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Sittmann, John
Liu, Zhongchi
Photosynthetic organisms often have limited mobility and rely on a variety of environmental, physiological, and chemical signals to regulate aspects of growth and development. In this thesis, I investigated how two such organisms, one a flowering plant and the other a heterokont alga, incorporate external signaling cues to make decisions regarding reproduction. My dissertation research is focused on 1) investigating molecular mechanisms of crosstalk between photoperiod and shade in regulating asexual reproduction in the wild strawberry Fragaria vesca, and 2) elucidating the mechanism of a bacterium-derived agent in the stimulation of cell division in the marine diatom Phaeodactylum tricornutum. First, strawberry, including woodland strawberry Fragaria vesca, is capable of a form of asexual reproduction by producing horizontal stems with daughter plants at the nodes. These horizontal stems, referred to as stolon, are derived from axillary meristems at the base of the leaves. Depending on the signals the axillary meristem receives, it will give rise to either a branch crown (a flowering shoot) or a stolon. Stolon allows for asexual reproduction to maintain the superior hybrid genotype and hence is of great significance agriculturally. Daughter plants derived from stolon are sold and propagated in strawberry farming. In this work, I have shown that a key regulatory protein FveRGA1 in GA signaling pathway functions as a repressor of stolon development. I further expanded this work by showing that the light quality (shade) signaling pathway interacts with the GA signaling to regulate stolon development. I identified and demonstrated FvePIF3 as a key transcription factor that positively regulates stolon initiation under far-red light (shade). Understanding the mechanisms underlying axillary meristem cell fate determination could advance biotechnology to increase strawberry production. Second, I have discovered and characterized a bacterium-based growth stimulation of the diatom Phaeodactylum tricornutum. Specifically, I noticed that a culture of P. tricornutum that had been accidentally contaminated with bacteria exhibited faster growth. I subsequently identified the responsible bacterium as Bacillus sp, which stimulated rapid Phaeodactylum cell division when added to the Phaeodactylum culture. I experimentally determined that the growth stimulating agent was heat labile and proteinase K-resistant. Further, I showed that the mother cell lysate of Bacillus sp. under sporulation was just as effective in promoting Phaeodactylum. In collaboration with Dr. Jon Clardy lab, we identified the growth-stimulating compounds as two distinct peptide-signaling molecules. The work revealed that the peptides may be previously under-reported signaling molecules for cross-kingdom communications. In addition to the fundamental discovery of novel signaling mechanisms between bacterium and algae, this work may facilitate large-scale diatom culture in biomass production for biofuel and biopharma.