2019 ASHS Annual Conference

Alternate bearing (AB), which is defined as an “ON” year with an overabundance of fruit followed by an “OFF” year with few if any fruit, can be a persistent production issue for pecans. Similarly, conditions which can induce AB have not been investigated at length. Key events that initiate and perpetuate an AB cycle are: (1) an environmental factor (such as temperature) that triggers a stress response, (2) a molecular cascade that follows a stress response, causing epigenetic changes and a disruption of homeostasis in favor of defense and resource conservation, and (3) persistence of homeostatic disruption that affects the following year’s physiological status. Our current research seeks to unveil the molecular mechanisms associated with initiating and maintaining an AB cycle. The relative mRNA levels of a selection of candidate genes will be tracked based on: (a) those involved in repression of floral induction, such as FLC and TEM1, in low-AB or regular-bearing (RB) cultivars subjected to different temperature stresses during dormancy to distinguish the onset of AB, and (b) those involved in reproductive organ abscission, like SEP and JOINTLESS, and vascular differentiation, like SUC1 and VND7, over a two-year period in RB and AB cultivars. Currently, we have identified putative temperature-responsive motifs including MYCCONSENSUSAT and MYB1AT in the promoter regions of apple abscission genes. We intend to use apple sequence data to distinguish related regions in pecan. We will also determine DNA methylation patterning, RNA-seq gene expression, and physiological differences between the two groups. Orchard management techniques such as mechanical and chemical thinning that are used to mitigate the impact of pecan AB are expensive, tedious, and raise health and environmental concerns. Thus, the long-term goals of this project are to: (i) work toward specifying a definitive measure of genotyping for AB severity, and (ii) lay the groundwork for developing RB cultivars through gene editing.