Plant hormones (Literature sources on phytohormones and plant signalling)

Authors: R. B. Lima and D. D. Figueiredo.

Plant and Cell Physiology (2024)

Abstract: "Since the discovery of brassinolide in the pollen of rapeseed, brassinosteroids (BRs) have consistently been associated with reproductive traits. However, compared to what is known for how BRs shape vegetative development, the understanding of how these hormones regulate reproductive traits is comparatively still lacking. Nevertheless, there is now considerable evidence that BRs regulate almost all aspects of reproduction, from ovule and pollen formation to seed and fruit development. Here, we review the current body of knowledge on how BRs regulate reproductive processes in plants, and what is known about how these pathways are transduced at the molecular level. We then discuss how the manipulation of BR biosynthesis and signaling can be a promising avenue for improving crop traits which rely on efficient reproduction. We thus propose that BR hold an untapped potential for plant breeding, which could contribute to attain food security in the coming years."

Authors: Chao Tan, Mengxiao Liang, Qiong Luo, Tan Zhang, Wenhui Wang, Suxin Li and Shuzhen Men.

Planta (2023)

Main conclusion: We found that auxin synthesis gene TAA1 and auxin polar transport genes AUX1 and PIN3 collectively maintain fertility and seed size in Arabidopsis. 

Abstract: "Auxin plays a vital role in plant gametophyte development and embryogenesis. The auxin synthesis gene TAA1 and the auxin polar transport genes AUX1 and PIN3 are expressed during Arabidopsis gametophyte and seed development. However, aux1, pin3, and taa1 single mutants only exhibit mild reproductive defects. We, therefore, generated aux1-T pin3 taa1-k2 and aux1-T pin3-2 taa1-k1 triple mutants by crossing or CRISPR/Cas9 technique. These triple mutants displayed severe reproductive defects with approximately 70% and 77%, respectively, of the siliques failing to elongate after anthesis. Reciprocal crosses and microscopy analyses showed that the development of pollen and ovules in the aux1 pin3 taa1 mutants was normal, whereas the filaments were remarkably short, which might be the cause of the silique sterility. Further analyses indicated that the development and morphology of aux1 pin3 taa1 seeds were normal, but their size was smaller compared with that of the wild type. These results indicate that AUX1, PIN3, and TAA1 act in concert to maintain fertility and seed size in Arabidopsis."

Authors: Kyoka Nagasaka, Hisayo Yamane, Soichiro Nishiyama, Shu Ebihara, Ryusuke Matsuzaki, Masakazu Shoji and Ryutaro Tao.

The Horticultural Journal (2022)

Abstract: "Pollination is an important factor affecting fruit development in highbush blueberry (Vaccinium corymbosum L.). In general, planting several different blueberry cultivars increases the chances of cross-pollination and ensures high-quality fruit production. However, little is known about the effects of the pollen source on fruit development in blueberry. The aims of this study were: 1) to understand the effects of the pollen source on fruit size and quality; and 2) to explore the mechanisms underlying fruit development affected by the pollen source. We first characterized the pollination effect on fruit development using 14 different pollination combinations for several years and found that the number of mature seeds and fruit size differed significantly among the fruit pollinated by different pollen sources. Significant correlations between the number of mature seeds and fruit size were found in most combinations, whereas the number of mature seeds was not correlated with other fruit quality parameters such as sugar concentration. Our results and those of previous reports showed that the number of mature seeds, which was influenced by the pollen source, was a primary determinant of fruit size. Time-course observation during fruit development revealed that fruit weight and cell size significantly differed between self-pollinated and cross-pollinated fruit from 30 days after pollination onwards. To explore the molecular mechanisms underlying berry growth affected by developing seeds, we compared gene expression changes between self-pollinated and cross-pollinated fruit. Transcriptome analysis of fruit at 10 days after pollination suggested that auxin signaling pathways were enhanced in cross-pollinated fruit compared with self-pollinated fruit. We thus hypothesize that activated auxin signal transduction underlying early stage seed and fruit development may promote fruit cell enlargement during the early stage of fruit growth in highbush blueberry."

