Functional analysis of the FORKED and FORKED-LIKE proteins and their role in the environmental control of leaf vein pattern

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Plants are exposed to a constantly changing environment from which they cannot escape. Instead, they alter development to produce a form better adapted to the new environment. The hormone auxin controls many important events in plant development such as defining where leaves form, where shoots and root branch and turning roots towards gravity. Auxin is directionally transported from cell to cell dependent upon the polar localization of PINFORMED (PIN) transport proteins to a particular side of the cell membrane. Many of the environmental responses of plant growth are achieved by changing PIN localization and hence cell polarity, tissue growth and organization.*** I am interested in the formation and organization of leaf veins, which also depends on polar localization of PIN within cells that will become veins. My group discovered the FORKED1 (FKD1) gene, that is important for localizing PIN within these cells. If the gene is mutated, PIN is never localized to the top side of cells, and rather than meeting to form the normal closed loops, the veins form an open, “forking” vein pattern. We found that FKD1 works in the secretory pathway, taking the PIN protein to the plasma membrane. I propose to use FKD1 to study other factors that direct proteins to the top side of cells, a previously proposed pathway that is little understood. *** FKD1 is in a gene family unique to plants and found in all plant genomes. The gene first appears in non-vascular liverworts, indicating that its function predates veins. A mutant that knocks out four members of the Arabidopsis gene family has short roots that do not respond to gravity and misshapen, lumpy leaves. Microscopic observation reveals that the bumps and hollows contain patches of randomly oriented cells. Cells in leaves are normally polarized with their long ends parallel to the midvein. While botanists have suggested that auxin might polarize the cells within the leaf, no one has been able to test the idea experimentally. The mutants provide a unique tool to explore the mechanism by which leaf cells are polarized.*** Vein pattern changes in response to the environment. For example, drought conditions cause a higher vein density with more joints, an adaptation that helps plants maintain photosynthesis. Interestingly, members of the gene family have sequences that suggest regulation by drought and the stress hormone ABA, and my students have shown that FKD1 responds to ABA. As well, mutation to FKD1 or other gene family members eliminates the vein pattern response to drought. Therefore, I propose that the gene family allows vein pattern adaptation to stress, through its regulation by stress-signaling pathways including ABA, and have designed experiments to test this idea. *** By revealing novel processes that achieve PIN localization and cell polarity and defining how the environment influences these processes, my research will allow manipulation of plant form better suited to the environment.