in the field is limited because their lasting time is shorter
than the existing period of virus infection. In addition, most of the
inducers consist of biological polysaccharides with a structure that is
easily degraded at the complicated environment in the field, rustling in
the reduced efficacy on the control of virus disease[5-6]. Therefore, there is an urgent need to
develop a novel inducer agent with the ability to last the release time
of loaded drugs for the field application.
Calcium ions (Ca2+) as an essential nutrient element,
can promote plant growth, improve photosynthesis, and increase the
synthesis and accumulation of organic matter [7].
Ca2+ as a second messenger is also involved in the
signal transduction of plant to stress responses[8-9]. Previous studies showed that
Ca2+ signals are perceived by four types of calcium
signaling sensor proteins, including calmodulin (CaM ),
calmodulin-like protein (CML ), calcium-dependent protein kinase
(CDPK), and calcium-like protein Calcineurin B-like (CBL)[10-11]. However, CML , CDPK, CBL are found
only in plants and some protists. CMLs , as one of the
plant-specific Ca2+ receptors, participates in plant
biological stress, abiotic stress as well as many developmental
processes [12-13]. It has been documented that
plant defense response, plant growth and development as well as cytokine
regulation, induce changes in intracellular calcium ion concentration,
which results in the induced expression of CMLs gene[14-15]. Accumulated evidence showed that the
up-regulation of CML gene expression is able to significantly
improve the plant resistance to biotic stress[16-17]. For example, CML41 was found to
reduce P. syringae infection by regulating plasmodesmal closure
through mediating a Ca2+ signaling in response to
bacterial pathogens[17]. Leba et al. showed
that CML9 is involved in plant resistance to pathogenic bacteria
through a flagellin-dependent pathway in plants[18]. Tobacco calmodulin-like protein, rgs-CaM,
binds to the RNA silencing (RNAi) suppressor of the tobacco etch virus
to inhibit RNAi and increase tobacco resistance to the virus[19]. CML30 was found to be regulated by
abiotic stress and play diverse roles in plant hormone synthesis and
disease defense [20]. Therefore, CML can be
regarded as an important target site to stimulate plant defense
response.
Recent studies showed that sustained-release hydrogel as a drug carrier
has attracted more interest in delivering therapeutic agents to control
human diseases [21]. Advantages of hydrogel
include a prolonged drug release and action time, increased stability of
loaded drugs and enhanced control efficiency of disease[22-23]. In addition, many studies showed that
sustained-release hydrogel exhibits the ability of carrying multiple
drugs and providing various control of multiple diseases. For example,
porphyrin photosensitizer sinoporphyrin sodium (DVDMS) and Polylactic
acid-copolymerized glycolic acid (PLGA) are loaded into the sodium
alginate-chitosan hydrogel that provides the dual action of
antibacterial and skin regeneration [24].
Therefore, the development of dual functions of hydrogel that could
deliver a range of drugs and provide the broad-spectrum resistance to
multiple diseases is required.
Our previous studies found that coating the surface of hydrogel with
amino oligosaccharides can prolong the release time of drugs loaded in
the hydrogel (ALA-gel)[25]. However, amino
oligosaccharides are highly soluble in water, resulting in the slow
dissolve of the ALA-gel surface and the reduced drug release time from
the ALA-gel. To overcome the drawbacks of ALA-gel and improve the
sustained release of drugs loaded in the hydrogel, a multiple-functional
sodium alginate-lentinan hydrogel (SL-gel) with a dense chitosan shell
(CSL-gel) was developed aiming at improving the control efficacy against
plant virus diseases. As expected, CSL-gel exhibited stable and
sustained LNT release and broad-spectrum anti-virus activities. The
sustainable and controlled release of calcium ions activated the
expression of CML30 that enhances the resistance of tobacco
against virus with a lasting time up to 30 days after CSL-gel treatment.