ABI3 controls embryo degreening through Mendel'sIlocus

Abstract

Chlorophyll (chl) is essential for light capture and is the starting point that provides the energy for photosynthesis and thus plant growth. Obviously, for this reason, retention of the green chlorophyll pigment is considered a desirable crop trait. However, the presence of chlorophyll in mature seeds can be an undesirable trait that can affect seed maturation, seed oil quality, and meal quality. Occurrence of mature green seeds in oil crops such as canola and soybean due to unfavorable weather conditions during seed maturity is known to cause severe losses in revenue. One recently identified candidate that controls the chlorophyll degradation machinery is the stay-green gene, SGR1 that was mapped to Mendel's I locus responsible for cotyledon color (yellow versus green) in peas. A defect in SGR1 leads to leaf stay-green phenotypes in Arabidopsis and rice, but the role of SGR1 in seed degreening and the signaling machinery that converges on SGR1 have remained elusive. To decipher the gene regulatory network that controls degreening in Arabidopsis, we have used an embryo staygreen mutant to demonstrate that embryo degreening is achieved by the SGR family and that this whole process is regulated by the phytohormone abscisic acid (ABA) through ABSCISIC ACID INSEN-SITIVE 3 (ABI3); a B3 domain transcription factor that has a highly conserved and essential role in seed maturation, conferring desiccation tolerance. Misexpression of ABI3 was sufficient to rescue cold-induced green seed phenotype in Arabidopsis. This finding reveals a mechanistic role for ABI3 during seed degreening and thus targeting of this pathway could provide a solution to the green seed problem in various oil-seed crops.freezing tolerance | nondormant T he success of angiosperms impinges on their ability to desiccate and protect their embryos in a dormant state until favorable conditions are perceived. In many angiosperms and oilseed plants such as Arabidopsis and canola, this desiccation process during seed maturation is intricately coupled to loss of chlorophyll (chl) from photosynthetically active embryos (1). During the embryo maturation phase, as the embryos begin to lose their chlorophyll, they concomitantly initiate the process of acquisition of desiccation tolerance and dormancy, thereby producing mature, brown (degreened) and dormant seeds. The persistence of chlorophyll in mature seeds has negative impacts on seed storability in many commercial plant species such as canola, cabbage, carrot, geranium, and soybean (2-4). Apart from contributing to reduced storability, prevalence of green seeds in mature oil seeds (canola and soybean) is also associated with reduced shelf life of oil and production of unfavorable odors and flavors. Particularly, in canola, which is one of the major global cash crops, the frost-induced green seed problem has been estimated to result in an annual loss of $150 million in revenue in North America alone (5, 6).During seed development, abscisic acid (ABA) is known to control mid to late stages of embryo maturation...

The Arabidopsis Polycomb Repressive Complex 1 (PRC1) Components AtBMI1A, B, and C Impact Gene Networks throughout All Stages of Plant Development

“…This is consistent with the developmental fate of the mutants, which are trapped in the seed maturation phase (Yang et al, 2013). During this phase, seeds acquire desiccation tolerance and accumulate storage reserves, prevailing in the form of lipids (VicenteCarbajosa and Carbonero, 2005), while chloroplast structure is disrupted (Delmas et al, 2013).…”

Different Mechanisms Are Responsible for Chlorophyll Dephytylation during Fruit Ripening and Leaf Senescence in Tomato

“…Surprisingly, however, the absence of PPH, the two CLHs, or all three genes in a triple mutant does not affect seed tocopherol content in Arabidopsis (Zhang et al, 2014), pointing to a different phytol hydrolytic activity. Furthermore, triple pph-1 clh1 clh2 mutants do not show an embryo stay-green phenotype (Zhang et al, 2014), contrary to mutants deficient in SGR or NYC1 (Nakajima et al, 2012;Delmas et al, 2013). Thus, it appears that SGR and some CCEs, such as NYC1 and PAO, are commonly active during chlorophyll degradation in different plant tissues, while PPH is active in leaf senescence but plays only a minor role during fruit ripening and seed development.…”

“…Chlorophyll breakdown, however, not only occurs during leaf senescence but also, for example, during leaf desiccation in resurrection plants (Craterostigma pumilum and Xerophyta viscosa), during fruit ripening and seed maturation (Armstead et al, 2007;Delmas et al, 2013;. Analysis of the dephytylation step in ripening fruits has been limited nearly exclusively to Citrus spp.…”

Regulatory network analysis reveals novel regulators of seed desiccation tolerance in Arabidopsis thaliana

“…Although some aspects of their function are specific, namely chlorophyll breakdown controlled by ABI3 (33) and production of somatic embryos controlled by LEC1 and LEC2 (12,16), their role in seed maturation is globally similar. A genetic analysis of single and double mutants suggests that these genes integrate an important regulatory network with overlapping functions, which could be explained by synergism or redundancy (19).…”