Gibberellins (GAs) are the second group of plant hormones to be characterised, about 30 years of auxin discovery. They were discovered by the Japanese scientists in the 1930s who were studying what they referred to as 'bakanae' or foolish disease in rice, where rice plants grew to be very tall without seed production. They discovered that this abnormal growth of rice was due to the chemicals secreted by the pathogenic fungi Gibberella fujikuroi, and they named it gibberellin A( GA). Scientists in the USA and in Britain isolated and purified a chemical from Gibberella culture and they elucidated its structure. Since then about 136 GAs have been identified and they all share a similar structure although they are not all biologically active. They are named using number as a subscript following a system initiated by Tokyo scientists who were able to isolate to isolate three different gibberellins from the initial sample that they referred to as gibberellins A, thus called them GA1,GA2 and GA3.
Characterisation and Bioactivities
Physiologists then continued to further characters GAs by determining their biological activities. They noticed that application of GAs to different plants resulted in dwarf plants growing taller. They then moved on to see if plants produced GAs in nature and indeed they discovered that most plants did and higher concentrations of GAs were found in immature seeds. The researchers discovered that gibberellins stimulate stem growth, they regulate the transition from juvenile to adult phases, they influence floral initiation and sex determination, promote pollen development ant tube growth, promote the setting of fruit and parthenocapy and they promote seed development and germination.
Commercially GAs are used to promote the growth of fruit crops. In most agricultural crops especially in cereal crops, it is ideal to get dwarf plants as the energy used to make the plant tall goes towards cereal production thus high yields. This led to the discovery of transcriptional regulators of GAs. But plants that responded negatively to gibberellins, like no change of phenotype even after being treated by gibberellins, took the interest of researchers. These plants did not respond to GA when it is applied. Different GA repressors which are the key to the loss of function or gain of function responses to GAs have since been discovered and some of these are GA insensitive (GAI), repressors of GA (RGA), repressors of GA insensitive plants (GAI-3) and slender rice (SLY).These transcription factors that repressed the activity of GAs are referred to as DELLA proteins.
DELLA proteins are GA activity repressors, named so after the first five amino acids at the N-terminal which are aspartic acid (D), glutamic acid (E), leucine (L), leucine (L) and alanine (A). DELLA proteins are negative regulators of gibberellins, as they repress signalling of GAs, thus they repress plant growth resulting in dwarf plants, which is ideal in cereal production in agriculture. There however are plants which are mutants which lack the GAI and RGA repressors. These plants show a phenotype just like that of the wild type even in the absence of GAs. GAs then act to oppose the function of DELLA as on applying GA ,DELLA protein get degraded by ubiquitination and proteolytic degradation.
Thus if enhanced plant growth such as long plants is an ideal situation , genetic engineers can the manipulate the genetic makeup of the plants are DELLA mutant whereas if dwarf plants are needed to increase yield, then genes that activates over-express the DELLA proteins are cloned into the plants of interest to make them dwarf and thus not being able to respond to GA, or GA activity repressing genes can be cloned into the plant whereby farmers will be able to reverse the dwarf phenotype of the plants if the need for tall plants arises. So far different DELLA proteins and DELLA protein repressors have been identified in different plants. Research has also shown that DELLA proteins do not only act on GA but on other enzyme as well which also in turn acts on DELLA proteins e.g. it has been shown that auxin and ethylene activates the degradation of the DELLA proteins. Thus further research is still being carried out to characterise the DELLA proteins further
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