Dominance induction of fruitlet shedding in Malus × domestica (L. Borkh): molecular changes associated with polar auxin transport

BMC Plant Biology Dominance induction of fruitlet shedding in Malus domestica (L. Borkh): molecular changes associated with polar auxin transport Valeriano Dal Cin 0 2 Riccardo Velasco 1 Angelo Ramina 2 0 Horticultural Sciences, University of Florida , Gainesville, PO Box 116090 , USA 1 Experimental Institute for Agriculture , via Mach 2 San Michele all'Adige, 38010 Trento , Italy 2 Department of Environmental Agronomy and Crop Science, University of Padova , Viale dell'Universita 16, 35020 Legnaro (Padova) , Italy Background: 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. Results: Seven 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. Conclusion: The 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. - Background Abscission is a coordinated process tightly regulated by the interplay of several factors, among which auxin and ethylene play a pivotal role [1,2]. Leaf deblading and ethylene application lead to premature abscission of the organ due to the disruption of the auxin flux and activation of the abscission zone (AZ) at the base of the petiole [3,4]. In the commonly accepted model, as demonstrated in the Arabidopsis etr1-1, ethylene coordinates abscission. In this mutant, flower abscission is significantly delayed because the ethylene receptor (ETR1) is hampered in ethylene binding activity, leading to partial ethylene insensitivity. Although ethylene accelerates abscission, it is strictly not necessary for shedding, indicating that a very complex interplay of events control the process [5-7]. Indeed, it has been shown that ethylene-dependent and independent pathways converge in determining flower abscission [8]. fruitlet, down to the peduncle through the AZ into the twig, depolarizes the weak auxin flows from the lateral fruitlets causing their abscission [42]. It has also been postulated that prevention of abscission requires a continuous and constant auxin transport through the AZ [1]. Besides preventing abscission, auxin regulates a tremendous number of processes, for instance root meristem activity, organogenesis, and vascular tissue differentiation [9-11]. Only recently the outstanding complex mode of action of auxin has been partially unraveled [12]. The most common auxin in plant, indol-3-acetic acid (IAA), binds and is perceived by TIR1, an F-box protein [13]. 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]. Auxin applications enhance the transcript amount of most of the Aux/IAAs in several species [16-19]. Another enthralling field concerns auxin transport [20], which can be classified as either polar (PAT) or non polar. However, the PAT is acquiring ever-growing interest and may be the most important means of auxin relocation [21]. IAA is taken up into the cell by a combination of lipophilic diffusion, symport via AUX and LAX (LIKE-AUX1) permeases, and ATP-dependent transport by a P-glycoprotein [22-25]. Auxin export is mediated by PIN-FORMED (PIN) facilitators and by ATP activated PGPs (Phosphoglycoproteins) [26-30]. PINs and PGPs were shown to characterize coordinated and independent auxin transport mechanisms, and function interactively in a tissue-specific manner [31]. Nevertheless, the function of the PGPs is nonspecific and mainly applies to auxin excess [32]. As a matter of fact, it is the asymmetric cellular localization of PIN proteins that determines the direction of the auxin flow [20]. Although 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]. The modes of action of auxin and ethylene elucidated in A. thaliana have been extended to other model species such as tomato [35,36]. Yet, little is known about the interactions between these two hormones during abscission induction of organs other than debladed leaves or senescing flowers. In particular, the apple cluster during the immature fruit drop represents an ideal system to study the shedding of actively growing organs [37]. At this developmental stage, the shedding process involves almost exclusively lateral fruitlets in which abscission is preceded by an increase in ethylene biosynthesis and sensitivity [38-40]. According to the correlative basis reknown model the central fruitlet exerts a dominant effect over lateral fruitlets because it is at a more advanced stage of development [37,41]. As assessed by the canalization theory the strong auxin flow coming from the central Apical dominance is a complex physiological process largely controlled by auxin and its interaction with two additional hormones: cytokinins and MAX (more axillary branching [43-45]. Cytokinins produced in the roots are directed to organs (shoot apical meristems, fruits, etc) whose sink strength is related to their ability in producing and exporting auxin [46]. This process directs more cytokinins which stimulate growth [47,48]. In the case of apical dominance of shoot meristems, lateral bud outgrowth occurs when the auxin flow from the apex is hampered, dominance is weakened, and cytokinins are redirected to axillary meristems [43,44]. Besides the main cytokinin stream coming from the roots, the hormone can also be produced in other tissues. For instance, following decapitation, a prompt increase in transcripts for the key enzyme in cytokinin biosynthesis, adenosine phosphate-isopentenyltransferase, occurs in the stem xylem [49]. The cytokinins produced here may then be translocated into the axill (...truncated)