Abstract
BackgroundApple fruitlet abscission is induced by dominance, a process in which hormones such as auxin, cytokinins and strigolactone play a pivotal role. The response to these hormones is controlled by transcription regulators such as Aux/IAA and ARR, whereas auxin transport is controlled by influx and efflux carriers.ResultsSeven partial clones encoding auxin efflux carriers (MdPIN1_A, MdPIN1_B, MdPIN10_A, MdPIN10_B, MdPIN4, MdPIN7_A and MdPIN7_B), three encoding auxin influx carriers (MdLAX1, MdLAX2 and MdLAX3) and three encoding type A ARR cytokinin response regulators (MdARR3, MdARR4 and MdARR6) were isolated by the use of degenerate primers. The organization of the PIN multigene family in apple is closer to Medicago truncatula than to Arabidopsis thaliana. The genes are differentially expressed in diverse plant organs and at different developmental stages. MdPIN1 and MdPIN7 are largely more expressed than MdPIN10 and MdPIN4. During abscission, the transcription of these genes increased in the cortex whereas in the seed a sharp fall was observed. The expression of these genes was found to be at least partially controlled by ethylene and auxin.ConclusionThe ethylene burst preceding abscission of fruitlets may be responsible for the decrease in transcript level of MDPIN1, MDARR5 and MDIAA3 in seed. This situation modulates the status of the fruitlet and its fate by hampering the PAT from the seeds down through the abscission zone (AZ) and this brings about the shedding of the fruitlet.
Highlights
Apple fruitlet abscission is induced by dominance, a process in which hormones such as auxin, cytokinins and strigolactone play a pivotal role
The alignment of the PINs presented here with the PINs of A. thaliana and M. truncatula, the isolation of the two genomic alleles of MdPIN10 and the differences at the level of the introns, and expression pattern of MdPIN1 and MdPIN10 indicate that they may be paralogous, as it has been reported for MtPIN4 and MtPIN5 [59]
A slight increase in MdLAX1, MdPIN4 and MdPIN7 transcripts was monitored in senescing flowers, as reported during daffodil flower senescence [66]. These results indicate that the expression of the Polar Auxin Transport (PAT) elements, besides being under auxin control, is developmentally and tissue regulated, as previously reported for hybrid aspen and A. thaliana, respectively [64,65]
Summary
Apple fruitlet abscission is induced by dominance, a process in which hormones such as auxin, cytokinins and strigolactone play a pivotal role The response to these hormones is controlled by transcription regulators such as Aux/IAA and ARR, whereas auxin transport is controlled by influx and efflux carriers. Abscission is a coordinated process tightly regulated by the interplay of several factors, among which auxin and ethylene play a pivotal role [1,2]. TIR1 interacts in the SCF complex to bring about the degradation of Aux/IAA transcriptional regulators [14] These proteins are active repressors of auxin responsive genes and are encoded by a large multigene family [15]. PINs and PGPs were shown to characterize coordinated and independent auxin transport mechanisms, and function interactively in a tissue-specific manner [31]. Different PINs are implicated in specific developmental processes, there seems to be redundancy as indicated by the ectopic expression of PIN proteins in some mutant combinations [20,33,34]
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