catechu is involved in fruitlet abscission

Procedural abscission of outer reproductive organs during flower and fruit development occurs in most plant lineages. Undesired abscission, such as fruitlet shedding causes considerable yield loss in many fruit-producing species. Ethylene is one of the key factors regulating organ abscission. However, the participants involved in the ethylene-mediated abscission pathway remains largely unidentified. In this study, we focused on the ethylene response transcription factors (ERFs) regulating fruitlet abscission in an industrial tree species, A. catechu. A total of 165 ERF genes have been found in the A. catechu genome and eight of these showed distinct expression between the "about-to-abscise" and "non-abscised" samples. An AcERF116 gene with high expression level in the fruit abscission zone (FAZ) was selected for further study. Overexpression of the AcERF116 gene accelerated cell separation in the abscission zone (AZ) and promoted pedicel abscission in transgenic tomato lines. The PG (ploygalacturonase) activity was enhanced in the FAZs of A. catechu fruitlets during ethylene-induced fruitlet abscission, while the PME (pectin methylesterase) activity was suppressed. In addition, cytosolic alkalization was observed in the AZs during abscission in both tomato and A. catechu. Our results suggest that AcERF116 plays a critical role in the crosstalk of ethylene and fruitlet abscission in A. catechu.

Phytohormones and candidate genes synergistically regulate fruitlet abscission in Areca catechu L

BACKGROUND

The fruit population of most plants is under the control of a process named "physiological drop" to selectively abort some developing fruitlets. However, frequent fruitlet abscission severely restricts the yield of Areca catechu. To reveal the physiological and molecular variations in this process, we detected the variation of phytohormone levels in abscised and non-abscised fruitlets in A. catechu.

RESULTS

The levels of gibberellin acid, jasmonic acid, salicylic acid, abscisic acid and zeatin were elevated, while the indole-3-acetic acid and indole-3-carboxaldehyde levels were declined in the "about-to-abscise" part (AB) of abscission zone (AZ) compared to the "non-abscised" part (CK). Then the differentially expressed genes (DEGs) between AB and CK were screened based on transcriptome data. DEGs involved in phytohormone synthesis, response and transportation were identified as key genes. Genes related to cell wall biosynthesis, degradation, loosening and modification, and critical processes during fruit abscission were identified as role players. In addition, genes encoding transcription factors, such as NAC, ERF, WRKY, MADS and Zinc Finger proteins, showed differentially expressed patterns between AB and CK, were also identified as candidates.

CONCLUSIONS

These results unraveled a phytohormone signaling cross talk and key genes involved in the fruitlet abscission process in A. catechu. This study not only provides a theoretical basis for fruitlet abscission in A. catechu, but also identified many candidate genes or potential molecular markers for further breeding of fruit trees.

LcERF10 functions as a positive regulator of litchi fruitlet abscission

Phytohormone ethylene is well-known in positive modulation of plant organ abscission. However, the molecular mechanism underlying ethylene-induced abscission remains largely unknown. Here, we identified an ethylene-responsive factor, LcERF10, as a key regulatory gene in litchi fruitlet abscission. LcERF10 was strongly induced in the fruitlet abscission zone (FAZ) during the ethylene-activated abscission. Silencing of LcERF10 in litchi weakened the cytosolic alkalization of the FAZ and reduced fruitlet abscission. Moreover, LcERF10 directly bound the promoter and repressed the expression of LcNHX7, a Na+/H+ exchanger that was down-regulated in FAZ following the ethylene-activated abscission and up-regulated after LcERF10 silencing. Additionally, ectopic expression of LcERF10 in Arabidopsis promoted the cytosolic alkalization of the floral organ AZ and accelerated the floral organ abscission. Collectively, our results suggest that the transcription factor LcERF10 plays a positive role in litchi fruitlet abscission.

Genome-wide identification and expression analysis of citrus fruitlet abscission-related polygalacturonase genes

Polygalacturonases (PGs) encoded by a relatively large gene family are involved in plant organ abscission, but few data is available in citrus. Here, to explore the role of PGs in citrus fruitlet abscission (CFA), we have obtained 38 citrus PG (CitPG) members, based on the citrus genome sequences. The ORF length varied from 378 to 2418 bp, encoding proteins with theoretical pI and molecular mass ranging from 4.83 to 9.92 and from 13,951.71 to 85,542.28, respectively. Most CitPGs contained no less than 3 introns, suggesting a high probability of alternative splicing. Phylogenetic tree revealed that all PGs could be divided into three groups, in which 9 CitPGs, including CitPG2, CitPG3, CitPG10, CitPG24, CitPG27, CitPG29, CitPG30, CitPG33 and CitPG34 possessed a close relationship with abscission-associated PGs, indicating their role in CFA. Expression analysis further demonstrated that CitPG2, CitPG29 and CitPG34 might be involved in CFA, the expression levels of which could be induced by ethylene, inhibited by IAA and increased during CFA. The findings in this study have provided a foundation for future studies to elucidate the roles of CitPGs in CFA.