Authors: Weifeng Chen, Zhengcao Xiao, Yule Wang, Jinxiao Wang, Rui Zhai, Kui Lin-Wang, Richard Espley, Fengwang Ma and Pengmin Li.

Horticulture Research (2021)

Abstract: "Flavonoids play important roles in regulating plant growth and development. In this study, three kaempferol 3-O-glycosides were identified and mainly accumulated in flowers but not in leaves or fruits of Malus. In Malus, flower petal color is normally white, but some genotypes have red flowers containing anthocyanin. Anthocyanin biosynthesis appears to be in competition with kaempferol 3-O-glycosides production and controlled by the biosynthetic genes. The white flower Malus genotypes had better-developed seeds than the red flower genotypes. In flowers, the overexpression of MYB10 in Malus domestica enhanced the accumulation of anthocyanin, but decreased that of kaempferol 3-O-glycosides. After pollination the transgenic plants showed slower pollen tube growth and fewer developed seeds. Exogenous application of different flavonoid compounds suggested that kaempferol 3-O-glycosides, especially kaempferol 3-O-rhamnoside, regulated pollen tube growth and seed set rather than cyanidin or quercetin 3-O-glycosides. It was found that kaempferol 3-O-rhamnoside might regulate pollen tube growth through effects on auxin, the Rho of plants (ROP) GTPases, calcium and the phosphoinositides signaling pathway. With the inhibition of auxin transport, the transcription levels of Heat Shock Proteins (HSPs) and ROP GTPases were downregulated while the levels were not changed or even enhanced when blocking calcium signaling, suggesting that HSPs and ROP GTPases were downstream of auxin signaling, but upstream of calcium signaling. In summary, kaempferol glycoside concentrations in pistils correlated with auxin transport, the transcription of HSPs and ROP GTPases, and calcium signaling in pollen tubes, culminating in changes to pollen tube growth and seed set."

In: Phys. org

Excerpts: "Researchers, led by Dr. Li Hongju from the Institute of Genetics and Developmental Biology (IGDB) of the Chinese Academy of Sciences, have revealed that the female gamete in flowering plants controls fertilization recovery by secreting pollen tube attractants.The researchers, who explored why ovules continue to attract pollen tubes after failed double fertilization, have also provided molecular clues for the survival of some plant species after the loss of synergid cells during evolution."

" However, recent research by Li's group has shown that Arabidopsis thaliana ovules, with their two synergid cells experimentally removed, can still attract pollen tubes and produce seeds. This observation suggested the existence of an alternative mechanism for fertilization recovery."

"To investigate whether the central cell could secrete a pollen tube attractant, the researchers measured the pollen tube attraction activity of more than a hundred secreted peptides expressed in the central cell. They eventually found that two of them, SALVAGER1 (SAL1) and SAL2, show pollen tube attraction activity and can bind to the pollen tube plasma membrane. Interestingly, they found that SAL1 and SAL2 localized in the central cell are secreted into the micropyle and funiculus when synergid cells are defective or eliminated by the gcs1 mutant pollen tubes, which carry infertile sperm cells. These results suggest that SALs are new types of pollen tube attractants."

Authors: Lei Guo, Xi Luo, Muzi Li, Dirk Joldersma, Madison Plunkert and Zhongchi Liu.

Nature Communications (2022)

Editor's view: In flowering plants, fertilization triggers auxin synthesis in the endosperm to promote seed and fruit development. Here the authors show that an MADS-box transcription factor AGL62 is required to activate auxin synthesis in the endosperms of Fragaria vesca, a diploid strawberry, and Arabidopsis.

