miR396 targets seven GROWTH-REGULATING FACTOR (GRF) genes and the BASIC HELIX-LOOP-HELIX (bHLH) TRANSCRIPTION FACTOR 74 gene (bHLH74) in Arabidopsis. Previous research revealed that the miR396 target module regulates cell proliferation and plays a critical role in leaf development. However, no additional biological functions of miR396 have been investigated in detail. In this study, T-DNA insertion mutants and transgenic plants with altered levels of miR396 or its target genes were used to characterize the regulatory role of miR396 in root development. We found that AtMIR396a was the predominant source for miR396 accumulation in the roots of seedlings, and that the mir396a-1 mutant had longer roots than wild-type seedlings. Overexpression of AtMIR396a decreased the transcript levels of target genes such as GRF genes and bHLH74, and resulted in a shorter root phenotype. Furthermore, the bhlh74-1 mutant had shorter roots, whereas overexpression of an miR396-resistant form of bHLH74 (mbHLH74) had an enhanced root growth phenotype. Moreover, MIR396a regulated root growth by affecting the elongation zone. Taken together, these data indicate that miR396a-mediated bHLH74 repression helps regulate root growth in Arabidopsis seedlings.

Soil salinity is a common environmental stress factor that limits agricultural production worldwide. Plants have evolved different strategies to achieve salt tolerance. miR393 has been identified as closely related to biotic and abiotic stresses, and targets F-box genes that encode auxin receptors. The miR393-TIR1/AFB2/AFB3 regulatory module was discovered to have multiple functions that manipulate the auxin response. This study focused on miR393 and one of its targets, TIR1, and found that they played potential roles in response to salt stress. Our results showed that overexpression of a miR393-resistant TIR1 gene (mTIR1) in Arabidopsis clearly enhanced salt stress tolerance, which led to a higher germination rate, less water loss, reduced inhibition of root elongation, delayed senescence, decreased death rate and stabilized Chl content. These plants accumulated more proline and anthocyanin, and displayed enhanced osmotic stress tolerance. The expression of some salt stress-related genes was altered, and sodium content can be reduced in these plants under salt stress. We proposed that highly increased auxin signaling by overexpression of mTIR1 may trigger auxin-mediated downstream pathways to enhance plant salt stress resistance by osmoregulation and increased Na(+) exclusion.

Summary A protein encoded by At1g32080 was consistently identified in proteomic studies of Arabidopsis chloroplast envelope membranes, but its function remained unclear. The protein, designated AtLrgB, may have evolved from a gene fusion of lrgA and lrgB. In bacteria, two homologous operons, lrgAB and cidAB, participate in an emerging mechanism to control cell death and lysis. We aim to characterize AtLrgB using reverse genetics and cell biological and biochemical analysis. AtLrgB is expressed in leaves, but not in roots. T‐DNA insertion mutation of AtLrgB produced plants with interveinal chlorotic and premature necrotic leaves. Overexpression of full‐length AtLrgB (or its LrgA and LrgB domains, separately), under the control of CaMV 35S promoter, produced plants exhibiting veinal chlorosis and delayed greening. At the end of light period, the T‐DNA mutant had high starch and low sucrose contents in leaves, while the 35S:AtLrgB plants had low starch and high sucrose contents. Metabolite profiling revealed that AtLrgB appeared not to directly transport triose phosphate or hexose phosphates. In yeast cells, AtLrgB could augment nystatin‐induced membrane permeability. Our work indicates that AtLrgB is a new player in chloroplast development, carbon partitioning and leaf senescence, although its molecular mechanism remains to be established.

UV Resistance Locus 8 (UVR8), an ultraviolet-B (UV-B; 280–315 nm) photoreceptor, participates in the regulation of various plant growth and developmental processes. UV-B radiation is an important factor enhancing the production of active components in medicinal plants. To-date, however, studies on UV-B photoreceptors have largely focused on Arabidopsis, and the functions of UVR8 in medicinal plants are still largely unknown. In the present study, a homolog of Arabidopsis UVR8, CmUVR8, was isolated from Chrysanthemum morifolium Ramat, and its structure and function were analyzed in detail. Protein sequence analysis showed that CmUVR8 contained nine conserved regulators of chromosome condensation 1 repeats, seven conserved bladed propellers, one C27 region, three “GWRHT” motifs and several crucial amino acid residues (such as 14 Trps and 2 Args), similar to AtUVR8. 3-D structural analysis of CmUVR8 indicated that its structure was similar to AtUVR8. Heterologous expression of CmUVR8 could rescued the deficient phenotype of uvr8-6, a mutant of UVR8 in Arabidopsis, indicating the role of CmUVR8 in the regulation of hypocotyl elongation and HY5 gene expression under UV-B irradiation. Moreover, CmUVR8 regulates UV-B-induced expression of four flavonoids biosynthesis-related genes and the UV-B-induced accumulation of flavonoids. Furthermore, the interaction between CmUVR8 and CmCOP1 were confirmed using a yeast two-hybrid assay. These results indicated that CmUVR8 plays important roles in UV-B signal transduction and the UV-B-induced accumulation of flavonoids, as a counterpart of AtUVR8.

Plant specialized metabolites (PSMs) play essential roles in the adaptation to harsh environments and function in plant defense responses. PSMs act as key components of defense-related signaling pathways and trigger the extensive expression of defense-related genes. In addition, PSMs serve as antioxidants, participating in the scavenging of rapidly rising reactive oxygen species, and as chelators, participating in the chelation of toxins under stress conditions. PSMs include nitrogen-containing chemical compounds, terpenoids/isoprenoids, and phenolics. Each category of secondary metabolites has a specific biosynthetic pathway, including precursors, intermediates, and end products. The basic biosynthetic pathways of representative PSMs are summarized, providing potential target enzymes of stress-mediated regulation and responses. Multiple metabolic pathways share the same origin, and the common enzymes are frequently to be the targets of metabolic regulation. Most biosynthetic pathways are controlled by different environmental and genetic factors. Here, we summarized the effects of environmental factors, including abiotic and biotic stresses, on PSM biosynthesis in various plants. We also discuss the positive and negative transcription factors involved in various PSM biosynthetic pathways. The potential target genes of the stress-related transcription factors were also summarized. We further found that the downstream targets of these Transcription factors (TFs) are frequently enriched in the synthesis pathway of precursors, suggesting an effective role of precursors in enhancing of terminal products. The present review provides valuable insights regarding screening targets and regulators involved in PSM-mediated plant protection in non-model plants.