Cold, salinity and drought stresses: an overview.

World wheat grain yields increased substantially in the 1960s and 1970s because farmers rapidly adopted the new varieties and cultivation methods of the so-called 'green revolution'. The new varieties are shorter, increase grain yield at the expense of straw biomass, and are more resistant to damage by wind and rain. These wheats are short because they respond abnormally to the plant growth hormone gibberellin. This reduced response to gibberellin is conferred by mutant dwarfing alleles at one of two Reduced height-1 (Rht-B1 and Rht-D1) loci. Here we show that Rht-B1/Rht-D1 and maize dwarf-8 (d8) are orthologues of the Arabidopsis Gibberellin Insensitive (GAI) gene. These genes encode proteins that resemble nuclear transcription factors and contain an SH2-like domain, indicating that phosphotyrosine may participate in gibberellin signalling. Six different orthologous dwarfing mutant alleles encode proteins that are altered in a conserved amino-terminal gibberellin signalling domain. Transgenic rice plants containing a mutant GAI allele give reduced responses to gibberellin and are dwarfed, indicating that mutant GAI orthologues could be used to increase yield in a wide range of crop species.

‘Green revolution’ genes encode mutant gibberellin response modulators

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms. It is now clear that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In some cases, the molecular mechanisms for fine-tuning the hormone levels are beginning to be uncovered. In this review, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss how GA concentrations in plant tissues are regulated during development and in response to environmental stimuli.

Gibberellin Metabolism and its Regulation

Gibberellins (GAs) are tetracyclic diterpenoid growth factors that are essential regulators of stem elongation and other plant developmental processes (Hooley 1994; Swain and Olszewski 1996). GA-related mutants have been identified in several plant species, including Arabidopsis (Ross 1994). GA-deficient Arabidopsis mutants display characteristic phenotypes, including dark green leaves and a dwarf growth habit attributable to reduced stem elongation (Koornneef and van der Veen 1980; Talon et al. 1990a; Sun and Kamiya 1994; Peng and Harberd 1997). gai is a semidominant mutation of Arabidopsis, which also confers a dark green, dwarf phenotype (Koornneef et al. 1985; Peng and Harberd 1993, 1997; Wilson and Somerville 1995). The gai mutation affects GA reception or subsequent signal transduction, and does not result in GA deficiency (Koornneef et al. 1985; Talon et al. 1990b; Wilson et al. 1992; Peng and Harberd 1993; Wilson and Somerville 1995).

Dominant mutations conferring visible phenotypes resembling those attributable to GA deficiency are also known in other plants, including maize (D8 allelic series; Harberd and Freeling 1989; Winkler and Freeling 1994) and wheat (Rht homeoallelic series; Gale et al. 1975). Previous genetic and physiological analyses of gai, D8, and Rht indicate that all are gain-of-function mutations (Gale et al. 1975; Harberd and Freeling 1989; Peng and Harberd 1993; Winkler and Freeling 1994; Wilson and Somerville 1995) conferring reduced GA responses and increased endogenous GA levels (Lenton et al. 1987; Fujioka et al. 1988; Talon et al. 1990b). The increased endogenous GA levels found in gai, D8, and Rht mutants are likely to arise through perturbation of the feedback control mechanisms by which GAs regulate in planta GA levels negatively (Croker et al. 1990; Chiang et al. 1995; Phillips et al. 1995; Xu et al. 1995). These dominant GA-response mutations are of considerable agricultural significance. The Rht mutations are especially important because they are the genetic basis of the high-yielding, semi-dwarf wheat varieties of the “green revolution” (Gale and Youssefian 1985). We cloned GAI to enhance our understanding of the mechanisms of GA signal transduction, and because of the potential use for gai in crop improvement.

