COMPARISONS OF DIVERSE PLANT-SPECIES REVEAL THAT ONLY GRASSES SHOW DRASTICALLY REDUCED LEVELS OF UBIQUITIN MONOMER IN MATURE POLLEN

Ubiquitin is a ubiquitous protein involved in targeting proteins for d egradation. Maize pollen was previously reported (Callis and Bedinger 1994) to show extremely low levels of ubiquitin monomer, and developme ntal significance was attributed to this surprising feature of maize p ollen. However, we had previously shown (Muschietti et al. 1994) that tomato pollen had high levels of ubiquitin monomer. Here we show that pollen from most plant families has high levels of ubiquitin monomer. Most grasses tested show reduced levels of ubiquitin monomer, but some maize inbred lines have higher levels of ubiquitin monomer than other inbreds. There was no correlation between the level of ubiquitin mono mer and either the monocotyledonous or tri-cellular condition of grass pollen or the dehydrated condition of mature pollen. Since many aspec ts of pollen development (i.e., wall formation, microspore mitosis, sy nthesis and storage of mRNAs and proteins, carbohydrates and lipids, d ehydration at maturity) are stereotypical among all plant families, th e reduced level of ubiquitin monomer in pollen of many grasses cannot be crucial for any feature of normal pollen development.