Authors: "The dominance of flowering plants on earth is owed largely to the evolution of maternal tissues such as fruit and seedcoat that protect and disseminate the seeds. The mechanism of how fertilization triggers the development of these specialized maternal tissues is not well understood. A key event is the induction of auxin synthesis in the endosperm, and the mobile auxin subsequently stimulates seedcoat and fruit development. However, the regulatory mechanism of auxin synthesis in the endosperm remains unknown. Here, we show that a type I MADS box gene AGL62 is required for the activation of auxin synthesis in the endosperm in both Fragaria vesca, a diploid strawberry, and in Arabidopsis. Several strawberry FveATHB genes were identified as downstream targets of FveAGL62 and act to repress auxin biosynthesis. In this work, we identify a key mechanism for auxin induction to mediate fertilization success, a finding broadly relevant to flowering plants." 

Authors: Joanna Rojek, Matthew R. Tucker, Michał Rychłowski, Julita Nowakowska and Małgorzata Gutkowska.

International Journal of Molecular Sciences (2021)

Abstract: "Auxin is a key regulator of plant development affecting the formation and maturation of reproductive structures. The apoplastic route of auxin transport engages influx and efflux facilitators from the PIN, AUX and ABCB families. The polar localization of these proteins and constant recycling from the plasma membrane to endosomes is dependent on Rab-mediated vesicular traffic. Rab proteins are anchored to membranes via posttranslational addition of two geranylgeranyl moieties by the Rab Geranylgeranyl Transferase enzyme (RGT), which consists of RGTA, RGTB and REP subunits. Here, we present data showing that seed development in the rgtb1 mutant, with decreased vesicular transport capacity, is disturbed. Both pre- and post-fertilization events are affected, leading to a decrease in seed yield. Pollen tube recognition at the stigma and its guidance to the micropyle is compromised and the seed coat forms incorrectly. Excess auxin in the sporophytic tissues of the ovule in the rgtb1 plants leads to an increased tendency of autonomous endosperm formation in unfertilized ovules and influences embryo development in a maternal sporophytic manner. The results show the importance of vesicular traffic for sexual reproduction in flowering plants, and highlight RGTB1 as a key component of sporophytic-filial signaling."

Authors: Yoon Kim, Sun-Ho Kim, Dong-Min Shin and Soo-Hwan Kim.

Frontiers in Plant Science (2021)

Abstract: "ATBS1-INTERACTING FACTOR 2 (AIF2) is a non-DNA-binding basic-helix-loop-helix (bHLH) transcription factor. Here, we demonstrate that AIF2 negatively modulates brassinosteroid (BR)-induced, BRASSINAZOLE RESISTANT 1 (BZR1)-mediated pollen and seed formation. AIF2-overexpressing Arabidopsis plants (AIF2ox) showed defective pollen grains and seed production while two AIF2 knockout mutants, aif2-1 and aif2-1/aif4-1, displayed opposite phenotypes. Genes encoding BZR1-regulated positive factors of seed size determination (SHB1, IKU1, MINI3) were suppressed in AIF2ox and genes for negative factors (AP2 and ARF2) were enhanced. Surprisingly, BZR1-regulated pollen genes such as SPL, MS1, and TDF1 were aberrantly up-regulated in AIF2ox plants. This stage-independent abnormal expression may lead to a retarded and defective progression of microsporogenesis, producing abnormal tetrad microspores and pollen grains with less-effective pollen tube germination. Auxin plays important roles in proper development of flower and seeds: genes for auxin biosynthesis such as TCPs and YUCCAs as well as for positive auxin signalling such as ARFs were suppressed in AIF2ox flowers. Moreover, lipid biosynthesis- and sucrose transport-related genes were repressed, resulting in impaired starch accumulation. Contrarily, sucrose and BR repressed ectopic accumulation of AIF2, thereby increasing silique length and the number of seeds. Taken together, we propose that AIF2 is negatively involved in pollen development and seed formation, and that sucrose- and BR-induced repression of AIF2 positively promotes pollen production and seed formation in Arabidopsis."