Regulated Expression of a Cytokinin Biosynthesis Gene IPT Delays Leaf Senescence and Improves Yield under Rainfed and Irrigated Conditions in Canola (Brassica napus L.)

Abstract

Delay of leaf senescence through genetic modification can potentially improve crop yield, through maintenance of photosynthetically active leaves for a longer period. Plant growth hormones such as cytokinin regulate and delay leaf senescence. Here, the structural gene (IPT) encoding the cytokinin biosynthetic enzyme isopentenyltransferase was fused to a functionally active fragment of the AtMYB32 promoter and was transformed into canola plants. Expression of the AtMYB32xs::IPT gene cassette delayed the leaf senescence in transgenic plants grown under controlled environment conditions and field experiments conducted for a single season at two geographic locations. The transgenic canola plants retained higher chlorophyll levels for an extended period and produced significantly higher seed yield with similar growth and phenology compared to wild type and null control plants under rainfed and irrigated treatments. The yield increase in transgenic plants was in the range of 16% to 23% and 7% to 16% under rainfed and irrigated conditions, respectively, compared to control plants. Most of the seed quality parameters in transgenic plants were similar, and with elevated oleic acid content in all transgenic lines and higher oil content and lower glucosinolate content in one specific transgenic line as compared to control plants. The results suggest that by delaying leaf senescence using the AtMYB32xs::IPT technology, productivity in crop plants can be improved under water stress and well-watered conditions.

Role and Regulation of Cytokinins in Plant Response to Drought Stress

“…CKs modulate plant growth through changes in plant morphology and metabolism [104,[121][122][123][124], acting as the indirect factors to limit damage caused by drought stress, hence increasing the chance of plant survival under water shortage conditions (Table 1) [39,44,[125][126][127][128][129][130]. Abundant evidence exists to indicate that CKs play important roles in root growth and development in different plant species.…”

“…alter transcriptional factor-encoding genes involved in stress signaling, oxidative protection and protein modification; enhance drought tolerance [125,126] IPT (Agrobacterium tumefaciens) inducible expression stress-and maturation-induced promoter (SARK) Rice (Oryza sativa) enhance sink strength; improve drought tolerance and increase grain yield [56] IPT (Agrobacterium tumefaciens) inducible expression stress-and maturation-induced promoter (SARK) Peanut (Arachis hypogaea) maintain higher photosynthetic rates, stomatal conductance and transpiration; improve drought tolerance and increase yield under field conditions. [38] IPT (Agrobacterium tumefaciens) overexpression modified developmentally regulated transcription factor AtMYB32 (AT4G34990) promoter from Arabidopsis Canola (Brassica napus) increase higher chlorophyll levels; delay leaf senescence; enhance yield under rain-fed and irrigated conditions [127] IPT (Agrobacterium tumefaciens) inducible expression stress-and maturation-induced promoter (SARK) Cotton (Gossypium hirsutum) delay senescence; enhance root and shoot biomass; maintain higher chlorophyll content and photosynthetic rates under water deficit conditions [109] IPT (Agrobacterium tumefaciens) inducible expression stress-and maturation-induced promoter (SARK) Rice (Oryza sativa) increase drought tolerance through the coordinated regulation of carbon and nitrogen assimilation [39] IPT (Agrobacterium tumefaciens) overexpression modified developmentally regulated transcription factor AtMYB32 (AT4G34990) promoter from Arabidopsis Wheat (Triticum aestivum) increase grain yield under water deficit [44] On the other hand, an enhanced root system with less cellular damage in transgenic plants compared with the wild-type plants under drought conditions could also be achieved through increasing CK levels by using an inducible promoter to drive the expression of IPT target gene [109,110]. Another defense mechanism that has been employed by this approach is the delay of drought-induced senescence [109,111,112].…”

The Senescence (Stay-Green)—An Important Trait to Exploit Crop Residuals for Bioenergy

“…SG trait is highly genetically regulated and dependent on expression of specific genes [14,33]. Senescence in maize was firstly described as a single dominant locus [34] where the delay in senescence was dominant over senescence satisfying goodness of fit (3:1) ratio.…”

Flower-Specific Overproduction of Cytokinins Altered Flower Development and Sex Expression in the Perennial Woody Plant Jatropha curcas L.

“…We thus hypothesize that cytokinin has the potential to be a key driver of modifying flower traits in J. curcas through genetic manipulation. As in canola, the transgenic expression of ipt under the control of the AtMYB32 promoter produces more flowers and increases the seed yield [42].In this study, to investigate the effect of endogenous cytokinins on flower development in J. curcas, we elevated the levels of cytokinins specifically in flowers through the transgenic expression of AtIPT4 under the control of a J. curcas orthologue TOMATO MADS BOX GENE 6 (JcTM6) promoter, which is predominantly active in J. curcas flowers. The increase in endogenous cytokinins in transgenic J. curcas resulted in the production of a notably higher number of flowers with an increased proportion of bisexual flowers.…”

“…We thus hypothesize that cytokinin has the potential to be a key driver of modifying flower traits in J. curcas through genetic manipulation. As in canola, the transgenic expression of ipt under the control of the AtMYB32 promoter produces more flowers and increases the seed yield [42].…”