MicroRNAs are short RNA molecules, typically around 21 nucleotides long, that do not encode proteins. Instead, they act as guides, binding to complementary sequences in messenger RNAs and triggering their degradation or blocking their translation. In plants, the miR156 family is one of the most ancient and influential of these regulators, targeting a group of transcription factors known as SPL proteins, which control everything from flowering time to leaf shape. Because SPL transcription factors sit at the top of gene regulatory hierarchies, the miR156-SPL module functions as a powerful hub: by tuning how much SPL protein a cell produces, miR156 indirectly influences hundreds of downstream genes. Previous work in annual crops and model plants such as Arabidopsis, alfalfa, apple, and rice had implicated this module in drought and salt responses, but its role in long-lived woody species remained largely uncharted territory.

The Harbin team, led by corresponding authors Zhang Huihui and Liu Xuemei, set out to map this circuit in birch, an economically and ecologically important tree of northern forests. Using transgenic birch lines engineered to overexpress either bp-miR156c or BpSPL2, the researchers subjected the plants to drought stress and compared their performance against wild-type controls. The results were striking and, at first glance, counterintuitive. Plants with extra bp-miR156c fared worse under drought, showing reduced tolerance, while plants overexpressing BpSPL2, the very gene that bp-miR156c silences, became notably more drought resistant. This inverse relationship confirmed that BpSPL2 is a positive regulator of drought tolerance, and that bp-miR156c, by cleaving the BpSPL2 transcript, dampens the tree’s drought defenses rather than boosting them.

GUS staining experiments provided direct evidence of the regulatory relationship, showing that BpSPL2 is indeed a target gene of bp-miR156c and is subject to its cleavage. With that hierarchy established, the researchers dug into what BpSPL2 actually does when water becomes scarce. Under drought conditions, the BpSPL2-overexpressing lines displayed significantly alleviated photodamage in both photosystem II and photosystem I, the two light-harvesting complexes at the heart of photosynthesis. Drought typically forces plants to close their stomata to conserve water, which starves the photosynthetic apparatus of carbon dioxide and causes absorbed light energy to spill into dangerous side reactions. The BpSPL2 lines also showed reduced oxidative damage, suggesting that the transcription factor helps the tree keep its cellular chemistry in balance even as its water supply dwindles.

To understand the molecular basis of these protective effects, the team performed RNA sequencing on the transgenic and wild-type plants under drought stress. The analysis revealed that differentially expressed genes in the BpSPL2-overexpressing plants were significantly enriched in several key pathways: photosynthesis-related processes, tryptophan metabolism, redox processes, and glutathione metabolism. This pattern pointed toward two parallel mechanisms. The enrichment of redox and glutathione-related genes suggested that BpSPL2 bolsters the tree’s antioxidant arsenal, while the activation of tryptophan metabolism hinted at a connection to auxin, the plant hormone that drives root development and is synthesized from tryptophan.

The researchers then used a combination of yeast one-hybrid assays, chromatin immunoprecipitation followed by PCR, and dual-luciferase reporter assays to identify the direct targets of BpSPL2. These experiments demonstrated that the BpSPL2 transcription factor recognizes a specific DNA sequence motif, known as the GTAC motif, and binds to the promoters of two critical genes. The first is BpGSTF3, which encodes a glutathione-S-transferase, an enzyme family famous for its role in detoxifying reactive oxygen species and xenobiotic compounds. The second is BpASA1, which encodes anthranilate synthase, the rate-limiting enzyme in tryptophan biosynthesis and therefore a gatekeeper of auxin production. By binding to both promoters, BpSPL2 enhances the transcription of these genes, setting in motion the two-pronged drought defense.

The consequences of this dual activation are elegant. On one hand, upregulating BpGSTF3 increases glutathione-S-transferase and antioxidant enzyme activities, allowing the leaves to mop up the reactive oxygen species that accumulate during drought-induced photosynthetic stress. This mitigates the photodamage to photosystems II and I and reduces oxidative injury to cellular membranes and proteins. On the other hand, activating BpASA1 ramps up tryptophan synthesis, which feeds into the production of indole-3-acetic acid, the principal naturally occurring auxin. Elevated auxin stimulates the formation of lateral roots, the branching side roots that dramatically expand a plant’s absorptive surface area. Indeed, overexpression of BpSPL2 significantly promoted root system development in birch, with particularly pronounced effects on lateral root growth, giving the trees a better architectural foundation for scavenging moisture from drying soil.

