Carotenoid Pigment Accumulation in Horticultural Plants

Carotenoids are a group of widely distributed natural pigments.They give many horticultural plants the bright red,orange,and yellow colors,as well as the aroma and flavor.Carotenoids enhance the health value and represent an essential quality trait of horticultural products.Significant efforts have been made to correlate specific carotenoid production with pathway gene expression.Some transcription factors that directly regulate transcription of the pathway genes have been identified.Horticultural crops have evolved with complicated and multifaceted regulatory mechanisms to generate the enormous diversity in carotenoid content and composition.However,the diverse and complex control of carotenoid accumulation is still not well understood.In this review,we depict carotenoid accumulation pathways and highlight the recent progress in the regulatory control of carotenoid accumulation in horticultural plants.Because of the critical roles of chromoplasts for carotenoid hyperproduction,we evaluate chromoplast ultrastructures and carotenoid sequestrations.A perspective on carotenoid research in horticultural crops is provided.

Recent progress on mechanisms that allocate cellular space to plastids

Mechanisms that allocate cellular space to organelles are of fundamental importance to biology but remain poorly understood.A detailed understanding of mechanisms that allocate cellular space to plastids,such as chloroplasts,will lead to high-yielding crops with enhanced nutritional value.The HIGH PIGMENT(HP)genes in tomato contribute to regulated proteolysis and abscisic acid metabolism.The HP1 gene was the first gene reported to influence the amount of cellular space occupied by chloroplasts and chromoplasts almost 20 years ago.Recently,our knowledge of mechanisms that allocate cellular space to plastids was enhanced by new information on the influence of cell type on the amount of cellular space occupied by plastids and the identification of new genes that help to allocate cellular space to plastids.These genes encode proteins with unknown and diverse biochemical functions.Several transcription factors were recently reported to regulate the numbers and sizes of chloroplasts in fleshy fruit.If these transcription factors do not induce compensating effects on cell size,they should affect the amount of cellular space occupied by plastids.Although we can now propose more detailed models for the network that allocates cellular space to plastids,many gaps remain in our knowledge of this network and the genes targeted by this network.Nonetheless,these recent breakthroughs provide optimism for future progress in this field.

The Or Gene Enhances Carotenoid Accumulation and Stability During Post-Harvest Storage of Potato Tubers

Provitamin A carotenoids in staple crops are not very stable during storage and their loss compromises nutritional quality. To elucidate the fundamental mechanisms underlying carotenoid accumulation and stability, we investigated transgenic potato tubers that expressed the cauliflower Orange （Or） gene. We found that the Or transgene not only promoted retention of 13-carotene level, but also continuously stimulated its accumulation during 5 months of cold storage. In contrast, no increased levels of carotenoids were observed in the tubers of vector-only controls or a yellow- flesh variety during the same period of storage. The increased carotenoid accumulation was found to be associated with the formation of lipoprotein-carotenoid sequestering structures, as well as with the enhanced abundance of phytoene synthase, a key enzyme in the carotenoid biosynthetic pathway. Furthermore, the provitamin A carotenoids stored were shown to be stable during simulated digestion and accessible for uptake by human intestinal absorptive cells. Proteomic analysis identified three major functional groups of proteins （i.e. heat shock proteins, glutathione-S-transferases, and carbohydrate metabolic proteins） that are potentially important in the Or-regulated carotenoid accumulation. Our results show that regulation of carotenoid sequestration capacity is an important mechanism by which carotenoid stability is regulated. Our findings suggest that induction of a proper sink structure formation in staple crops may provide the crops with a unique ability to promote and/or stabilize provitamin A accumulation during plant growth and post-harvest storage.

OR^His, a Natural Variant of OR, Specifically Interacts with Plastid Division Factor ARC3 to Regulate Chromoplast Number and Carotenoid Accumulation

Chromoplasts are colored plastids that synthesize and store massive amounts of carotenoids.Chromoplast number and size define the sink strength for carotenoid accumulation in plants.However,nothing is known about the mechanisms controlling chromoplast number.Previously,a natural allele of Orange(OR),OR^His,was found to promote carotenoid accumulation by activating chromoplast differentiation and increasing carotenoid biosynthesis,but cells in orange tissues in melon fruit and cauliflower OR mutant have only one or two enlarged chromoplasts.In this study,we investigated an OR^His variant of Arabidopsis OR,genetically mimicking the melon OR^His allele,and found that it also constrains chromoplast number in Arabidopsis calli.Both in vitro and in vivo experiments demonstrate that OR^His specifically interacts with the Membrane Occupation and Recognition Nexus domain of ACCUMULATION AND REPLICATION OF CHLOROPLASTS 3(ARC3),a crucial regulator of chloroplast division.We further showed that OR^His interferes with the interaction between ARC3 and PARALOG OF ARC6(PARC6),another key regulator of chloroplast division,suggesting a role of OR^His in competing with PARC6 for binding to ARC3 to restrict chromoplast number.Overexpression or knockout of ARC3 in Arabidopsis OR^His plants significantly alters total carotenoid levels.Moreover,overexpression of the plastid division factor PLASTID DIVISION 1 greatly enhances carotenoid accumulation.These division factors likely alter carotenoid levels via their influence on chromoplast number and/or size.Taken together,our findings provide novel mechanistic insights into the machinery controlling chromoplast number and highlight a potential new strategy for enhancing carotenoid accumulation and nutritional value in food crops.