Ethylene biosynthesis in higher plants is regulated developmentally and environmentally. To investigate the regulation of ACC synthase gene expression, the promoters of Arabidopsis ACS genes, AtACS4, AtACS5, and AtACS7, were fused to a GUS reporter gene, and the recombinant transgenes were introduced into Arabidopsis to produce three groups of AtACS∷GUS transgenic plants. Histochemic and fluorometric study of these transgenic plants revealed that promoters of AtACS4, AtACS, and AtACS7 are all active in dark-germinated seedlings. AtACS5 has the highest promoter activity in leaves of 2-week-old light-grown seedlings among the three AtACS genes studied. In the mature leaves, AtACS4 and AtACS7 genes are expressed in both veins and areoles, whereas AtACS5 is expressed at a higher level in the areoles and epidermal cells surrounding trichomes. The promoter activities of all these AtACS genes are found in the reproductive organs. AtACS5 and AtACS7 are highly expressed in petals, sepals, carpels, stamens, cauline leaves, inflorescence stems, and siliques, while AtACS4 expression is undetectable in the petals of open flowers. All three AtACS genes are expressed in root tissue. In the 2-week-old light-grown Arabidopsis, the AtACS4 promoter is responsive to the plant hormones IAA, ethylene, and ABA, and to darkness and wounding; the AtACS5 promoter to IAA, ABA, salt, high temperature, and wounding; and the AtACS7 promoter to GA₃, ethylene, and ABA, and to darkness and salt. Low-temperature treatment abolishes the darkness-induced AtACS7 gene expression, but not that of AtACS4. Each AtACS gene has a unique expression profile during growth and development. It appears that at any developmental stage or any growth period of Arabidopsis, there is always a member of AtACS multigene family that is actively expressed.

The phytohormones ethylene and abscisic acid (ABA) play essential roles in the abiotic stress adaptation of plants, with both cross-talk of ethylene signalling and ABA biosynthesis and signalling reported. Any reciprocal effects on each other's biosynthesis, however, remain elusive. ACC synthase (ACS) acts as the key enzyme in ethylene biosynthesis. A pilot study on changes in ACS promoter activities in response to abiotic stresses revealed the unique involvement in abiotic stress responses of the only type 3 ACC synthase, ACS7, among all nine ACSs of Arabidopsis. Hence an acs7 mutant was characterized and its abiotic stress responses were analysed. The acs7 mutant germinated slightly faster than the wild type and subsequently maintained a higher growth rate at the vegetative growth stage. Ethylene emission of acs7 was merely one-third of that of the wild type. acs7 exhibited enhanced tolerance to salt, osmotic, and heat stresses. Furthermore, acs7 seeds were hypersensitive to both ABA and glucose during germination. Transcript analyses revealed that acs7 had elevated transcript levels of the stress-responsive genes involved in the ABA-dependent pathway under salt stress. The ABA level was also higher in acs7 following salt treatment. Our data suggest that ACS7 acts as a negative regulator of ABA sensitivity and accumulation under stress and appears as a node in the cross-talk between ethylene and ABA.

Nitrogen is an essential element for plant growth and yield. Improving Nitrogen Use Efficiency (NUE) of crops could potentially reduce the application of chemical fertilizer and alleviate environmental damage. To identify new NUE genes is therefore an important task in molecular breeding. Macroautophagy (autophagy) is an intracellular process in which damaged or obsolete cytoplasmic components are encapsulated in double membraned vesicles termed autophagosomes, then delivered to the vacuole for degradation and nutrient recycling. One of the core components of autophagosome formation, ATG8, has been shown to directly mediate autophagosome expansion, and the transcript of which is highly inducible upon starvation. Therefore, we postulated that certain homologs of Saccharomyces cerevisiae ATG8 (ScATG8) from crop species could have potential for NUE crop breeding. A soybean (Glycine max, cv. Zhonghuang-13) ATG8, GmATG8c, was selected from the 11 family members based on transcript analysis upon nitrogen deprivation. GmATG8c could partially complement the yeast atg8 mutant. Constitutive expression of GmATG8c in soybean callus cells not only enhanced nitrogen starvation tolerance of the cells but accelerated the growth of the calli. Transgenic Arabidopsis over-expressing GmATG8c performed better under extended nitrogen and carbon starvation conditions. Meanwhile, under optimum growth conditions, the transgenic plants grew faster, bolted earlier, produced larger primary and axillary inflorescences, eventually produced more seeds than the wild-type. In average, the yield was improved by 12.9%. We conclude that GmATG8c may serve as an excellent candidate for breeding crops with enhanced NUE and better yield.

As the last stage of leaf development, senescence is a fine-tuned process regulated by interplays of multiple signaling pathways. We have previously identified soybean (Glycine max) SENESCENCE-ASSOCIATED RECEPTOR-LIKE KINASE (SARK), a leucine-rich repeat-receptor-like protein kinase from soybean, as a positive regulator of leaf senescence. Here, we report the elucidation of the molecular mechanism of GmSARK-mediated leaf senescence, especially its specific roles in senescence-inducing hormonal pathways. A glucocorticoid-inducible transcription system was used to produce transgenic Arabidopsis (Arabidopsis thaliana) plants for inducible overexpression of GmSARK, which led to early leaf senescence, chloroplast destruction, and abnormal flower morphology in Arabidopsis. Transcript analyses of the GmSARK-overexpressing seedlings revealed a multitude of changes in phytohormone synthesis and signaling, specifically the repression of cytokinin functions and the induction of auxin and ethylene pathways. Inhibition of either auxin action or ethylene biosynthesis alleviated the senescence induced by GmSARK. Consistently, mutation of either AUXIN RESISTANT1 or ETHYLENE INSENSITIVE2 completely reversed the GmSARK-induced senescence. We further identified a homolog of GmSARK with a similar expression pattern in Arabidopsis and named it AtSARK. Inducible overexpression of AtSARK caused precocious senescence and abnormal floral organ development nearly identical to the GmSARK-overexpressing plants, whereas a T-DNA insertion mutant of AtSARK showed significantly delayed senescence. A kinase assay on recombinant catalytic domains of GmSARK and AtSARK revealed that these two leucine-rich repeat-receptor-like protein kinases autophosphorylate on both serine/threonine and tyrosine residues. We inferred that the SARK-mediated pathway may be a widespread mechanism in regulating leaf senescence.

A protein phosphatase negatively regulates Arabidopsis leaf senescence through dephosphorylating a senescence-promoting receptor-like kinase.