Malus species with diverse bloom times exhibit variable rates of floral development

Authors: GOECKERITZ, CHARITY - Michigan State University; Gottschalk, Christopher; HOLLENDER, COURTNEY - Michigan State University; VAN NOCKER, STEVE - Michigan State University

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2023
Publication Date: 2/9/2023
Citation: Goeckeritz, C., Gottschalk, C.C., Hollender, C., Van Nocker, S. 2023. Malus species with diverse bloom times exhibit variable rates of floral development. Journal of the American Society for Horticultural Science. 148:2. https://doi.org/10.21273/JASHS05236-22.
DOI: https://doi.org/10.21273/JASHS05236-22

Interpretive Summary: The timing of spring bloom is a valuable trait to target for improved resilience in apple trees to climate change. Currently, many commercial apple cultivars are prone to blossom-killing spring frost events due to their early bloom times or are late-blooming which coincides with conditions suitable for infection by Erwinia, the bacteria responsible for the devastating disease known as fire blight. Much of the previous research into mechanisms that govern bloom time has focused on dormancy with little attention given to the processes preceding it – floral initiation and development. In this study, we employed anatomical characterization and gene expression analysis to study the variation in the timing of floral initiation and develop in a set of extreme early- and late-blooming Malus accessions. Anatomical characterizations found variation in the timing of floral initiation, developmental pace, and the developmental stage in which accessions entered dormancy. Initiation timing, developmental pace, and pre-dormancy stage were not always correlated with the spring bloom timing. We also conducted a census for well-studied floral development genes, identifying 73 apple genes with potential roles in floral development. Of those 73 genes, 36 were found to increase in expression over the period in which flowers were initiated and developing in five of the six accessions studied. In contrast, nine genes were identified as repressed over that same period. We assigned these two groups as being ‘floral promotive’ and ‘floral repressive’, respectively. When clustering the gene expression profiles of the 45 genes, we found that the two extremely late-blooming M. angustifolia accessions group separately from the other four accessions. This result suggests M. angustifolia has a genetic mechanism that delays floral initiation and development relative to the other accessions. The results obtained from this work lay the groundwork for understanding the variation in flowering processes as they relate to spring bloom timing in the Malus genus.

Technical Abstract: Commercial apple (Malus x. domestica) production, along with other tree fruit, is increasingly impacted by climate change. In response, breeding programs must quickly develop more resilient cultivars. One strategy is to breed for various bloom times. Members of the Malus genus, including the domesticated apple, wild species, and hybrids, exhibit striking variation in bloom date. Although bloom time is known to be strongly influenced by chilling requirements, other aspects of floral development in Malus and their contributions toward bloom time have not been well studied. Here, we performed a phenological analysis of flower development in wild and domesticated Malus species with extreme differences in bloom time. Over the course of one developmental season, we tracked histological changes in the floral apex of three extremely early-blooming genotypes (M. domestica ‘Anna’, M. orthocarpa, M. sylvestris) and three extremely late-blooming genotypes (M. angustifolia PI 589763, M. angustifolia PI 613880, and M. x domestica ‘Koningszuur’) capturing the timing of floral meristem initiation and organ development. In addition, we expanded upon a current staging system for apple flower development to qualitatively describe the changes we observed. The developmental trajectories of each genotype did not group according to bloom category, and we observed variation in the floral development stage at the time of dormancy onset. Our results suggest previous observations that early development was not a major factor for final bloom time may have been a result of comparing genotypes with narrow genetic variability.