Previous experiments had identified a T-DNA insertion, genetically linked to GAI, which contained a Ds transposable element (Peng and Harberd 1993). This Ds was used to clone GAI through targeted insertional mutagenesis. Comparison of GAI and gai DNA sequences shows that the predicted mutant protein (gai) lacks a short (17-amino-acid) segment of the GAI protein sequence. We propose that this structural alteration is responsible for the dominant, gain-of-function properties of gai. In addition, presumed null alleles of GAI confer increased resistance to the growth-retarding effects of paclobutrazol (PAC), an inhibitor of GA biosynthesis. These observations suggest the following hypotheses to explain the role of GAI in GA signaling. First, GAI is proposed to be a negative regulator that represses GA responses but whose activity is opposed by GA. Second, gai is proposed to be a mutant repressor that is relatively resistant to the effects of GA and, therefore, maintains repression irrespective of the presence of GA.

Several recent publications have described extragenic mutations that suppress the phenotype conferred by gai (Carol et al. 1995; Wilson and Somerville 1995; Jacobsen et al. 1996) or by GA deficiency mutations (Jacobsen and Olszewski 1993; Silverstone et al. 1997). Here we extend the analysis of the phenotypes conferred by two of these suppressors (spy-7 and gar2-1). First, we compare the effects of spy-7 and gar2-1 (alone and in combination) on the growth of and PAC resistance of plants containing gai. We have also investigated the effects of spy-7 and gar2-1 on the regulation of GA biosynthesis, by comparing the steady-state levels of gene transcripts encoding GA C-20 oxidase, the enzyme that catalyzes the penultimate step in the synthesis of biologically active GAs (Phillips et al. 1995; Xu et al. 1995). Finally, we have investigated the effects of spy-7 and gar2-1 on steady-state levels of gai transcripts.

The results of the above experiments indicate that GAI, SPY, and GAR2 operate within, or modulate, a signal-transduction pathway that represses growth and whose activity is opposed by GA. Because of the existence of mutations having comparable effects to gai and spy in other plant species (Swain and Olszewski 1996), and because GA is an essential growth regulator in a wide variety of plant species (Hooley 1994), it seems likely that the Arabidopsis GAI, SPY, and GAR2 genes define a system for GA-mediated growth regulation that is common to all higher plants.

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The Arabidopsis GAI gene defines a signaling pathway that negatively regulates gibberellin responses

The power of molecular genetics has dramatically advanced our understanding of all aspects of gibberellin (GA) signaling. Many genes encoding GA response pathway components have been identified using Arabidopsis and cereal mutants, and more elaborate genetic screens are producing additional mutants

/pdf/gibberellin-signaling-biosynthesis-catabolism-and-response-4ze6agdblf.pdf

Gibberellin signaling: biosynthesis, catabolism, and response pathways.

https://www.pnas.org/content/pnas/42/4/185.full.pdf

Growth response of single-gene dwarf mutants in maize to gibberellic acid.

Gibberellins A12 (GA12), GA53, GA44, GA19, GA17, GA20, GA29, GA1, and GA8 have been identified from extracts of vegetative shoots of normal (wild type) maize using full scan capillary gas chromatography-mass spectrometry and Kovats retention indices. Seven of these gibberellins (GAs) have been quantified by capillary gas chromatography-selected ion monitoring using internal standards of [14C4]GA53, [14C4]GA44, [2H2] GA19, [13C1]GA20, [13C1]GA29, [13C1]GA1, and [13C1]GA8. Quantitative data from extracts of normal, dwarf-1, dwarf-2, dwarf-3, and dwarf-5 seedlings support the operation of the early 13-hydroxylation pathway in vegetative shoots of Zea mays. These data support the positions in the pathway blocked by the mutants, previously assigned by bioassay data and metabolic studies. The GA levels in dwarf-2, dwarf-3, and dwarf-5 were equal to, or less than, 2.0 nanograms per 100 grams fresh weight, showing that these mutants are blocked for steps early in the pathway. In dwarf-1, the level of GA1 was very low (0.23 nanograms per 100 grams fresh weight) and less than 2% of that in normal shoots, while GA20 and GA29 accumulated to levels over 10 times those in normals; these results confirm that the dwarf-1 mutant blocks the conversion of GA20 to GA1. Since the level of GAs beyond the blocked step for each mutant is greater than zero, each mutated gene probably codes for an altered gene product, thus leading to impaired enzyme activities.

