Effects of plant stress on facultative apomixis in Boechera (Brassicaceae)

In angiosperms, apomixis is asexual reproduction by seed. In gametophytic apomixis, every nucleus in the female gametophyte is unreduced, and the embryo forms parthenogenetically. Different types of gametophytic apomixis occur depending on the origin of the embryo. In this research we documented that both aposporous and diplosporous apomixis occur in the genus Boechera (Brassicaceae). Our results from cytological analysis show that in B. gunnisoniana, B. lignifera, B. retrofracta x exilis, and B. retrofracta x stricta the type of apomixis is diplospory; and in B. microphylla it is apospory. Sexual reproduction was observed in B. stricta. By using quantitative real time PCR (qRT-PCR), we obtained the same pattern of expression of four stress-related genes and one ribosomal gene that had been previously documented in a project carried out with microarray analysis in our lab. In this part of the project, we could appreciate differences in gene expression in pistils and ovules of sexual and apomictic Boechera species. A major objective of this study was to evaluate the effects of stress on megasporogenesis, seed set, and gene expression in sexual and apomictic species of the genus Boechera. Cytology of ovule development, flow cytometry of single seeds and qRT-PCR of pistil samples were conducted on the five apomictic Boechera species and the one sexual species mentioned above. Sets of plants from each species were grown under ideal control conditions and other sets were exposed to drought stress or drought plus heat stress. The cytological evaluation of ovules showed an increase in the frequency of sexual ovule development (meiosis instead of apomeiosis) under stress in some species. However, no alteration was detected in frequencies of sexual and apomictic seed set regardless as to whether the plants were grown with or without stress. Drought stress triggered changes in gene expression analyzed by qRT-PCR. While these changes may be related to the switch from apomeiotic to meiotic megasporogenesis, these changes did not cause an increase in sexual seed formation. Low frequency sexual seed formation (relative sexual sterility) may be the result of the interspecific or triploid nature of the apomictic plants being tested.