Cloning and Genomic Organization of a Rhamnogalacturonase Gene from Locally Isolated Strain of Aspergillus niger

Expression of an endo-rhamnogalacturonase from Aspergillus aculeatus enhances release of Arabidopsis transparent mucilage

Mucilage is a gelatinous and sticky hydrophilic polysaccharide released from epidermal cells of seed coat after the hydration of mature seeds and is composed primarily of unbranched rhamnogalacturonan I (RG-I). In this study, we produced a recombinant endo-RG-I hydrolase from Aspergillus aculeatus (AaRhgA) in the fission yeast Schizosaccharomyces pombe and examined its substrate preference for pyridylaminated (PA) RG-I with the various degrees of polymerization (DP). Recombinant AaRhgA requires PA-RG-I with a DP of 10 or higher for its hydrolase activity. We heterologously expressed the AarhgA gene under the strong constitutive promoter, cauliflower mosaic virus 35S promoter, in Arabidopsis thaliana. In a series of biochemical analyses of each mucilage fraction released from the water-imbibed seeds of the transgenic plants, we found the enhanced deposition of the transparent mucilage layer that existed in the peripheral regions of the adherent mucilage and was not stained with ruthenium red. This study demonstrated the feasibility of manipulating the mucilage organization by heterologous expression of the endo-RG-I hydrolase.

Host Recognition and Specific Infection of Endomelanconiopsis endophytica during Early Infection

Coevolution between the pathogen and host plant drives pathogenic effector diversity. However, the molecular mechanism behind host-specific pathogenesis remains to be explored. Here, we present a 43 Mb whole-genome sequence of Endomelanconiopsis endophytica strain LS29, a host-specific pathogen of the common subtropical tree Castanopsis fissa. We described its genome annotations and identified its effector candidates. By performing temporal transcriptome sequencing of E. endophytica on C. fissa during early infection, we found that E. endophytica repressed other microbes in order to attack the tissue of the host by producing antibiotics earlier than 24 h post-inoculation (hpi). Simultaneously, a variety of effectors were secreted to recognize the host plant, but most of them showed a significantly opposing expression regulation trend after 24 hpi, indicating that 24 hpi represents a key time point between host recognition and specific infection. Furthermore, a comparison of isoenzymes showed that only a few effectors were identified as specific effectors, which were involved in hydrolyzing the compounds of the plant cell wall and releasing fatty acids during the early infection of C. fissa. Our results determined host recognition timing and identified a specific catalog of effectors, which are crucial for revealing the molecular mechanism of host-specific pathogenesis.

A Profusion of Molecular Scissors for Pectins: Classification, Expression, and Functions of Plant Polygalacturonases

In plants, the construction, differentiation, maturation, and degradation of the cell wall are essential for development. Pectins, which are major constituents of primary cell walls in eudicots, function in multiple developmental processes through their synthesis, modification, and degradation. Several pectin modifying enzymes regulate pectin degradation via different modes of action. Polygalacturonases (PGs), which function in the last step of pectin degradation, are a crucial class of pectin-modifying enzymes. Based on differences in their hydrolyzing activities, PGs can be divided into three main types: exo-PGs, endo-PGs, and rhamno-PGs. Their functions were initially investigated based on the expression patterns of PG genes and measurements of total PG activity in organs. In most plant species, PGs are encoded by a large, multigene family. However, due to the lack of genome sequencing data in early studies, the number of identified PG genes was initially limited. Little was initially known about the evolution and expression patterns of PG family members in different species. Furthermore, the functions of PGs in cell dynamics and developmental processes, as well as the regulatory pathways that govern these functions, are far from fully understood. In this review, we focus on how recent studies have begun to fill in these blanks. On the basis of identified PG family members in multiple species, we review their structural characteristics, classification, and molecular evolution in terms of plant phylogenetics. We also highlight the diverse expression patterns and biological functions of PGs during various developmental processes, as well as their mechanisms of action in cell dynamic processes. How PG functions are potentially regulated by hormones, transcription factors, environmental factors, pH and Ca²⁺ is discussed, indicating directions for future research into PG function and regulation.