Defined nuclear changes accompany the reprogramming of the microspore to embryogenesis

The switch of the gametophytic developmental program toward pollen embryoge nesis to form a haploid plant represents an important alternative for plant breeding. In the present study, the switch of the gametophytic development al program toward a sporophytic pathway, "embryogenesis," has been studied in three different plant species, Brassica, tobacco, and pepper. The switch has been induced by stress (heat shock) at the very responsive stage of th e microspore, which is the vacuolate period. As a result, the cell nucleus undergoes striking structural changes with regard to late gametophytic deve lopment, including alterations of biosynthetic activities and proliferative activity. An enrichment in HSP70 heat-shock protein and in the presence of Ntf6-MAP kinase was observed after inductive treatment in the nuclei durin g early embryogenesis. This apparently reflected the possible roles of thes e proteins, specifically the protective role of HSP70 for the nuclear machi nery, and signal transduction of Ntf6-MAPK for the entry of cells into prol iferation. Importantly, the observed nuclear changes were similar in the th ree species investigated and represented convenient markers for early monit oring of embryogenesis and selection purposes for obtaining double-haploid plants in plant breeding. (C) 2000 Academic Press.