Location: Physiology and Pathology of Tree Fruits Research
Project Number: 2094-21220-003-003-T
Project Type: Trust Fund Cooperative Agreement
Start Date: Apr 1, 2021
End Date: Mar 31, 2025
Objective:
1. Transform pear rootstock germplasm with a flowering-activating, chemically-induced system. Introduce flowering genes into fire-blight resistant pear rootstock germplasm whose expression can be induced by an inexpensive agrochemical, allowing early flowering for rapid breeding without the negative phenotypes seen in other rapid-cycle breeding (RCB) systems.
2. Early molecular and phenotypic characterization of transformants. Confirm the presence and location of the inducible flower genes. Test lines for flowering response.
3. In-depth characterization and optimization of RCB plants. Characterize flowering gene expression and flowering response to agrochemical in detail. Determine optimal dose and delivery of chemical induction. Test viability of flowers to be pollinated and begin crossing with germplasm containing additional traits of interest.
Approach:
Pears are grown as composite trees, consisting of a clonally propagated scion (which allows for maintenance of existing desirable varieties), grafted onto a rootstock, which are bred and chosen for a variety of desirable traits. Rootstocks are typically selected based on their compatibility with the scion variety and regional soil type, disease resistance, precocity, and architectural traits they confer, such as dwarfing. For Pacific Northwest (PNW) pears specifically, Pyrus-compatible, fire blight resistant, cold-hardy, precocious, dwarfing rootstocks would be ideal, however existing varieties to choose from are quite limited in these traits. A major limitation for rootstock breeding is the long juvenility period (reaching ~10 years in European pear trees), particularly for combining, or stacking, multiple traits, as this requires numerous rounds of crossing. While DNA-informed breeding can speed the selection process, breeding cycles are still 5-10 years, meaning variety releases are 20-40 years from initial crosses, depending upon the breeding scheme. To speed up this process, researchers working in other temperate trees including plum, apple, and citrus have used biotechnology to develop rapid-cycle breeding (RCB) tools to stack traits in a greatly reduced amount of time (reviewed below). Existing RCB tools work by modifying flowering gene expression, leading to premature or even continuous flowering, dramatically reducing the lengths of breeding cycles, and enabling year-round breeding within the greenhouse.
While RCB systems are functional in apple, plum, and citrus, continuous flowering throughout breeding cycles can create challenges. These include balancing vegetative and reproductive growth, as well as the accumulation of terminal floral meristems that lead to growth cessation. Early flowering has been demonstrated in pear by groups in Israel and Japan, however an RCB system has not been developed in the US, nor in rootstock germplasm that would be of most interest to the PNW region. To develop a system for rapid breeding of pear rootstocks and avoid the drawbacks in existing systems, we propose to introduce an inducible RCB system, the tools for which were recently developed in the Cutler lab and successfully applied to citrus. This system introduces an induction switch that allows a low-cost agrochemical to control the expression of the flowering gene FLOWERING LOCUS T (FT). In citrus plants containing this inducible FT system (iFT), researchers were able to induce flowering within 2-3 weeks. Importantly, transgenic plants are morphologically normal in the absence of induction treatment, avoiding the well-documented negative effects of continuous FT overexpression and flowering.
