SummaryPlant laccase (LAC) enzymes belong to the blue copper oxidase family and polymerize monolignols into lignin. Recent studies have established the involvement of microRNAs in this process; however, physiological functions and regulation of plant laccases remain poorly understood. Here, we show that a laccase gene, LAC4, regulated by a microRNA, miR397b, controls both lignin biosynthesis and seed yield in Arabidopsis. In transgenic plants, overexpression of miR397b (OXmiR397b) reduced lignin deposition. The secondary wall thickness of vessels and the fibres was reduced in the OXmiR397b line, and both syringyl and guaiacyl subunits are decreased, leading to weakening of vascular tissues. In contrast, overexpression of miR397b-resistant laccase mRNA results in an opposite phenotype. Plants overexpressing miR397b develop more than two inflorescence shoots and have an increased silique number and silique length, resulting in higher seed numbers. In addition, enlarged seeds and more seeds are formed in these miR397b overexpression plants. The study suggests that miR397-mediated development via regulating laccase genes might be a common mechanism in flowering plants and that the modulation of laccase by miR397 may be potential for engineering plant biomass production with less lignin.

SUMMARYhMMS21 is a SUMO E3 ligase required for the prevention of DNA damage-induced apoptosis, and acts by facilitating DNA repair in human cells. The Arabidopsis genome contains a putative MMS21 homologue capable of interacting with the SUMO E2 conjugating enzyme AtSCE1a, as indicated by a yeast two-hybrid screen and bimolecular fluorescence complementation experiments. In vitro and in vivo data demonstrated that AtMMS21 was a SUMO E3 ligase. We identified the Arabidopsis AtMMS21 null T-DNA insertion mutant mms21-1, which had a short-root phenotype, and affected cell proliferation in the apical root meristem, as indicated by impaired expression of the cell division marker CYCB1:GUS in mms21-1 roots. The mms21-1 roots had reduced responses to exogenous cytokinins, and decreased expression of the cytokinin-induced genes ARR3, ARR4, ARR5 and ARR7, compared with the wild type. Thus, our findings suggest that the AtMMS21 gene is involved in root development via cell-cycle regulation and cytokinin signalling.

Flowering is a critical event in the life cycle of plants and is regulated by a combination of endogenous controls and environmental cues. In the present work, we provide clear genetic evidence that GASA5, a GASA family gene in Arabidopsis (Arabidopsis thaliana), is involved in controlling flowering time and stem growth. GASA5 expression was present in all tissues of Arabidopsis plants, as detected by RT-PCR, and robust GUS staining was observed in the shoot apex of 8-day-old seedlings and inflorescence meristems during reproductive development. Phenotypic analysis showed that a GASA5 null mutant (gasa5-1) flowered earlier than wild type with a faster stem growth rate under both long-day (LD) and short-day (SD) photoperiods. In contrast, transgenic plants overexpressing GASA5 demonstrated delayed flowering, with a slower stem growth rate compared to wild-type plants. However, neither the GASA5 null mutants nor the GASA5 overexpressing plants revealed obvious differences in flowering time upon treatment with gibberellic acid (GA(3)), indicating that GASA5 is involved in gibberellin (GA)-promoted flowering. GAI (GA INSENSITIVE), one of the five DELLAs in Arabidopsis, was more highly expressed in GASA5-overexpressing plants, but it was lower in gasa5-1. Further transcript profiling analysis suggested that GASA5 delayed flowering by enhancing FLOWERING LOCUS C (FLC) expression and repressing the expression of key flowering-time genes, FLOWERING LOCUS T (FT) and LEAFY (LFY). Our results suggest that GASA5 is a negative regulator of GA-induced flowering and stem growth.

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The DELLA protein REPRESSOR OF ga1-3-LIKE2 (RGL2) plays an important role in seed germination under different conditions through a number of transcription factors. However, the functions of the structural genes associated with RGL2-regulated germination are less defined. Here, we report the role of an Arabidopsis (Arabidopsis thaliana) cell wall-localized protein, Gibberellic Acid-Stimulated Arabidopsis6 (AtGASA6), in functionally linking RGL2 and a cell wall loosening expansin protein (Arabidopsis expansin A1 [AtEXPA1]), resulting in the control of embryonic axis elongation and seed germination. AtGASA6-overexpressing seeds showed precocious germination, whereas transfer DNA and RNA interference mutant seeds displayed delayed seed germination under abscisic acid, paclobutrazol, and glucose (Glc) stress conditions. The differences in germination rates resulted from corresponding variation in cell elongation in the hypocotyl-radicle transition region of the embryonic axis. AtGASA6 was down-regulated by RGL2, GLUCOSE INSENSITIVE2, and ABSCISIC ACID-INSENSITIVE5 genes, and loss of AtGASA6 expression in the gasa6 mutant reversed the insensitivity shown by the rgl2 mutant to paclobutrazol and the gin2 mutant to Glc-induced stress, suggesting that it is involved in regulating both the gibberellin and Glc signaling pathways. Furthermore, it was found that the promotion of seed germination and length of embryonic axis by AtGASA6 resulted from a promotion of cell elongation at the embryonic axis mediated by AtEXPA1. Taken together, the data indicate that AtGASA6 links RGL2 and AtEXPA1 functions and plays a role as an integrator of gibberellin, abscisic acid, and Glc signaling, resulting in the regulation of seed germination through a promotion of cell elongation.