The study’s findings fit into a broader and rapidly evolving picture of the miR156-SPL module as a versatile stress-regulation toolkit in plants. Research in alfalfa has shown that miR156 improves drought tolerance by silencing SPL13, while work in apple has linked the module to both salt stress tolerance and flavonoid synthesis. In rice, variations in the SPL gene OsSPL10 confer drought tolerance by regulating reactive oxygen species production, and recent studies in poplar have implicated a miR156g-SPL module in drought responses. The birch work adds an important dimension because trees face drought over years and decades rather than a single growing season, and their survival depends on integrating root architecture, photosynthetic resilience, and antioxidant capacity in a way that annual crops do not. Demonstrating that a single transcription factor can simultaneously tune all three of these systems in a woody species is a significant conceptual advance.

There are also practical implications. As climate change intensifies drought frequency and severity across the boreal and temperate forests where birch thrives, understanding the genetic levers of drought tolerance becomes essential for both conservation and forestry. The bp-miR156c-BpSPL2 module identified in this study provides a theoretical foundation for breeding or engineering birch trees with enhanced drought resistance, whether through marker-assisted selection for favorable alleles or through biotechnological approaches that modulate SPL activity. Because the module acts upstream of both antioxidant defense and root development, manipulating it could deliver coordinated improvements that would be difficult to achieve by targeting either trait alone. The work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and related institutional funds, reflecting the strategic importance of forest resilience research in the region.

For now, the Harbin team’s contribution is a detailed molecular map: a microRNA that silences, a transcription factor that activates, two target genes that execute, and a physiological outcome that keeps a tree alive when the rain stops. It is a reminder that in the quiet chemistry of a leaf and the hidden architecture of a root system, plants have evolved regulatory circuits of remarkable sophistication. Decoding them, one module at a time, may prove essential as forests around the world confront a hotter, drier future.

Subject of Research: The bp-miR156c-BpSPL2 regulatory module controlling drought tolerance in birch through reactive oxygen species scavenging and lateral root development

Article Title: The bp-miR156c–BpSPL2 module positively regulates drought tolerance by mediating lateral root development and reactive oxygen species scavenging in Betula platyphylla

Article References: Peng, H., Hongrui, Z., Jiaqian, A., Zhongjia, Y., Huilei, D., Huihui, Z., & Xuemei, L. (2026). The bp-miR156c–BpSPL2 module positively regulates drought tolerance by mediating lateral root development and reactive oxygen species scavenging in Betula platyphylla. Plant Cell Reports, 45(10), Article 282. https://doi.org/10.1007/s00299-026-03924-4

Image Credits: AI Generated

DOI: 10.1007/s00299-026-03924-4

Keywords: drought tolerance, birch, Betula platyphylla, miR156, SPL transcription factor, BpSPL2, reactive oxygen species, lateral roots, auxin, glutathione-S-transferase, tryptophan metabolism, plant molecular biology

The biology of birch reproduction makes this achievement particularly striking. Unlike annual plants that complete flowering within weeks, the male inflorescence of B. platyphylla develops over nearly an entire year. The catkins initiate in spring, then overwinter in a dormant state as uninucleate microspores, suspended in time until they resume development and release pollen the following season. Any gene that governs this long, cold-interrupted program must therefore act with precise timing, and understanding where that timing goes wrong is essential if breeders want to deliberately shut down pollen production without harming the tree.

The research began with an unusual natural specimen. A single spontaneous mutant individual of B. platyphylla was found bearing two distinct types of abnormal male inflorescences, designated WLM and MM, alongside comparison with wild-type plants. At three key developmental stages, the microspore mother cell stage, the tetrad and microspore stage, and the mature pollen stage just before pollination, the team carried out detailed phenotypic observations paired with transcriptome sequencing. Both mutant inflorescence types showed impaired pollen grain maturation and defective anther dehiscence at stage 6, confirming that the mutation disrupts the male reproductive program at multiple points.

As floral development progressed, the scale of the molecular disruption became increasingly dramatic. The number of differentially expressed genes between the mutant inflorescences and wild type rose markedly from stage to stage, indicating that the underlying defect compounds over time rather than resolving. Pathway enrichment analysis of these differentially expressed genes pointed to several biochemical routes as potentially critical for birch male flower development, most notably phenylpropanoid biosynthesis and ABC transporter activity. Both categories make biological sense: phenylpropanoid metabolism supplies building blocks for sporopollenin, the extraordinarily tough polymer that forms the pollen exine, while ABC transporters ferry lipidic precursors from the tapetum to the developing microspores.

