Poor shoot and leaf growth in Huanglongbing-affected sweet orange is associated with increased investment in defenses

Answiya Neupane; Faisal Shahzad; Chiara Bernardini; Amit Levy; Tripti Vashisth

doi:10.3389/fpls.2023.1305815

Poor shoot and leaf growth in Huanglongbing-affected sweet orange is associated with increased investment in defenses

Front Plant Sci. 2023 Dec 19:14:1305815. doi: 10.3389/fpls.2023.1305815. eCollection 2023.

Authors

Answiya Neupane¹, Faisal Shahzad¹, Chiara Bernardini^{1

2}, Amit Levy^{1

2}, Tripti Vashisth¹

Affiliations

¹ Citrus Research and Education Center, University of Florida, Lake Alfred, FL, United States.
² Department of Plant Pathology, University of Florida, Gainesville, FL, United States.

Abstract

Citrus disease Huanglongbing (HLB) causes sparse (thinner) canopies due to reduced leaf and shoot biomass. Herein, we present results demonstrating the possible mechanisms behind compromised leaf growth of HLB-affected 'Valencia' sweet orange trees by comparing morphological, transcriptome, and phytohormone profiles at different leaf development phases (1. buds at the start of the experiment; 2. buds on day 5; . 3. leaf emergence; 4. leaf expansion; and 5. leaf maturation) to healthy trees. Over a period of 3 months (in greenhouse conditions), HLB-affected trees had ≈40% reduction in growth traits such as tree height, number of shoots per tree, shoot length, internode length, and leaf size compared to healthy trees. In addition, buds from HLB-affected trees lagged by ≈1 week in sprouting as well as leaf growth. Throughout the leaf development, high accumulation of defense hormones, salicylic acid (SA) and abscisic acid (ABA), and low levels of growth-promoting hormone (auxin) were found in HLB-affected trees compared to healthy trees. Concomitantly, HLB-affected trees had upregulated differentially expressed genes (DEGs) encoding SA, ABA, and ethylene-related proteins in comparison to healthy trees. The total number of cells per leaf was lower in HLB-affected trees compared to healthy trees, which suggests that reduced cell division may coincide with low levels of growth-promoting hormones leading to small leaf size. Both bud dieback and leaf drop were higher in HLB-affected trees than in healthy trees, with concomitant upregulated DEGs encoding senescence-related proteins in HLB-affected trees that possibly resulted in accelerated aging and cell death. Taken together, it can be concluded that HLB-affected trees had a higher tradeoff of resources on defense over growth, leading to sparse canopies and a high tree mortality rate with HLB progression.

Keywords: Huanglongbing; canopy; citrus; hormones; leaf growth.

Grants and funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This research was funded by USDA NIFA ECDRE AWD 2018-70016-27412.