The plant cell wall has a diversity of functions. It provides a structural framework to support plant growth and acts as the first line of defence when the plant encounters pathogens. The cell wall must also retain some flexibility, such that when subjected to developmental, biotic or abiotic stimuli it can be rapidly remodelled in response. Genes encoding enzymes capable of synthesising or hydrolysing components of the plant cell wall show differential expression when subjected to different stresses, suggesting they may facilitate stress tolerance through changes in cell wall composition. In this review we summarise recent genetic and transcriptomic data from the literature supporting a role for specific cell wall-related genes in stress responses, in both dicot and monocot systems. These studies highlight that the molecular signatures of cell wall modification are often complex and dynamic, with multiple genes appearing to respond to a given stimulus. Despite this, comparisons between publically available datasets indicate that in many instances cell wall-related genes respond similarly to different pathogens and abiotic stresses, even across the monocot-dicot boundary. We propose that the emerging picture of cell wall remodelling during stress is one that utilises a common toolkit of cell wall-related genes, multiple modifications to cell wall structure, and a defined set of stress-responsive transcription factors that regulate them.

▪ Abstract Seed development requires coordinated expression of embryo and endosperm and has contributions from both sporophytic and male and female gametophytic genes. Genetic and molecular analyses in recent years have started to illuminate how products of these multiple genes interact to initiate seed development. Imprinting or differential expression of paternal and maternal genes seems to be involved in controlling seed development, presumably by controlling gene expression in developing endosperm. Epigenetic processes such as chromatin remodeling and DNA methylation affect imprinting of key seed-specific genes; however, the identity of many of these genes remains unknown. The discovery of FIS genes has illuminated control of autonomous endosperm development, a component of apomixis, which is an important developmental and agronomic trait. FIS genes are targets of imprinting, and the genes they control in developing endosperm are also regulated by DNA methylation and chromatin remodeling genes. These results define some exciting future areas of research in seed development.

Female gamete development in Arabidopsis ovules comprises two phases. During megasporogenesis, a somatic ovule cell differentiates into a megaspore mother cell and undergoes meiosis to produce four haploid megaspores, three of which degrade. The surviving functional megaspore participates in megagametogenesis, undergoing syncytial mitosis and cellular differentiation to produce a multicellular female gametophyte containing the egg and central cell, progenitors of the embryo and endosperm of the seed. The transition between megasporogenesis and megagametogenesis is poorly characterised, partly owing to the inaccessibility of reproductive cells within the ovule. Here, laser capture microdissection was used to identify genes expressed in and/or around developing megaspores during the transition to megagametogenesis. ARGONAUTE5 (AGO5), a putative effector of small RNA (sRNA) silencing pathways, was found to be expressed around reproductive cells during megasporogenesis, and a novel semi-dominant ago5-4 insertion allele showed defects in the initiation of megagametogenesis. Expression of a viral RNAi suppressor, P1/Hc-Pro, driven by the WUSCHEL and AGO5 promoters in somatic cells flanking the megaspores resulted in a similar phenotype. This indicates that sRNA-dependent pathways acting in somatic ovule tissues promote the initiation of megagametogenesis in the functional megaspore. Notably, these pathways are independent of AGO9, which functions in somatic epidermal ovule cells to inhibit the formation of multiple megaspore-like cells. Therefore, one somatic sRNA pathway involving AGO9 restricts reproductive development to the functional megaspore and a second pathway, inhibited by ago5-4 and P1/Hc-Pro, promotes megagametogenesis.