2 research outputs found
Adsorption of Insecticidal Cry1Ab Protein to Humic Substances. 1. Experimental Approach and Mechanistic Aspects
Adsorption is a key process affecting the fate of insecticidal
Cry proteins (<i>Bt</i> toxins), produced by genetically
modified <i>Bt</i> crops, in soils. However, the mechanisms
of adsorption to soil organic matter (SOM) remain poorly understood.
This work assesses the forces driving the adsorption of Cry1Ab to
Leonardite humic acid (LHA), used as a model for SOM. We studied the
effects of solution pH and ionic strength (<i>I</i>) on
adsorption using a quartz crystal microbalance with dissipation monitoring
and optical waveguide lightmode spectroscopy. Initial Cry1Ab adsorption
rates were close to diffusion-limited and resulted in extensive adsorption,
even at pH >6, at which LHA and Cry1Ab carry negative net charges.
Adsorption increased with decreasing <i>I</i> at pH >6,
indicating Cry1AbāLHA patch-controlled electrostatic attraction
via positively charged domains of Cry1Ab. Upon rinsing, only a fraction
of Cry1Ab desorbed, suggesting a range of interaction energies of
Cry1Ab with LHA. Different interaction energies likely resulted from
nonuniformity in the LHA surface polarity, with higher Cry1Ab affinities
to more apolar LHA regions due to the hydrophobic effect. Contributions
from the hydrophobic effect were substantiated by comparison of the
adsorption of Cry1Ab and the reference proteins albumin and lysozyme
to LHA and to apolar and polar model surfaces
Adsorption of Insecticidal Cry1Ab Protein to Humic Substances. 2. Influence of Humic and Fulvic Acid Charge and Polarity Characteristics
Assessing the fate and potential risks of transgenic
Cry proteins
in soils requires understanding of Cry protein adsorption to soil particles.
The companion paper provided evidence that patch-controlled electrostatic
attraction (PCEA) and the hydrophobic effect contributed to Cry1Ab
protein adsorption to an apolar humic acid (HA). Here, we further
assess the relative importance of these contributions by comparing
Cry1Ab adsorption to seven humic substances varying in polarity and
charge, at different solution pH and ionic strength, <i>I</i>. Cry1Ab adsorption to relatively apolar HAs at <i>I</i> = 50 mM exhibited rapid initial rates, was extensive, and was only
partially reversible at pH 5ā8, whereas adsorption to more
polar fulvic acids was weak and reversible or absent at pH >6.
The
decrease in adsorption with increasing HS polarity at all tested pH
strongly supports a large contribution from the hydrophobic effect
to adsorption, particularly at <i>I</i> = 50 mM when PCEA
was effectively screened. Using insect bioassays, we further show
that Cry1Ab adsorbed to a selected HA retained full insecticidal activity.
Our results highlight the need to consider adsorption to soil organic
matter in models that assess the fate of Cry proteins in soils