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Summary: Ethylene plays a critical role in many diverse processes in plant development. Recent studies have demonstrated that overexpression of the maize ARGOS8 gene reduces the plant's response to ethylene by decreasing ethylene signaling and enhances grain yield in transgenic maize plants. The objective of this study was to determine the effects of ethylene on the development of nodal roots, which are primarily responsible for root‐lodging resistance in maize. Exogenous application of the ethylene precursor 1‐aminocyclopropane‐1‐carboxylic acid (ACC) was found to promote the emergence of nodal roots. Transcriptome analysis of nodal tissues revealed that the expression of genes involved in metabolic processes and cell wall biogenesis was upregulated in response to ACC treatment, supporting the notion that ethylene is a positive regulator for the outgrowth of young root primordia. In BSV::ARGOS8 transgenic plants with reduced ethylene sensitivity due to constitutive overexpression of ARGOS8, nodal root emergence was delayed and the promotional effect of ACC on nodal root emergence decreased. Field tests showed that the BSV::ARGOS8 plants had higher root lodging relative to non‐transgenic controls. When ARGOS8 expression was controlled by the developmentally regulated promoter FTM1, which conferred ARGOS8 overexpression in adult plants but not in the nodal roots and nodes in juvenile plants, the FTM1::ARGOS8 plants had no significant difference in root lodging compared with the wild type but produced a higher grain yield. These results suggest that ethylene has a role in promoting nodal root emergence and that a delay in nodal root development has a negative effect on root‐lodging resistance in maize. Significance Statement: Root‐lodging resistance is a critically important trait for high‐yield maize hybrids and has been selected in maize breeding programs over the past 100 years. In maize, the stem‐borne nodal roots are formed in the normal developmental process and play a major role in root‐lodging resistance. Using the ethylene precursor 1‐aminocyclopropane‐1‐carboxylic acid (ACC) and ARGOS8 transgenic plants which have reduced ethylene sensitivity, we found that ACC promotes nodal root emergence and that ARGOS8 transgenic plants have delayed nodal root development and increased root lodging. These results suggest a positive role for ethylene in root‐lodging resistance. These findings have applications in maize breeding for improved agronomic traits. [ABSTRACT FROM AUTHOR]

Ethylene influences many aspects of plant growth and development. The biosynthesis of ethylene is highly regulated by a variety of internal and external cues. A key target of this regulation is 1-aminocyclopropane-1-carboxylic acid ( ACC) synthases ( ACS), generally the rate-limiting step in ethylene biosynthesis, which is regulated both transcriptionally and post-transcriptionally. Prior studies have demonstrated that cytokinin and brassinosteroid ( BR) act as regulatory inputs to elevate ethylene biosynthesis by increasing the stability of ACS proteins. Here, we demonstrate that several additional phytohormones also regulate ACS protein turnover. Abscisic acid, auxin, gibberellic acid, methyl jasmonic acid, and salicylic acid differentially regulate the stability of ACS proteins, with distinct effects on various isoforms. In addition, we demonstrate that heterodimerization influences the stability of ACS proteins. Heterodimerization between ACS isoforms from distinct subclades results in increased stability of the shorter-lived partner. Together, our study provides a comprehensive understanding of the roles of various phytohormones on ACS protein stability, which brings new insights into crosstalk between ethylene and other phytohormones, and a novel regulatory mechanism that controls ACS protein stability through a heterodimerization of ACS isoforms. [ABSTRACT FROM AUTHOR]

Flowering plants utilize different floral structures to develop flesh tissue in fruits. Here we show that suppression of the homeologous SEPALLATA1/2-like genes MADS8 and MADS9 in the fleshy fruit apple ( Malus x domestica) leads to sepaloid petals and greatly reduced fruit flesh. Immunolabelling of cell-wall epitopes and differential staining showed that the developing hypanthium (from which the apple flesh develops) of MADS8/9-suppressed apple flowers lacks a tissue layer, and the remaining flesh tissue of fully developed apples has considerably smaller cells. From these observations, it is proposed that MADS8 and MADS9 control the development of discrete zones within the hypanthium tissue, and therefore fruit flesh, and also act as foundations for development of different floral organs. At fruit maturity, the MADS8/9-suppressed apples do not ripen in terms of both developmentally controlled ripening characters, such as starch degradation, and ethylene-modulated ripening traits. Transient assays suggest that, like the RIN gene in tomato, the MADS9 gene acts as a transcriptional activator of the ethylene biosynthesis enzyme, 1-aminocyclopropane-1-carboxylate (ACC) synthase 1. The existence of a single class of genes that regulate both flesh formation and ripening provides an evolutionary tool for controlling two critical aspects of fleshy fruit development. [ABSTRACT FROM AUTHOR]

E3 ubiquitin ligases select specific proteins for ubiquitin conjugation, and the modified proteins are commonly degraded through the 26S proteasome. XBAT32 is a RING-type E3 ligase involved in maintaining appropriate levels of ethylene. Previous work has suggested that XBAT32 modulates ethylene production by ubiquitinating two ethylene biosynthesis enzymes, ACS4 (type-II isoform) and ACS7 (type-III isoform). In Arabidopsis, conserved sequences within the C-terminal tail of type-I and -II 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) isoforms influence ubiquitin-dependent proteolysis. ACS7, the sole Arabidopsis type-III ACS, contains a truncated C-terminal tail that lacks all known regulatory sequences, which suggests that this isoform may not be subject to ubiquitin-mediated proteasomal degradation. Here we demonstrate in planta that ACS7 is turned over in a 26S proteasome-dependent manner and that degradation of ACS7 requires the E3 ligase XBAT32. Furthermore, the ethylene-related phenotypes that result from overexpression of ACS7 in wild-type plants are greatly exaggerated in xbat32-1, suggesting that XBAT32 is required to attenuate the effect of overexpression of ACS7. This observation is consistent with a role for XBAT32 in the ubiquitin-mediated degradation of ACS7. The dark-grown phenotype of xbat32-1 seedlings overexpressing ACS7 can be effectively rescued by aminoethoxyvinylglycine, an inhibitor of ACS activity. The degradation rate of ACS4 is also significantly slower in the absence of XBAT32, further implicating XBAT32 in the ubiquitin-mediated degradation of ACS4. Altogether, these results demonstrate that XBAT32 targets ethylene biosynthetic enzymes for proteasomal degradation to maintain appropriate levels of hormone production. [ABSTRACT FROM AUTHOR]