Research |
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What is the mechanistic link between LATD/NIP and ABA signaling? The rescue of the latd mutant root phenotype by ABA indicates an intimate connection between LATD/NIP function and ABA, but not the nature of this interaction. The simplest interpretation for this observation is that latd mutant plants are unable to synthesize enough ABA, but we found no significant difference in ABA levels between wild-type and latd mutant plants, ruling out a synthesis defect. More likely, latd mutants experience too low a concentration of ABA at the root meristems, which inhibits root growth. The fact that root growth can be rescued by adding ABA to the environment, suggests that latd roots fail to grow because they are unable to transport or accumulate ABA in this tissue, and exogenous ABA application bypasses this defect. We are trying to distinguish between these possibilities by using ABA quantification and immunolocalization to follow ABA movement in root tips of wild-type and latd mutants, and to test the effect of nitrate on patterns of ABA accumulation. In this way, we hope to reveal a mechanism for ABA rescue of the latd root defect.
Determining the role of ABA in nodule development. ABA affects the earliest responses of the host to the rhizobial symbiont, modulating the frequency of nuclear calcium spiking and affecting early symbiotic signaling. Nodulation is exquisitely sensitive to various aspects of the root environment, such as nitrate and salt. ABA is known to mediate the responses to these environmental signals. Do nitrate and salt inhibit nodule development and promote senescence by signaling via ABA? How does ABA regulate nodule development? We know that LATD/NIP is required for nodule meristem function. Does LATD/NIP interact with ABA to control nodule development and rhizobial infection? We are using a combination of imaging, gene expression analysis and ABA localization and quantification to examine nodule development in wild-type and latd mutant roots, to address this question. Our goal is to ultimately understand the way in which hormone signaling mediates environmental regulation of nodule development at a mechanistic level.
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Evolution of nodulation. The ability to form a symbiotic organ dedicated to nitrogen fixation on the roots of host plants is restricted to a clade of plants in the Rosids, which includes the legumes as well as actinorhizal plants. This symbiosis with nitrogen-fixing bacteria appears to have piggybacked on the ancient mycorrhizal symbiosis, mimicking the fungal signaling molecule and activating the same common symbiosis signaling pathway. However, most plants form a mycorrhizal symbiosis, and yet nodulation is limited to a small group of flowering plants. The evolutionary innovation that allowed the development of nodulation and distinguished it from the ubiquitous mycorrhizal symbiosis is the formation of a dedicated symbiotic organ, the root nodule. Parallels in development and shared genetic components, such as LATD/NIP, between lateral roots and nodules suggest co-option of lateral root development as a key step in the evolution of the nodule organ.
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How does a belowground symbiosis influence an aboveground symbiosis? In this collaborative project with ecologists Alison Brody and Taylor Ricketts, we are examining the effect of belowground interactions with fungi, including the ancient mycorrhizal symbiosis, on traits affecting pollination. We are testing this question in blueberry, using ITS sequencing to identify the different fungi associated with the roots. In years 2 and 3 of this project, the plants will be moved to different blueberry farms to examine the effect of different root treatments on pollination in farms that have different pollinator diversity and numbers. How does the mycorrhizal association in plant roots affect floral traits? What effect do these floral traits have on attractiveness to pollinators? We are testing whether different root fungal communities have different effects on floral traits and whether treatment with a commercial mycorrhizal inoculum alters the association of plant roots with local communities.