Molecular analysis of transfer cell development in maize endosperm

For thousands of years, humans and animals have depended on plant seeds as a food source important for their survival. In fact plant seeds, especially seeds from crop plants (such as rice, barley, wheat and maize), are of great cultural importance, as they are thought to be responsible for the rise of human civilizations and the beginning of agriculture and animal domestication. Today, crop seeds account for two-thirds of the world's caloric intake in the form of human feed and animal fodder, and they also have an important commercial value as a source of energy or biofuel. Plant seeds are formed as a result of sexual reproduction, and represent the beginning of the next generation; for seeds germinate and grow into seedlings, thereafter becoming a mature plant. Seeds develop within the deeply embedded female sexual organs of the mother plant. During early stages of development, seeds are highly dependent on obtaining their nutrients from the maternal tissues, and the uptake of such nutrients is mediated by a specialized seed transfer tissue. If this transfer tissue is not formed properly or functions incorrectly, seed development is affected, often leading to smaller seeds being formed, or seeds aborting. Recent studies of crop seeds have shown the existence of several genes that are only expressed in the transfer cell tissue, although the function of these genes remains unknown. We have identified a small group of transfer cell-specific genes that encode very small proteins also located in this tissue in maize. We have found that by altering the function of these genes and proteins, the transfer cell tissue develops abnormally, resulting in smaller maize seeds being formed. These results suggest that these genes are necessary for the correct development of the maize seed transfer tissue. We therefore propose to investigate the biological function of these proteins during seed development, with the aim of gaining insight into how development of this tissue is regulated. Because we have also identified other proteins that interact with these specific proteins, we will analyze whether they are equally important in regulating the development of the maize seed transfer tissue. By understanding both how the transfer cell tissue is formed and how it is able to fulfil its role in regulating the uptake of nutrients from the maternal tissue to the developing seed, we can use this information to optimise grain yield and grain size not just in maize, but also in other cereals.

At the maternal interface of most seeds is located a group of specialised cells, termed transfer cells. Transfer cells are an essential component of the developing seed, for they function as the primary source of nutrient acquisition. However, little is known about how this tissue is formed and developmentally regulated. This proposal focuses on understanding the function of a transfer cell-specific small glycosylated cystein-rich peptide, MEG1, which shares homology to other proteins found only in the grasses. Preliminary data show that altering expression of MEG1 leads to abnormal transfer cell development and a drastic reduction in seed size, with seeds often aborting. Therefore MEG1 is implicated in regulating transfer cell development, function, and ultimately, seed size. This proposed research will address the precise role of MEG1 by analysing existing missexpression lines and by generating chemically-inducible transgenic lines to adopt a more refined approach to the study of MEG1 proteins in maize. Using conventional microscopy and immunohistochemical methods, we will also determine the exact subcellular localisation of MEG1 proteins and assess whether MEG1 glycosylation is essential for MEG1 localisation and function. The nature of MEG1 peptides is suggestive for their role in signalling, and we have identified three MEG1-interacting proteins. Using biochemical and microscopy techniques, we will assess the nature of these interactions. We will also identify and characterise mutations in MEG1-interacting partners and assess their role in regulating transfer cell identity and development. Collectively, these studies will provide novel data on factors regulating transfer cell identity and differentiation, as well as uncovering a novel signalling pathway in early seed development. Increased understanding of how transfer cells are formed and how they regulate nutrient uptake is an essential first step in improving cereal grain compositions and grain yield.