**13. Induction of ligninases**

**12. Induction of hemicellulases and pectinases**

222 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

to induce the expression of a wide range of hemicellulases.

lytic genes in *A. terreus* [80].

from it in *A. niger* [48, 90].

glucuronidase gene [97].

Hemicellulase expression has been studied mostly in Aspergilli and *T. reesei*. However, a comparison of the genome sequences of *T. reesei* [87] and *Aspergillus niger* [88] demonstrated that *A. niger* is more versatile in the range of hemicellulases, and therefore this specie has become a very useful model fungus for basic studies on CWDEs in recent years. It is known that the presence of the hemicelluloses xylan, xyloglucan, arabinan and mannan usually in‐ duces a high production of hemicellulases, though the mechanism of sensing is still lacking, as assigned for cellulase induction. Usually, small hemicellulose-derived molecules are able

The monosaccharide D-xylose is a well-known inducer of xylanolytic enzymes in *Aspergillus* species. In *A. niger*, D-xylose appears to induce other hydrolase genes rather than xylanase genes, such as the accessory enzymes α-glucuronidase (*aguA*), acetylxylan esterase (*axeA*) and feruloyl esterase (*faeA*) [89, 90]. Some results have been demonstrated that xylose can act as a repressor carbon source of hemicellulase induction at high concentrations [91], whereas other studies demonstrated that utilization of a high D-xylose concentration was beneficial for the induction of hemicellulase-encoding genes [92]. In addition to xylose, xylo‐ biose and D-glucose-β-1,2-D-xylose have been demonstrated to induce expression of xylano‐

The genes encoding enzymes responsible for the degradation of arabinoxylan in *A. niger* were induced by arabinose and L-arabitol. These genes encode enzymes such as arabinofuranosi‐ dases (*afbA* and *afbB*) and arabinoxylan arabinofuranosidases (*axhA*) [6]. High intracellular ac‐ cumulation of L-arabitol in *A. nidulans* mutant strains was able to induce higher amounts of arabinofuranosidases and endoarabinases than in the wild type strain [93]. The arabinolytic system appears to be independent from xylanolytic system in *A. niger*, as demonstrated by the isolation of *ara* mutants of *A. niger* defective for the induction of the genes encoding arabinolyt‐

Regarding to pectinolytic enzymes, D-galacturonic acid, polygalacturonate and sugar beet pectin have been shown to induce virtually all the genes encoding for pectin degradation enzymes in *A. niger*, suggesting that these genes are under the control of a general pectino‐ lytic regulatory system responding to D-galacturonic acid or a metabolic product derived

In *T. ressei*, the induction of hemicellulases was observed during growth in the presence of cellulose, xylan, sophorose, xylobiose and L-arabitol [6]. A xylanase gene (*xyn2*) was in‐ duced by sophorose and xylobiose [95, 96]. Xylobiose was able to induce genes involved in xylan degradation in *T. reesei*, such as the xylanase genes *xyn1* and *xyn2* and the β-xilosidase gene *bxl1*. In addition, sophorose induced some genes encoding enzymes that cleave the side chains of xylan such as *agl1* and *agl2* (α-galactosidase genes), and *glr1*, encoding a α-

Complex mixtures of polysaccharides have been shown to induce a wide range of cellulases/ hemicellulases genes in *A. niger*. For instance, sugarcane bagasse, a by-product of sugar/

ic enzymes, but not for the induction of enzymes belonging to xylanolytic system [94].

The ligninolytic system of many fungi appears to be induced under nutrient deprivation, main‐ ly nitrogen, carbon and sulphur. Therefore, the expression of most of the ligninolytic genes is regarded as a stress response to nutrient deprivation. Also, the presence of Mn(II) is required for induction of manganese peroxidase (MnPs) genes in the white-rot fungi *P. chrysosporium* [99 - 101]. Besides nutrient depletion and the presence of Mn(II), MnP genes were found to be expressed under heat shock in nitrogen limited cultures [102], and in the presence of H2O2, chemical stress or molecular oxygen [103]. Similarly, lignin peroxidase (LiP) genes are also ex‐ pressed under carbon and nitrogen limitation in *P. chrysosporium* [104].

Laccases are multicopper oxidase proteins, and therefore can be induced by copper, al‐ though other metals can induce the expression of laccase genes as well, such as manganese and cadmium [105, 106]. Many natural and xenobiotic aromatic compounds, which are often related to lignin or humic substances, were shown to induce genes related to laccases [107]. In general, it has been postulated that laccases are the first enzymes degrading lignin, and possible further degradation products released from the polymer could act as inducers to amplify laccase expression, and subsequently induce other ligninolytic genes [105].
