Abstract
Invertases (INVs) play essential roles in plant growth in response to environmental cues. Previous work showed that plant invertases can be post-translationally regulated by small protein inhibitors (INVINHs). Here, this study characterizes a proteinaceous inhibitor of INVs in maize (Zm-INVINH4). A functional analysis of the recombinant Zm-INVINH4 protein revealed that it inhibited both cell wall and vacuolar invertase activities from maize leaves. A Zm-INVINH4::green fluorescent protein fusion experiment indicated that this protein localized in the apoplast. Transcript analysis showed that Zm-INVINH4 is specifically expressed in maize sink tissues, such as the base part of the leaves and young kernels. Moreover, drought stress perturbation significantly induced Zm-INVINH4 expression, which was accompanied with a decrease of cell wall invertase (CWI) activities and an increase of sucrose accumulation in both base parts of the leaves 2 to 7 days after pollinated kernels. In summary, the results support the hypothesis that INV-related sink growth in response to drought treatment is (partially) caused by a silencing of INV activity via drought-induced induction of Zm-INVINH4 protein.
Highlights
To search invertase inhibitor homologs and elucidate the role of cell walls or/and vacuolar invertase inhibitors in maize, previously characterized monocot invertase inhibitor sequences were used as queries to search in the maize genome systematically with a basic local alignment search tool (BLAST)
A deduced amino acid sequence of Zm-INVINH4 was aligned with previously reported genes from other species using the CLUSTAL W2 program [23]
The result of the comparison of Zm-INVINH4 with INVINH from other species showed that there was four conserved Cys residues in all of the sequences (Figure 1), which was a hallmark of all known plant invertase inhibitors [17]
Summary
Acid invertases regulate cell divisions of the endosperm and embryo, especially during the early kernel development [10]. An invertase inhibitor, named Zm-INVINH1, localized to the embryo surrounding the region during early kernel development, has been found, and its enzyme activity was identified in vitro [22]. This research presents an apoplast-localized proteinaceous inhibitor of INV in maize, named Zm-INVINH4, which has inhibitory activity by reducing maize invertase activity and is expressed in the base part of leaves and young kernels after drought stimulus, accompanied by the decrease of cell wall invertase (CWI) activities and the increase of sucrose accumulation. Results suggest that Zm-INVINH4 may modulate the INV-related source to sink growth and the development in maize under the drought stress regime
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