Germination and early seedling development are critical for successful stand establishment of plants. Following germination, the cotyledons, which are derived from embryonic tissue, emerge from the seed. Arabidopsis seedlings at post-germinative stages are supported mainly by the supply of nutrition from the cotyledons until vegetative leaves emerge and initiate photosynthesis. The switch to autotrophic growth is a significant transition at the post-germinative stage. Here, we provide evidence that down-regulation of SQUAMOSA PROMOTER-BINDING PROTEIN LIKE13 (SPL13) by microRNA156 (miR156) plays an important role in the regulation of the post-germinative switch from the cotyledon stage to the vegetative-leaf stage. Silent mutations created in the SPL13 sequence in the region that is complementary to the miR156 sequence caused the deregulation of the mutant form of SPL13 (mSPL13) mRNA from miR156. Mutant seedlings over-accumulated miRNA-resistant messages and exhibited a delay in the emergence of vegetative leaves compared to wild-type seedlings. The delay was not observed in control transgenic plants expressing non-mutated SPL13, indicating that the phenotype was caused specifically by the silent mutations and deregulation of SPL13 from miR156. Characterization of the SPL13 promoter indicated that this gene is expressed mainly in the hypocotyl and affects leaf primordium development. These results suggest that the repression of SPL13 by miR156 is essential for normal post-germinative growth in Arabidopsis.

MicroRNAs (miRNAs) are involved in developmental programmes of plants, including seed germination and post-germination. Here, we provide evidence that two different miRNA pathways, miR156 and miR172, interact during the post-germination stages inArabidopsis. Mutant seedlings expressing miR156-resistantSQUAMOSA PROMOTER-BINDING PROTEIN-LIKE13(mSPL13), which has silent mutations in the miR156 complementary sequence, over-accumulatedSPL13mRNA and exhibited a delay in seedling development. Microarray analysis indicated thatSCHNARCHZAPFEN(SNZ), anAP2-like gene targeted by miR172, was down-regulated in these mutants. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) and miRNA gel blot analyses showed that theMIR172genes were up-regulated inmSPL13mutants. These results suggest that the miRNA regulation cascades (miR156⊣SPL13 → miR172⊣SNZ) play a critical role during the post-germination developmental stages inArabidopsis.

Coronalon (6-ethyl indanoyl isoleucine), a synthetic jasmonate mimic, is known to regulate levels of transcripts and secondary metabolites that are commonly elicited by methyl jasmonate (MeJA) in a variety of plants. The ability of coronalon and its derivative (In-L-Ile-Me) to elicit MeJA-activated transcriptional and defence responses [nicotine and trypsin proteinase inhibitors (TPIs)] was compared in treated and systemic untreated tissues of wild-type (WT) and NaLOX3-silenced Nicotiana attenuata plants which are unable to activate either local or systemic defence responses. Coronalon and its derivative significantly regulated 71% and 86% of genes up-regulated by MeJA and 53% and 66% of the genes down-regulated by MeJA in the treated leaves, but only 3% and 7% of all regulated genes in untreated, but phylotactically connected, leaves of WT plants. Consistent with their ability to elicit transcriptional responses in treated tissues, coronalon and In-L-Ile-Me increased nicotine and TPIs when applied to the tissues in which these metabolites are produced, namely roots and leaves. However, treating roots elicited TPI activity in leaves in both WT and NaLOX3-silenced plants, suggesting that mimics can be transported apoplastically from roots to leaves in the xylem. This response was lower in NaLOX3-silenced plants, suggesting that the ability of coronalon and In-L-Ile-Me to elicit TPI responses in leaves after root treatments requires intact jasmonic acid (JA) signalling. Treating leaves did not elicit detectable changes in endogenous JA levels but did decrease free salicylic acid contents. It is concluded that coronalon and In-L-Ile-Me elicit jasmonate responses in treated tissues and could be valuable tools for dissecting local and systemic jasmonate signalling networks in plants.

The objective of the present study was to recognize the molecular background of the accumulation of raffinose family oligosaccharides (RFOs) in pea (Pisum sativum L.) seedlings under osmotic stress conditions. The exposure of 5-day-old pea seedlings to osmotic stress for 48 h created by immersing roots in PEG8000 solution (−1.5 MPa) induced synthesis of galactinol and RFOs (raffinose and stachyose) in the epicotyl and root tissues, but not in cotyledons. After 24 h of recovery, galactinol completely disappeared, raffinose decreased fourfold and stachyose decreased twofold in roots, but increased in epicotyls. The temporary accumulation of RFOs resulted from a dramatic increase in the enzymatic activity and changes in expression of galactinol synthase (PsGolS), raffinose synthase (PsRS) and stachyose synthase (PsSTS) genes. PsGolS was induced by osmotic stress in both epicotyls as well as in roots. PsRS and PsSTS were induced only in epicotyls, but repressed or remained unaffected in roots, respectively. During recovery, the expression and activity of PsGolS, PsRS and PsSTS dramatically decreased. The expression of PsGolS gene, that level of mRNA transcript significantly decreased during recovery and whose promoter region was identified to contain some stress-related regulating elements, seems to play a crucial role in the biosynthesis of RFOs under osmotic stress. Possible signals that may trigger the induction of expression of PsGolS, PsRS and PsSTS genes and accumulation of RFOs in pea seedlings are discussed.