Abstract
BackgroundBowman-Birk inhibitors (BBI) are a family of serine-type protease inhibitors that modulate endogenous plant proteolytic activities during different phases of development. They also inhibit exogenous proteases as a component of plant defense mechanisms, and their overexpression can confer resistance to phytophagous herbivores and multiple fungal and bacterial pathogens. Dicot BBIs are multifunctional, with a “double-headed” structure containing two separate inhibitory loops that can bind and inhibit trypsin and chymotrypsin proteases simultaneously. By contrast, monocot BBIs have a non-functional chymotrypsin inhibitory loop, although they have undergone internal duplication events giving rise to proteins with multiple BBI domains.ResultsWe used a Hidden Markov Model (HMM) profile-based search to identify 57 BBI genes in the common wheat (Triticum aestivum L.) genome. The BBI genes are unevenly distributed, with large gene clusters in the telomeric regions of homoeologous group 1 and 3 chromosomes that likely arose through a series of tandem gene duplication events. The genomes of wheat progenitors also contain contiguous clusters of BBI genes, suggesting this family underwent expansion before the domestication of common wheat. However, the BBI gene family varied in size among different cultivars, showing this family remains dynamic. Because of these expansions, the BBI gene family is larger in wheat than other monocots such as maize, rice and Brachypodium.We found BBI proteins in common wheat with intragenic homologous duplications of cysteine-rich functional domains, including one protein with four functional BBI domains. This diversification may expand the spectrum of target substrates. Expression profiling suggests that some wheat BBI proteins may be involved in regulating endogenous proteases during grain development, while others were induced in response to biotic and abiotic stresses, suggesting a role in plant defense.ConclusionsGenome-wide characterization reveals that the BBI gene family in wheat is subject to a high rate of homologous tandem duplication and deletion events, giving rise to a diverse set of encoded proteins. This information will facilitate the functional characterization of individual wheat BBI genes to determine their role in wheat development and stress responses, and their potential application in breeding.
Highlights
Bowman-Birk inhibitors (BBI) are a family of serine-type protease inhibitors that modulate endogenous plant proteolytic activities during different phases of development
A second search using a custom Hidden Markov Model (HMM) profile built from an alignment of BBIs from the first screen yielded additional BBIs in every species analyzed, and for wheat, included 13 BBI proteins not associated with a BBI protease inhibitor family (Pfam) domain in their International wheat genome sequencing consortium (IWGSC) RefSeq v1.0 gene model annotations [55]
Our study revealed that BBI domain duplication, possibly originating from incomplete gene duplication followed by gene fusion or internal duplication, resulted in further diversification of encoded wheat BBI proteins, potentially enlarging the spectrum of their protease substrates (Fig. 6)
Summary
Bowman-Birk inhibitors (BBI) are a family of serine-type protease inhibitors that modulate endogenous plant proteolytic activities during different phases of development They inhibit exogenous proteases as a component of plant defense mechanisms, and their overexpression can confer resistance to phytophagous herbivores and multiple fungal and bacterial pathogens. PIs regulate the activity of endogenous proteases to prevent proteolytic degradation, for example, by controlling the mobilization of storage proteins in seeds and kernels, and regulating senescence [5, 6] They play important roles in plant defense by regulating the activity of exogenous proteases from different types of pests and pathogens to prevent cellular damage [7]. The latest PI classifications are maintained in the MEROPS database [15]
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