JXB article: When trees make memories

21 Apr 2021 - By: Mareike Jezek

This article highlights the following publication:
Alternate bearing in fruit trees: fruit presence induces polar auxin transport in citrus and olive stem and represses IAA release from the bud

Dor Haim, Liron Shalom, Yasmin Simhon, Lyudmila Shlizerman, Itzhak Kamara, Michael Morozov, Alfonso Albacete, Rosa M Rivero, Avi Sadka

Journal of Experimental Botany, 72, 2021

When trees make memories


JXB article April 2021
Citrus fruits in on-crops are located apical to the buds of the following year (left), whereas olives are at a basal position relative to second-year buds (right). Unlike in olive, in citrus, the buds can be from the same year as the fruit. Auxin transported in the stem acts as a messenger between fruits and buds to signal fruit presence in both crops.

Biennial yield fluctuation is a common phenomenon in fruiting trees and economically challenging for farmers. In alternately bearing crops such as apple, mango, and coffee heavy fruit load in an “on” year is followed by a so-called “off” year with minimal fruit onset. Off years are marked by little flower induction yet profound vegetative growth, indicating that a lack of resources is an unlikely reason for low fruit development. Removing fruits in on years can result in a return bloom the following year and equalize fruitfulness, yet precise timing is crucial for this strategy to be successful, and the critical timepoint usually lies outside the normal harvest period.

Many generations of biologists have been controversially debating the cause of this “vexatious propensity” (Butler, 1917), but the underlying signal that emanates from fruits and determines the fate of second-year buds has remained enigmatic. The phytohormone gibberellic acid (GA) is considered as a potential candidate, since exogenous GA application is known to prevent flowering. Recent discoveries, however, point to another key player in the alternate bearing signalling pathway.

Previously, Shalom et al. (2014) analysed the transcriptome and hormone levels in citrus tree buds and revealed profound changes in auxin metabolism and Ca2+-dependent and Ca2+-independent transport after fruit removal from the stem, indicating its potential role in bud development. The phytohormone auxin enables shoots to grow towards the light and roots to respond to gravity, but has rarely been linked to alternate bearing so far.

In this follow-up study, Haim et al. (2021) used radiolabeled auxin to monitor its transport routes in branches and buds of on- and off-crops. Exogenously applied to the bud, auxin transport into the stem was markedly reduced when fruits were present on the branch. These buds were more likely to later develop into vegetative shoots. The absence of fruits in off-crops or after removal, however, allowed the release of auxin from the bud and consequently, inflorescence shoots emerged in the following year. In stems, however, the opposite was observed – labelled auxin applied to the apical part of a stem was transported to its basal end more rapidly in on-crops compared to fruitless trees. The authors suppose a causative link between these processes in that strong polar auxin transport emanating from the fruit “traps” auxin in buds, which consequently prevents flowering induction. Interestingly, similar results were obtained in biennially bearing olive branches. In contrast to citrus fruits that sit at the apical end of a stem, first-year olives are in a basal position relative to the second-year buds. In these stems, the directionality of auxin flow was reversed, and strong basal-to-apical transport was detected. These findings are in agreement with the so-called auxin transport autoinhibiton theory (Bangerth, 1989), which suggests that auxin export from earlier developed sinks (in this case fruits) inhibits the efflux from later developed organs (the buds) irrespective of their exact positioning.

This work corroborates that auxin is an important messenger underlying the fruit load “memory”, and its cross-talk with other phytohormones needs further exploration in the future.

 

 

Bangerth F. 1989. Dominance among fruits/sinks and the search for a correlative signal. Physiologia Plantarum, Volume 76, Issue 4. https://doi.org/10.1111/j.1399-3054.1989.tb05487.x

Butler O. 1917. On the Cause of Alternate Bearing in the Apple. Bulletin of the Torrey Botanical Club, Volume 44, Issue 2. https://doi.org/10.2307/2479535

Haim D, et al. 2021. Alternate bearing in fruit trees: fruit presence induces polar auxin transport in citrus and olive stem and represses IAA release from the bud. Journal of Experimental Botany, Volume 72, Issue 7. https://doi.org/10.1093/jxb/eraa590

Shalom L, et al. 2014. Fruit load induces changes in global gene expression and in abscisic acid (ABA) and indole acetic acid (IAA) homeostasis in citrus buds. Journal of Experimental Botany, Volume 65, Issue 12. https://doi.org/10.1093/jxb/eru148

Mareike Jezek
JXB 

Author: Mareike Jezek
Category: JXB
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Mareike Jezek

Mareike Jezek

Mareike Jezek is the publication Advisor (Institute of Molecular Cell & Systems Biology) of the University of Glasgow and also the Assistant Editor of the Journal of Experimental Botany 

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