Abstract
BackgroundPectin is an abundant component in many fruit and vegetable wastes and could therefore be an excellent resource for biorefinery, but is currently underutilized. Fungal pectinases already play a crucial role for industrial purposes, such as for foodstuff processing. However, the regulation of pectinase gene expression is still poorly understood. For an optimal utilization of plant biomass for biorefinery and biofuel production, a detailed analysis of the underlying regulatory mechanisms is warranted. In this study, we applied the genetic resources of the filamentous ascomycete species Neurospora crassa to screen for transcription factors that play a major role in pectinase induction.ResultsThe pectin degradation regulator-1 (PDR-1) was identified through a transcription factor mutant screen in N. crassa. The Δpdr-1 mutant exhibited a severe growth defect on pectin and all tested pectin-related poly- and monosaccharides. Biochemical as well as transcriptional analyses of WT and the Δpdr-1 mutant revealed that while PDR-1-mediated gene induction was dependent on the presence of l-rhamnose, it also strongly affected the degradation of the homogalacturonan backbone. The expression of the endo-polygalacturonase gh28-1 was greatly reduced in the Δpdr-1 mutant, while the expression levels of all pectate lyase genes increased. Moreover, a pdr-1 overexpression strain displayed substantially increased pectinase production. Promoter analysis of the PDR-1 regulon allowed refinement of the putative PDR-1 DNA-binding motif.ConclusionsPDR-1 is highly conserved in filamentous ascomycete fungi and is present in many pathogenic and industrially important fungi. Our data demonstrate that the function of PDR-1 in N. crassa combines features of two recently described transcription factors in Aspergillus niger (RhaR) and Botrytis cinerea (GaaR). The results presented in this study contribute to a broader understanding of how pectin degradation is orchestrated in filamentous fungi and how it could be manipulated for optimized pectinase production.
Highlights
Pectin is an abundant component in many fruit and vegetable wastes and could be an excellent resource for biorefinery, but is currently underutilized
Identification of NCU09033 as putative pectin degradation regulator Three enzymes, namely the endo- and exo-polygalacturonases GH28-1, GH28-2, and the pectin methylesterase (PME) CE8-1 have previously been shown to be essential for growth of N. crassa on pectin [21], indicating that HG/d-Galacturonic acid (d-GalA) could be the main carbon source for N. crassa in this polysaccharide
To increase the specificity of our query for a putative pectin degradation regulator-1 (PDR-1)-binding site, a search was performed in the promoter regions of five major pectinase genes regulated by PDR-1 (NCU09034, NCU09035, gh28-1, gh105-1, and pdr-1 itself ), in which we looked for the Gal4p-like inverted repeat with a TCGG motif, and allowing for imperfect matches
Summary
Pectin is an abundant component in many fruit and vegetable wastes and could be an excellent resource for biorefinery, but is currently underutilized. The most structurally heterogeneous type of plant polysaccharides, are able to form highly dynamic gelling structures and are the major polysaccharides of the middle lamella, which is the shared interface and connecting layer between plant cells. They play an integral role in plant cell growth and morphogenesis [4, 5]. The production of pectinases by plants during leaf abscission [8] or by phytopathogenic fungi during host colonization [9,10,11] highlights the importance of targeted degradation of pectin during maceration processes. The constituent sugars of pectin are a potentially valuable feedstock for biomaterials or biofuels, if efficiently liberated and converted [15, 16]
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