Melanin Is Not Required for Turgor Generation but Enhances Cell-Wall Rigidity in Appressoria of the Corn Pathogen Colletotrichum graminicola

Abstract

The ascomycete and causative agent of maize anthracnose and stem rot, Colletotrichum graminicola, differentiates melanized infection cells called appressoria that are indispensable for breaching the plant cell wall. High concentrations of osmolytes accumulate within the appressorium, and the internal turgor pressure of up to 5.4 MPa provides sufficient force to penetrate the leaf epidermis directly. In order to assess the function of melanin in C. graminicola appressoria, we identified and characterized the polyketide synthase 1 (CgPKS1) gene which displayed high similarity to fungal polyketide synthases (PKS) involved in synthesis of 1,3,6,8-tetrahydronaphthalene, the first intermediate in melanin biosynthesis. Cgpks1 albino mutants created by targeted gene disruption were unable to penetrate intact leaves and ruptured frequently but, surprisingly, were able to penetrate ultrathin polytetrafluoroethylene membranes mimicking the plant surface. Nonmelanized Cgpks1 appressoria were sensitive to externally applied cell-wall-degrading enzymes whereas melanized appressoria were not affected. Expression studies using a truncated CgPKS1 fused to green fluorescent protein revealed fluorescence in immature appressoria and in setae, which is in agreement with transcript data obtained by RNA-Seq and quantitative polymerase chain reaction. Unexpectedly, surface scans of mutant and wild-type appressoria revealed considerable differences in cell-wall morphology. Melanization of appressoria is indispensable for successful infection of intact leaves. However, cell collapse experiments and analysis of the appressorial osmolyte content by Mach-Zehnder interferometry convincingly showed that melanin is not required for solute accumulation and turgor generation, thus questioning the role of melanin as a barrier for osmolytes in appressoria of C. graminicola.

Transcriptome Profiling of the Phaseolus vulgaris - Colletotrichum lindemuthianum Pathosystem

“…In total, these studies have led to a better understanding of the early defense signaling mechanisms during pathogen infection and proliferation. For example, Ludwig et al [48] reported melanin as indispensable for pathogenesis in C . graminicola .…”

Effect of Temperature on Rice Blast Infection Process with Emphasis on Appressoria Formation by Magnaporthe oryzae

“…This indicated that osmolytes barrier was not provided by melanin in C. graminicola, may be same way it does in M. oryzae but melanin plays crucial role in rupture of cell wall (Ludwig et al, 2014).…”

“…Recent experiment on Colletotrichum graminicola has shown that if melanin biosynthesis was inhibited than also pathogen could able to the penetration in leaves Meanwhile cell collapse assays showed that melanin was not required for turgour pressure in C. graminicola (Ludwig et al, 2014). This indicated that osmolytes barrier was not provided by melanin in C. graminicola, may be same way it does in M. oryzae but melanin plays crucial role in rupture of cell wall (Ludwig et al, 2014).…”

Infection process of Phakopsora gossypii in cotton leaves

“…According to Vittal et al (2014), the early events of uredospore germination, appressorium formation, and penetration of P. pachyrhizi on the soybean leaf occurred by 24 hai. The formation of appressoria is a pivotal event that allows a number of fungi to penetrate the leaf cuticle and establish a successful infection in their hosts (Chang et al, 2014;Dean, 1997;Loehrer et al, 2014;Ludwig et al, 2014;Mendgen and Deising, 1993). Certain biotrophic and hemibiotrophic fungi, such as those causing powdery mildews and anthracnose, respectively, can directly penetrate the cuticle through the action of cutinases, but in most cases, the strong osmotic pressure exerted by the melanized or not appressoria becomes detrimental (Dean, 1997;Ludwig et al, 2014;Mendgen and Deising, 1993).…”

“…The formation of appressoria is a pivotal event that allows a number of fungi to penetrate the leaf cuticle and establish a successful infection in their hosts (Chang et al, 2014;Dean, 1997;Loehrer et al, 2014;Ludwig et al, 2014;Mendgen and Deising, 1993). Certain biotrophic and hemibiotrophic fungi, such as those causing powdery mildews and anthracnose, respectively, can directly penetrate the cuticle through the action of cutinases, but in most cases, the strong osmotic pressure exerted by the melanized or not appressoria becomes detrimental (Dean, 1997;Ludwig et al, 2014;Mendgen and Deising, 1993). Other biotrophic fungi such as those causing rust can penetrate their host indirectly by entering the leaf mesophyll through stomata (Dean, 1997;Mendgen and Deising, 1993).…”