The molecular events of IAA inhibiting citrus fruitlet abscission revealed by digital gene expression profiling

Citrus fruits possess two abscission zones (AZ), AZ A and AZ C located at the pedicel and calyx, respectively. Early citrus fruitlet abscission (CFA) exclusively occurs at AZ A. Previous data have shown that indole-3-acetic acid (IAA) could inhibit fruitlet abscission. However, its role in CFA remains vague. In this study, we first removed the ovaries of fruitlets in order to exclude their interferences. Then, the calyxes were treated with IAA, gibberellin 3 (GA₃) and 6-benzylaminopurine (6-BA), respectively. The results have shown that IAA could prevent CFA from taking place, while either GA₃ or 6-BA could not. When IAA concentration decreased to a value between 30 mg/L and 40 mg/L, CFA occurred, showing a concentration-dependent manner. Digital gene expression analysis revealed that 2317 corresponded to IAA treatment, of which 1226 genes were closely related to CFA. The most affected genes included those related to biosynthesis, transport and signaling of phytohormones, primarily ethylene (ET), abscisic acid (ABA) and auxin as well as protein ubiquitination, ROS response, calcium signal transduction, cell wall and transcription factors (TFs). The results obtained in this study suggested that the IAA in AZ A could suppress ethylene biosynthesis and signaling, and then inhibit abscission signaling. To our knowledge, it is the first time to reveal the key role of IAA in CFA, which will contribute to a better understanding for the mechanism underlying CFA.

Regulation of polar auxin transport in grapevine fruitlets (Vitis vinifera L.) and the proposed role of auxin homeostasis during fruit abscission

BACKGROUND

Indole-3-acetic acid (IAA), the most abundant auxin, is a growth promoter hormone involved in several developmental processes. Auxin homeostasis is very important to its function and this is achieved through the regulation of IAA biosynthesis, conjugation, degradation and transport. In grapevine, IAA plays an essential role during initial stages of berry development, since it delays fruitlet abscission by reducing the ethylene sensitivity in the abscission zone. For this reason, Continuous polar IAA transport to the pedicel is required. This kind of transport is controlled by IAA, which regulates its own movement by modifying the expression and localization of PIN-FORMED (PIN) auxin efflux facilitators that localize asymmetrically within the cell. On the other hand, the hormone gibberellin (GA) also activates the polar auxin transport by increasing PIN stability. In Vitis vinifera, fruitlet abscission occurs during the first two to three weeks after flowering. During this time, IAA and GA are present, however the role of these hormones in the control of polar auxin transport is unknown.

RESULTS

In this work, the use of radiolabeled IAA showed that auxin is basipetally transported during grapevine fruitlet abscission. This observation was further supported by immunolocalization of putative VvPIN proteins that display a basipetal distribution in pericarp cells. Polar auxin transport and transcripts of four putative VvPIN genes decreased in conjunction with increased abscission, and the inhibition of polar auxin transport resulted in fruit drop. GA₃ and IAA treatments reduced polar auxin transport, but only GA₃ treatment decreased VvPIN transcript abundance. When GA biosynthesis was blocked, IAA was capable to increase polar auxin transport, suggesting that its effect depends on GA content. Finally, we observed significant changes in the content of several IAA-related compounds during the abscission period.

CONCLUSIONS

These results provide evidence that auxin homeostasis plays a central role during grapevine initial fruit development and that GA and IAA controls auxin homeostasis by reducing polar auxin transport.

The ARF, AUX/IAA and GH3 gene families in citrus: genome-wide identification and expression analysis during fruitlet drop from abscission zone A

Completion of the whole genome sequencing of citrus enabled us to perform genome-wide identification and functional analysis of the gene families involved in agronomic traits and morphological diversity of citrus. In this study, 22 CitARF, 11 CitGH3 and 26 CitAUX/IAA genes were identified in citrus, respectively. Phylogenetic analysis revealed that all the genes of each gene family could be subdivided into three groups and showed strong evolutionary conservation. The GH3 and AUX/IAA gene families shrank and ARF gene family was highly conserved in the citrus genome after speciation from Arabidopsis thaliana. Tissue-specific expression profiles revealed that 54 genes were expressed in at least one tissue while just 5 genes including CitARF07, CitARF20, CitGH3.04, CitAUX/IAA25 and CitAUX/IAA26 with very low expression level in all tissues tested, suggesting that the CitARF, CitGH3 and CitAUX/IAA gene families played important roles in the development of citrus organs. In addition, our data found that the expression of 2 CitARF, 4 CitGH3 and 4 AUX/IAA genes was affected by IAA treatment, and 7 genes including, CitGH3.04, CitGH3.07, CitAUX/IAA03, CitAUX/IAA04, CitAUX/IAA18, CitAUX/IAA19 and CitAUX/IAA23 were related to fruitlet abscission. This study provides a foundation for future studies on elucidating the precise role of citrus ARF, GH3 and AUX/IAA genes in early steps of auxin signal transduction and open up a new opportunity to uncover the molecular mechanism underlying citrus fruitlet abscission.