Qualitative and Quantitative Analyses of Gibberellins in Vegetative Shoots of Normal, dwarf-1, dwarf-2, dwarf-3, and dwarf-5 Seedlings of Zea mays L.

Studies on the biosynthesis of gibberellins from (−)-kaurenoic acid in cultures of Gibberella Fujikuroi

The endogenous gibberellins (GAs) were examined from young vegetative shoots of the dominant mutant, Dwarf-8, a GA-nonresponder, and normal maize; GA44, GA17, GA19, GA20, GA29, GA1, and GA8, members of the early-13-hydroxylation pathway, were identified from both kinds of shoots by full-scan mass spectra and Kovats retention indices. In addition, we report the identification of 3-epi-GA1, GA3, GA4, GA5, GA7, GA9, GA12, GA15, GA24, GA34, and GA53 by using the same criteria. [1,7,12,18-14C4]GA53 and -GA44, [17-2H2]GA19, and [17-13C,3H2]GA20, -GA29, -GA1, -GA8, and -GA5 were used as internal standards to determine the endogenous levels of these GAs by measurement of isotope dilution, using capillary gas chromatography and selected ion monitoring. Shoots of Dwarf-8 accumulate relatively high levels of GA20, GA1, and GA8. The accumulation of GA1 appears to be related to gene dosage. Since Dwarf-8 contains the same pattern of GAs as normals (including GA1 and GA3), the genetic control point probably lies after GA1 (and GA3). Thus Dwarf-8 may be a GA receptor mutant or a mutant that controls a product downstream from the binding of the bioactive GA to a receptor.

https://www.pnas.org/content/pnas/85/23/9031.full.pdf

The dominant non-gibberellin-responding dwarf mutant (D8) of maize accumulates native gibberellins.

Gibberellins are compounds that produce a wide variety of growth responses in flowering plants. Among these responses are stimulation of shoot growth for many plant species,' root elongation in maize,2 induction of bolting and flowering in some biennial and long-day plants,3-resumption of normal growth in certain genetically dwarfed genotypes of maize,6 growth stimulation in dwarf strains of peas and beans,7 and the overcoming of physiological dwarfing of apple seedlings.8 Gibberellins also stimulate parthenocarpy in tomatoes,9 promote the germination of light-requiring lettuce seed in the dark,'0 and reverse the light inhibition of pea stem growth.\"I For the interpretation of these and other effects, it is important to know whether gibberellins or \"gibberellin-like\" substances occur naturally in flowering plants. The three known gibberellins, gibberellic acid (gibberellin A3), Cj9H2206; gibberellin A (gibberellin Al), C19H2406; and gibberellin A2, C19H2606, are products of certain strains of the imperfect stage of the fungus Gibberella fujikuroi (Saw.) Wr.12-B6 No other organism has been found to contain these compounds. The present paper will report evidence for the occurrence of \"gibberellin-like\" substances from species of several different families of flowering plants.'7 These substances produce the same growth response in maize dwarfs as do the known gibberellins. RJ values for these substances will also be reported and compared with those for the known gibberellins.

https://www.pnas.org/content/pnas/43/5/398.full.pdf

Bernard O. Phinney

Papers

Growth response of single-gene dwarf mutants in maize to gibberellic acid.

Qualitative and Quantitative Analyses of Gibberellins in Vegetative Shoots of Normal, dwarf-1, dwarf-2, dwarf-3, and dwarf-5 Seedlings of Zea mays L.

Studies on the biosynthesis of gibberellins from (−)-kaurenoic acid in cultures of Gibberella Fujikuroi

The dominant non-gibberellin-responding dwarf mutant (D8) of maize accumulates native gibberellins.

Evidence for “gibberellin-like” substances from flowering plants