The role of cell wall phenolics during the early remodelling of cellulose-deficient maize cells

Feruloyl esterase activity and its role in regulating the feruloylation of maize cell walls

Cell walls of grasses have ferulic acid (FA) ester-linked to the arabinosyl substitutions of arabinoxylan (AX). Feruloyl esterases (FAE) are carboxylic acid esterases that release FA from cell walls and synthetic substrates. Despite the importance of FA for cell wall recalcitrance and in response to biotic and abiotic stresses, the physiological function of plant FAEs remains unclear. Here, we developed a simple method for the determination of FAE activity (ZmFAE) in maize using the total protein extract and investigated its role in regulating the feruloylation of cell wall. The method includes a single protein extraction and enzymatic reaction with protein concentration as low as 65 μg at 35 °C for 30 min, using methyl ferulate as the substrate. The methodology allowed the determination of the apparent K_m (392.82 μM) and V_max (79.15 pkat mg^-1 protein). We also found that ZmFAE activity was correlated (r = 0.829) with the levels of FA in seedling roots, plant roots and leaves of maize. Furthermore, the exposure to osmotic stress resulted in a 50% increase in ZmFAE activity in seedling roots. These data suggest that FAE-catalyzed reaction is important for cell wall feruloylation during plant development and in response to abiotic stress. We conclude proposing a model for the feruloylation and deferuloylation of AX, which explains the role of FAE in regulating the levels of ester-linked FA. Our model might orient further studies investigating the role of plant FAEs and assist strategies for genetic engineering of grasses to obtain plants with reduced biomass recalcitrance.

Probing the role of cell wall feruloylation during maize development by differential expression of an apoplast targeted fungal ferulic acid esterase

While many aspects of the growth of maize are well understood, the role of cell wall feruloylation particularly during internode elongation has not been firmly established, but results so far indicate that it has significant implications for both biofuel feedstock conversion and for crop yield. The growth of the cell wall is achieved by synthesis, integration and cross-linking between wall polymers. As ferulate oxidative coupling of arabinoxylan side chains constitutes a significant type of cross-link in grass cell walls, it is expected to have a crucial role in plant growth. Making use of plants expressing an apoplast targeted Aspergillus niger FAEA under the control of either a constitutive or an inducible promoter, the role of cell wall feruloylation in maize internode expansion was investigated. Analysis of FAEA expressing plants showed that where FAEA was targeted to the apoplast under a constitutive promoter, plants varied in stature either from semi-dwarf plants with a 40-60% height reduction, to extreme dwarf mutants with over 90% reduction in plant heights compared to controls. Results indicate that disruption of cell wall feruloylation by FAEA occurs before the start of rapid internode expansion is initiated and affects the normal course of internode elongation, resulting in short internodes and dwarfed plants. In contrast, when under the inducible Lm See1 senescence promoter, FAEA activity was found to be low up to the VT stage of development but increased significantly at the VR stage as plants began to senesce, strongly suggesting that normal cell wall feruloylation is required for the process of internode expansion. In addition, with apoplast targeted expression of FAEA under control of the senescence enhanced promoter it was possible to demonstrate decreased cell wall feruloylation without affecting internode expansion or other aspects of plant development.

Phloroglucinol Mediated Plant Regeneration of Ornithogalum dubium as the Sole "Hormone-Like Supplement" in Plant Tissue Culture Long-Term Experiments

Tissue culture is an essential requirement in plant science to preserve genetic resources and to expand naturally occurring germplasm. A variety of naturally occurring and synthetic hormones are available to induce the processes of dedifferentiation and redifferentiation. Not all plant material is susceptible to tissue culture, and often complex media and hormone requirements are needed to achieve successful plant propagations. The availability of new hormones or chemicals acting as hormones are critical to the expansion of tissue culture potentials. Phloroglucinol has been shown to have certain hormone-like properties in a variety of studies. Ornithogalum dubium, an important geophyte species, was used to characterise the potential of phloroglucinol as the sole plant-like hormone in a tissue culture experiment. Tissue culture, plant regeneration, total phenolic and genetic variability were established by applying a variety of methods throughout long-term experiments. Phloroglucinol did induce callus formation and plant regeneration when used as the sole supplement in the media at a rate of 37%, thus demonstrating auxin/cytokines-like properties. Callus formation was of 3 types, friable and cellular, hard and compact, and a mixture of the two. The important finding was that direct somatogenesis did occur albeit more frequently on younger tissue, whereby rates of induction were up to 52%. It is concluded that phloroglucinol acts as a "hormone-like" molecule and can trigger direct embryogenesis without callus formation.