Early defoliation induces auxin redistribution, promoting paradormancy release in pear buds

Abstract

Paradormancy of fruit trees occurs in summer and autumn when signals from adjacent organs stimulate buds to develop slowly. This stage has received less attention that the other stages of dormancy, and the underlying mechanism remains uncharacterized. Early defoliation in late summer and early autumn is usually followed by out-of-season blooming in pear (Pyrus spp.), which substantially decreases the number of buds the following spring and negatively affects fruit production. This early bud flush is an example of paradormancy release. Here, we determined that flower bud auxin content is stable after defoliation; however, polar distribution of the pear (Pyrus pyrifolia) PIN-FORMED auxin efflux carrier 1b (PpyPIN1b) implied that auxin tends to be exported from buds. Transcriptome analysis of floral buds after artificial defoliation revealed changes in auxin metabolism, transport, and signal transduction pathways. Exogenous application of a high concentration of the auxin analog 1-naphthaleneacetic acid (300 mg/L) suppressed PpyPIN1b expression and its protein accumulation in the cell membrane, likely leading to decreased auxin efflux from buds, which hindered flower bud sprouting. Furthermore, carbohydrates and additional hormones also influenced out-of-season flowering. Our results indicate that defoliation-induced auxin efflux from buds accelerates bud paradormancy release. This differs from release of apical-dominance-related lateral bud paradormancy after the apex is removed. Our findings and proposed model further elucidate the mechanism underlying paradormancy and will help researchers to develop methods for inhibiting early defoliation-induced out-of-season bud sprouting.

Figure 1

“Cuiguan” pear spur flower bud development under natural conditions in 2019. Photographs and scanning electron micrographs of spur flower buds on June 19 (A), July 31 (B), August 28 (C), September 11 (D), and September 25 (E). The bottom panel for each time-point is a magnified image of one of the flower buds in the upper panel. sp, sepal primordium; pp, petal primordium; triangles, stamen primordium; and asterisks, pistil primordium.

Figure 2

Early defoliation induced “Cuiguan” pear spur flower bud break in autumn. A–C, Bud break rate after defoliation in 2018 (A), 2019 (B), and 2020 (C). D, Representative images of spur bud break on mock control and defoliated trees on day 20. Error bars indicate the SEs of three biological replicates and asterisks indicate significant differences between the mock control and defoliated trees (Student’s t test). *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 3

Defoliation accelerated flower bud development. A, Photographs of dormant flower buds from mock control trees (top) and sprouted flower buds in different states from defoliated trees (bottom) at 10 days after defoliation in 2019. B–E, Vertical sections of flower buds collected at 6 days (B, C) and 11 days (D, E) after defoliation from mock control trees and defoliated trees in 2018. Red arrowheads indicate the pistil primordia of buds after defoliation. White arrowheads indicate the center of flower primordia without the pistil primordia of buds in the mock control group. F–O, Scanning electron micrographs of fruit spur flower buds from mock control trees (F–J) and defoliated trees (K–O) at 0, 5, 10, 16, and 20 days after defoliation in 2019. The central floral primordium is labeled with a white dashed line, with magnified images of the petal primordia, stamen primordia, and pistil primordia (bottom). Scale bars, 5 mm (A) and 200 μm (B–E). pp, petal primordium; triangles, stamen primordium; and asterisks, pistil primordium.

Figure 4

Changes in the IAA content in spur flower buds and stems at the base of flower buds and immunofluorescence localization of PpyPIN1b in flower buds at 7 days after defoliation. A, Flower bud IAA contents at 0, 5, 10, and 20 days after defoliation in 2018. B, Flower bud IAA contents at 0, 1, 3, 5, 7, and 10 days after defoliation in 2019. C, Stem IAA contents at 0, 3, 7, and 10 days after defoliation. **P < 0.01. D, Left and right panels, respectively, present flower buds from the mock control and defoliated groups at 7 days after defoliation. The fluorescence in the upper images indicates the nuclear and PpyPIN1b fluorescence of flower buds. The fluorescence in the lower magnified images represents the PpyPIN1b protein immunofluorescence localization signal. White arrowheads indicate the cell membrane in which PpyPIN1b was distributed.

Figure 5

Analyses of the metabolic activities and the expression of key DEGs involved in auxin metabolism and signaling pathways during defoliation-induced paradormancy release. A, MapMan analysis of the metabolic activities associated with DEGs in three defoliation-induced paradormancy release stages. B, Relative expression profiles of genes involved in auxin biosynthesis, storage, catabolism, transport, and signaling pathways. TAA1, tryptophan aminotransferase; YUCCA, indole-3-pyruvate monooxygenase; CYP79B2/CYP79B3, tryptophan N-monooxygenase; NIT, deaminated glutathione amidase; AMI1, amidase; TDC, tyrosine decarboxylase; ALDH, acetaldehyde dehydrogenase; UGT, UDP-glycosyltransferase; TGW6, indole-3-acetic acid-glucose hydrolase; IAMT1, indole-3-acetate O-methyltransferase; MES17, methylesterase; ILR1/IAR3/ILL, IAA-amino acid hydrolase; GH3, indole-3-acetic acid-amido synthetase; DAO, 2-oxoglutarate-dependent dioxygenase; UGT74D1, UDP-glycosyltransferase; AUX/IAA, auxin-responsive protein; PIN, auxin efflux carrier component; ABCB, ABC transporter B family member; and AUX/LAX, auxin influx transporter.

Figure 6

Inhibitory effects of a high auxin concentration on flower bud break after defoliation and expression patterns of hormone-related genes after an NAA treatment. A, Bud break percentage of current-year long shoots after a 300 mg/L NAA treatment. B, Representative images of bud break on day 20 on defoliated (left) and NAA-treated current-year long shoots (right). Scale bars, 5 cm. C and D, Scanning electron micrographs of flower buds of current-year long shoots on day 5 in the control group (C) and the NAA-treated group (D). pp, petal primordium; triangles, stamen primordium; and asterisks, pistil primordium. E and F, Expression patterns of genes related to auxin (E) and other hormones (F) after the NAA treatment. Error bars indicate the SEs of three biological replicates and asterisks indicate significant differences between control (defoliated) and NAA-treated branches (Student’s t test). *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 7

Proposed model of the physiological process of autumn flowering before (left) and after (right) defoliation. Decreased auxin contents in the stems at the base of flower buds after defoliation promote auxin efflux from buds, which activates flower buds. The effects of auxin combined with the stimulatory influence of CK and the diminished inhibitory effect of SL activate the PIN auxin efflux carrier and downregulate the expression of the branching inhibitor gene BRC1 to further break bud paradormancy. (Elements in the figure are not drawn to scale.)