Genome-Wide Analysis of the Growth-Regulating Factor (GRF) Family in Aquatic Plants and Their Roles in the ABA-Induced Turion Formation of Spirodela polyrhiza

The Developmental Cycle of Spirodela polyrhiza Turions: A Model for Turion-Based Duckweed Overwintering?

Duckweeds are widely distributed small, simply constructed aquatic higher plants (the Lemnaceae) found on quiet freshwater surfaces. Species inhabiting temperate climates may have to cope with long periods of severe cold during the winter season. Several duckweeds form compact resting structures from the assimilatory fronds of the growing season that can bridge inhospitable conditions in a quiescent state. Of these, turions separate from the mother fronds and overwinter on the water body bottom in a dormant state. They can surface, germinate, and sprout to resume active growth upon warming in the spring. The turions of the largest duckweed, Spirodela polyrhiza, have been intensively examined as to ultrastructure, the factors governing their formation and release from dormancy, and the signals driving their germination and sprouting and the accompanying starch degradation. Comparative transcriptomics of assimilatory fronds and dormant turions are revealing the molecular features of this developmental cycle. The results illustrate an elegant sequence of reactions that ensures aquatic survival of even severe winters by frost avoidance in a vegetative mode. Since little is known about other duckweed resting fronds, the S. polyrhiza turion developmental cycle cannot be considered to be representative of duckweed resting fronds in general but can serve as a reference for corresponding investigations.

Plant developmental oddities

MAIN CONCLUSION

Plants lacking shoot apical meristem develop with unique body shapes, suggesting rewiring of developmental genes. This loss of the meristem is likely influenced by a combination of environmental factors and evolutionary pressures. This study explores the development of plant bodies in three families (Podostemaceae, Lemnaceae, and Gesneriaceae) where the shoot apical meristem (SAM), a key structure for growth, is absent or altered. The review highlights alternative developmental strategies these plants employ. Also, we considered alternative reproduction in those species, namely through structures like turions, fronds, or modified leaves, bypassing the need for a SAM. Further, we report on studies based on the expression patterns of genes known to be involved in SAM formation and function. Interestingly, these genes are still present but expressed in atypical locations, suggesting a rewiring of developmental networks. Our view on the current literature and knowledge indicates that the loss or reduction of the SAM is driven by a combination of environmental pressures and evolutionary constraints, leading to these unique morphologies. Further research, also building on Next-Generation Sequencing, will be instrumental to explore the genetic basis for these adaptations and how environmental factors influence them.

Hormonal profiles in dormant turions of 22 aquatic plant species: do they reflect functional or taxonomic traits?

BACKGROUND AND AIMS

Turions are vegetative, dormant overwintering organs formed in aquatic plants in response to unfavourable ecological conditions. Contents of cytokinin (CK), auxin metabolites and abscisic acid (ABA) as main growth and development regulators were compared in innately dormant autumnal turions of 22 aquatic plant species of different functional ecological or taxonomic groups with those in non-dormant winter apices in three aquatic species and with those in spring turions of four species after their overwintering.

METHODS

The hormones were analysed in miniature turion samples using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry.

KEY RESULTS

In innately dormant turions, the total contents of each of the four main CK types, biologically active forms and total CKs differed by two to three orders of magnitude across 22 species; the proportion of active CK forms was 0.18-67 %. Similarly, the content of four auxin forms was extremely variable and the IAA proportion as the active form was 0.014-99 %. The ABA content varied from almost zero to 54 µmol kg-1 dry weight and after overwintering it usually significantly decreased. Of all functional traits studied, hormone profiles depended most on the place of turion sprouting (surface vs bottom) and we suggest that this trait is crucial for turion ecophysiology.

CONCLUSIONS

The key role of ABA in regulating turion dormancy was confirmed. However, the highly variable pattern of the ABA content in innately dormant and in overwintered turions indicates that the hormonal mechanism regulating the innate dormancy and its breaking in turions is not uniform within aquatic plants.

Characteristics of turion development in two aquatic carnivorous plants: Hormonal profiles, gas exchange and mineral nutrient content

Turions are vegetative, dormant, and storage overwintering organs formed in perennial aquatic plants in response to unfavorable ecological conditions and originate by extreme condensation of apical shoot segments. The contents of cytokinins, auxins, and abscisic acid were estimated in shoot apices of summer growing, rootless aquatic carnivorous plants, Aldrovanda vesiculosa and Utricularia australis, and in developing turions at three stages and full maturity to reveal hormonal patterns responsible for turion development. The hormones were analyzed in miniature turion samples using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry. Photosynthetic measurements in young leaves also confirmed relatively high photosynthetic rates at later turion stages. The content of active cytokinin forms was almost stable in A. vesiculosa during turion development but markedly decreased in U. australis. In both species, auxin content culminated in the middle of turion development and then decreased again. The content of abscisic acid as the main inhibitory hormone was very low in growing plants in both species but rose greatly at first developmental stages and stayed very high in mature turions. The hormonal data indicate a great strength of developing turions within sink-source relationships and confirm the central role of abscisic acid in regulating the turion development.

Genome-wide identification of GRF gene family and their contribution to abiotic stress response in pitaya (Hylocereus polyrhizus)

Growth-regulating factors (GRFs) are plant-specific transcription factors identified in many land plants. Recently, their indispensable roles in stress response are highlighted. In present work, 11 HpGRFs were cloned in pitaya. Segmental duplication is considered essential for the expansion of HpGRFs. A phylogenetic tree suggested that GRFs could be divided into eight categories, among which G-I was a Caryophyllales-specific one. The categorization was further evidenced by differences in the gene structure, collinearity, protein domain of HpGRFs. Five miR396 hairpins giving rise to two types of matured miR396s were identified in pitaya via sRNA-Seq in combination with bioinformatic analysis. Parallel analysis of RNA ends proved that HpGRFs except HpGRF5 were degraded by miR396-directed cleavages at the regions which code the conserved WRC motifs of HpGRFs. Multiple cis-regulatory elements were discovered in the promoters of HpGRFs. Among the elements, most are involved in stress and phytohormone response as well as plant growth, indicating a crosstalk between them. Expression analysis showed the responsive patterns of the miR396-GRF module under abiotic stresses. To conclude, our work systematically identified the miR396-targeted HpGRFs in pitaya and confirmed their involvement in stress response, providing novel insights into the comprehensive understanding of the stress resistance of pitaya.