Appropriately Timing Seed Germination- New Players and Upstream Controllers

Abstract

Seed germination is a most critical developmental phase change, particularly under abiotic stress conditions such as drought or salinity. The timing of germination plays a major role in setting the timeline of the plant developmental cycle, and in conditioning seedling survival and adaption to unfavourable environments until completion of the plant developmental cycle and successful reproduction. Over recent decades, great progress has been made in understanding seed dormancy, its establishment during maturation on the mother plant, and the factors controlling its release after shedding. In contrast, little is known about the genetic determinants of germination in non-dormant seeds, especially under abiotic stresses. The Arabidopsis ERECTA family AtERf is demonstrated master regulator of various aspects of aerial plant growth and development, and is also involved in responses to environmental stresses, and some immune responses. A recent study (Nanda et al., 2019) uncovered another, unsuspected function, especially in the sensitivity of seed germination to salinity and osmotic stress. The germination of seeds with reduced ERECTA family signalling was found to slowed down, delayed or even halted by salinity and osmotic stress, through largely reversible mechanisms, so that when conditions became more favourable again, germination generally resumed, without significant loss of seed viability. That adaptive function was shown to be primarily under maternal control, and to involve the three family members, with specificity, among ERf members. The present study followed from that work, with a two-fold aim: a) Better understand the physiological and molecular bases of the maternal inheritance of the ERf-mediated germination phenotype; b) Test conservation across species of the newly discovered ERf regulatory function in Arabidopsis seed germination, with a focus on barley (Hordeum vulgare, Hv), a major staple food and pillar of the brewing industry. The first Chapter of this thesis describes the discovery of a novel molecular interactor of the Arabidopsis AtERf - the MYB family transcription factor AtGL1- previously unknown to be acting in seeds, nor to interact with the extensively documented ERf signalling pathways in aerial organs. GL1 is here shown to synergistically interact with the ERf members, in a gene combination-dependent manner, to regulate the initiation, progression, and timing of germination completion under saline conditions. A second Chapter reports on a genome-wide analysis of ERf- and GL1-mediated changes in transcriptional networks during early seed development and differentiation through RNAseq analysis of developing siliques. The results indicate that wide-spread ERf and GL1 separately act and also interact in the transcriptional control of hormone signalling, transcription factors, cell wall biogenesis, the formation of cuticular and wax compounds, stress response pathways. The last part of the thesis presents the analysis of a range of barley mutant lines obtained through CRISPR-Cas9 editing of the putative barley HvERf and HvGL1 orthologues. They show that these genes, separately and together, exert a major regulatory role in vegetative development, fertility and the anatomical and structural characteristics of the maternal seed enclosing tissues, and in the regulation of seed germination under salinity but also constitutively. In conclusion, the findings reported in this thesis overall highlight the potential of the barley ERECTA genes and putative orthologue to AtGL1 for genetic improvement, especially with respect of more uniform germination within seed cohorts, and regulation of its timing to mitigate risks of germination failure or seedling death, thus enhancing yield. Show more