Within the gene ontology analysis, the researchers identified 31 candidate genes linked to pollen exine formation, the sporopollenin biosynthetic process, and anther development. Among them were BpSPL8, encoding a SQUAMOSA PROMOTER BINDING PROTEIN-LIKE transcription factor; BpAPY7, an apyrase previously implicated in exine patterning and anther dehiscence in other species; and BpCYP703A2, a cytochrome P450 enzyme known from land plants as a catalyst for in-chain hydroxylation of lauric acid, providing essential building blocks for sporopollenin synthesis. The presence of these familiar players confirmed that birch uses a conserved genetic toolkit for pollen wall construction, while the mutant context revealed how that toolkit is coordinated.

The decisive insight came from phylogenetic and co-expression analyses, which converged on BpSPL2 as a core regulator sitting at the top of the network. SPL genes belong to a family of SBP-box transcription factors with well-documented roles in flowering time, leaf morphology, and fertility across the plant kingdom, and previous work in Arabidopsis had shown that both miR156-targeted and non-targeted SPL genes act together to secure male fertility. To test whether BpSPL2 was genuinely causal rather than merely correlated with sterility, the team manipulated its expression in birch and observed the consequences directly.

The results were unambiguous and, in one direction, dramatic. When BpSPL2 was overexpressed, microsporogenesis was arrested at the microsporocyte stage itself, before meiosis could proceed to tetrad formation. The arrested cells subsequently degraded during the overwintering period, ultimately yielding no microspores and no pollen whatsoever. In other words, elevating this single transcription factor completely abolishes pollen production by stopping development at its earliest cellular step. Conversely, suppressing BpSPL2 produced the opposite phenotype: microsporogenesis was promoted, and the resulting inflorescences and anthers became enlarged. The gene thus behaves as a temporal switch whose dosage determines whether, when, and how far the pollen developmental program advances.

These findings establish BpSPL2 as a key temporal regulator of microsporogenesis in B. platyphylla and provide a mechanistic account of two distinct abnormal male flower phenotypes arising on a single spontaneous mutant. The study is also notable for its systematic design: by sampling the same mutant individual at three developmental stages across a nearly year-long cycle and integrating phenotypic cytology with transcriptome-wide expression profiling, the authors built a stage-resolved picture of how male sterility unfolds in a long-lived tree, a scale of analysis rarely achieved in forest reproductive genetics.

The practical implications extend well beyond basic biology. Birch pollen is a significant allergen, and sensitization to birch is strongly correlated with pollen-food allergy syndrome in northern China, where reactions to soy and apple frequently accompany birch pollen allergy. A pollen-free birch variety would remove the allergen at its source rather than treating symptoms, an attractive proposition for urban landscaping and allergy-prone regions. The new work provides both a theoretical foundation and concrete genetic resources for such breeding: the 31 candidate genes, the pathway maps, and above all the BpSPL2 switch itself give breeders molecular targets for generating sterile lines, whether through conventional marker-assisted selection, transgenic modulation, or genome editing approaches that have already proven effective for fertility genes in crops.

There are broader lessons here for tree biotechnology as well. Pollen-mediated gene flow has long been a concern for genetically modified forest plantations, and sterile varieties would simultaneously address allergenicity and containment. The birch study demonstrates that in long-cycle woody species, where traditional crossing and transformation are slow, a spontaneous mutant coupled with modern transcriptomics can shortcut decades of genetic analysis. By pinpointing one master regulator in a tree whose catkins spend a winter as dormant microspores, the researchers have shown that even the most patient developmental programs in plants can be understood, and potentially rewritten, at the level of a single gene.

Subject of Research: Molecular regulation of male sterility and microsporogenesis in Betula platyphylla

Article Title: Transcriptome analysis of a spontaneous male-sterile mutant in Betula platyphylla and functional characterization of the key gene BpSPL2 in regulating inflorescence development

Article References: Transcriptome analysis of a spontaneous male-sterile mutant in Betula platyphylla and functional characterization of the key gene BpSPL2 in regulating inflorescence development. (n.d.). https://doi.org/10.1007/s00299-026-03970-y

Image Credits: AI Generated

DOI: 10.1007/s00299-026-03970-y

Keywords: Betula platyphylla, male sterility, BpSPL2, microsporogenesis, pollen development, transcriptome analysis, SPL transcription factors, sporopollenin, anther development, forest tree breeding, birch pollen allergy, inflorescence development