Translocation in Legumes: Assimilates, Nutrients, and Signaling Molecules1

Translocation or long distance transport in plants is achieved by a vascular network that connects and is an integral part of all organs. The vasculature comprises two distinctly different and separate cellular translocation pathways: xylem and phloem. The principal xylem pathway is the transpiration stream that moves nutrients and water taken up by roots to the shoot. This stream also bears products of root metabolism and solutes that reflect features of the internal and external root environment. Phloem provides the means for redistributing xylem-delivered solutes to weakly transpiring organs, but most significantly phloem distributes the carbon assimilated by photosynthesis (principally as Suc) to heterotrophic organs like roots, vegetative and reproductive apices, flowers, fruits, and developing seeds. Together these two translocation streams provide all the nutrients and assimilates, in appropriate forms and proportions, to enable growth and development in an ordered and regulated fashion. Because translocation connects distant components of the plant body, xylem and phloem have long been considered to fulfill a role in communicating between organs, through the movement of plant hormones and other signaling molecules. Such signals are envisaged to move with assimilates by mass flow. However, phloem also transmits pressure/concentration (turgor) information at rates greatly in excess of mass flow of solutes (Thompson and Holbrook, 2004) and long distance electrical signaling is also thought to be directionally propagated via vascular bundles (Brenner et al., 2006). These action potential or osmotic signals may prove to have a significant regulatory role in terms of phloem function but are outside the scope of this article. Most recently our understanding of the functional significance of phloem has been extended with the realization that it also provides a conduit for trafficking macromolecules (nucleic acids and proteins), some of which may regulate gene expression as a consequence of their translocation (Banerjee et al., 2006; Lough and Lucas, 2006; Jones-Rhoades et al., 2006). Similarly root-derived signals that are postulated to regulate shoot processes are believed to move in xylem (Beveridge, 2006; Kinkema et al., 2006) together with a suite of secreted proteins (Buhtz et al., 2004). While the supporting evidence for these diverse roles of translocation has been gathered from many species, this article will highlight information that is specific to legumes where it is available, drawing particularly on data from the author's laboratory for members of the genus Lupinus. Much of our knowledge of what is found in and translocated by phloem comes from analyses of sieve tube (ST) exudate and in xylem by analyses of sap, displaced from the vasculature by applying either increased or decreased pressure. A range of techniques has been developed and exploited to sample transport fluids and it is appropriate to consider the likely limitations that collection methods impose on interpretation of the compositional data they have generated. The gold standard for phloem has been considered to be analyses of exudate collected from the detached stylets of sap sucking insects such as aphids. Aphid stylectomy has been applied mostly to woody and herbaceous dicotyledon species (Peel, 1975) but also to wheat (Triticum aestivum; Fisher et al., 1992) and using brown leaf hoppers to rice (Oryza sativa; Aoki et al., 2005). While stylet exudate may be regarded as ideal and least subject to artifact, stylets can be extremely variable in both rate and extent of exudation, and, even though the insect body is removed, salivary secretions are likely to persist as contaminants (Miles, 1999). The most common method has been to collect exudate from incisions in the bark of woody plants with compositional data for more than 500 species from 100 dicotyledonous families collated by Zimmermann and Ziegler (1975). Among herbaceous species there are very few that exude freely from severed vasculature, most occlude rapidly after damage preventing loss of phloem contents. The rapid blocking of ST has been interpreted as a phloem defense mechanism triggered by the release of Ca2+ causing constriction of sieve pores with extracellular callose (a 1,3-β-glucan polymer) and their plugging with coagulated structural phloem proteins (Will and van Bel, 2006). Interestingly, ST in legumes contains a unique protein crystalline body, the forisome, which disperses in response to damage or disturbance of phloem turgor to physically block the sieve plates, again in response to an increase in intracellular Ca2+ levels (Knoblauch et al., 2001). Known exceptions to rapid wound response are the hemophiliacs of the plant world. They include a number of cucurbits (cucumber [Cucumis sativus], pumpkin [Cucurbita pepo]), castor bean (Ricinus communis), Yucca flaccida, the axes of some palms, Brassica napus, and members of the genus Lupinus. Reasons for the sluggish response to vascular damage in these species are not known. Vascular contents have also been collected from nonspontaneous phloem bleeders by bathing wounds in a solution containing chelators (e.g. EDTA) to preclude rapid Ca2+-induced occlusion (King and Zeevart, 1974; Terce-Laforgue et al., 2004) or following rapid freezing and thawing that apparently also retards normal wound response (Pate et al., 1984). Pate (1976) outlined reasons why exudate collected from an incision may not accurately represent the solute composition moving in intact phloem and these considerations remain important in interpreting compositional data in relation to translocation. Even though an aphid or leaf hopper stylet may enter and draw initially on a single ST, each microliter of exudate is equivalent to the lumenal volume of about 2,500 STs (Dixon, 1975). On this basis in lupin (Lupinus albus), where 50 μL of sap containing approximately 10% Suc is readily collected within minutes from an incision in the bundle of vascular elements at the stylar tip or along the sutures of a fruit, the exudate could derive from more than 105 ST, extending some considerable distance beyond the wound. Initial piercing of a ST would cause a rapid fall in turgor pressure, decreasing local water potential and resulting in both longitudinal and lateral fluxes of water, diluting the exudate (Peel, 1975). It is reasonable to expect that the impact of a wound could potentially increase solute fluxes from closely associated companion cells (CCs) and phloem parenchyma. Pressure release on piercing may also result in exudate containing structural components of the STs that are not normally translocated. Because the exudate from a wound is typically high in and other the contents of cells from both vascular and are likely to be along with water and solutes from the this the of exudate from a vascular incision is typically to While this may the impact from analyses of solutes from collected of exudate that there is in these (Pate et al., 1974; and 1975). The of of phloem exudate has been by a number of et the protein of phloem exudate with that of and found that there very in between the two have for proteins or that would be in but not phloem. Most is or the for elements not but van Bel, and these that there is from outside cells et al., et al., et al., et al., 2006). contents are collected as sap by or from and from root root pressure, or by applying external in a following of the shoot. or to the of and it is reasonable to expect that water and solutes in the will be in the collected While or applied are not from the or in the xylem but not normally translocated are likely to be The contents of cells at the severed phloem be to xylem While these considerations will not have a significant impact on the and of the solutes in phloem and xylem they are likely to be important for and on the macromolecules that have been found in or and for which long distance signaling roles have been on their translocation. the in phloem exudate move a an important to prove translocation. The principal assimilates translocated from of to of their in growth and development are of carbon and of legumes Suc is the and and the and both in xylem and phloem some species the and or may particularly in as translocated products of in root legumes also unique solutes as result of the with and that can plant These include very high levels of et al., a of and and to be is a body of data that to translocation from the of ST collected from woody species through the that in the and to a of to of translocation expression and of specific for assimilates have the basis from which transport in phloem and and of translocation have been developed et al., van Bel, have an ideal in this particularly for of translocation of and to et al., et al., to the solutes in of from lupin species has an range of 1999). These include all protein acids as as and plant growth and many other solutes to be the of for each of these and all are normally translocated or are in as a consequence of collection is not The are on the data for lupin plants a after in growth from et The of carbon and and and are in the of as in each The of carbon and from of phloem collected from in each of of and each from the in each at the of the the lateral from the and from the in the of exudate to root collected from the root following of the shoot and from each of the as sap by of and the in carbon and in to the fluxes of these and of The of carbon or from xylem to from xylem to and from phloem to xylem as processes for the of the of These processes provide information as to the where of and regulate the extent of and where processes are likely to in the of and to some extent the movement and of that protein and the of in ST exudate and 1975) the and functional significance of these phloem proteins have for considerable and of protein may be as high as to in exudate from species and has to et al., 2004). levels in exudate collected by stylectomy in rice are but more than 100 et al., et recently proteins in phloem. of lupin phloem exudate also a number of proteins and and The most more than species in the phloem of and Lucas, 2006). ST are and in protein the proteins in sieve elements are most likely products of gene expression in and phloem through the normal of proteins through this may be as a consequence of and the range of in from a number of species and may be in part an these considerations that protein and composition that the impact of is et al., The of at either or levels of have to of proteins with and this has also been the for phloem et al., 2004). that have exploited expression of proteins as for the of in or plants et al., have also with the vasculature, of the many has been to phloem and the that such proteins be translocated. et a of a number of proteins a that the for between the and the ST and that and proteins in the range to and translocated in phloem. However, and of these proteins in root to a to and beyond this The data a of approximately to and for proteins the root these is not but they represent a reasonable many of the proteins in the phloem on the basis of of are to be beyond the vasculature in even they are translocated. is that proteins are in phloem and that there may be translocation. The of Aoki et in which two pumpkin phloem proteins and the vasculature of intact rice plants through severed leaf hopper stylets provide evidence for in protein translocation. of of the with other phloem proteins to regulate translocation of this protein to the roots et al., 2005). considerable about which proteins in in long distance transport and their is a resulting from their to The likely limitations and by and in their of macromolecules in phloem remain to be The about the functional significance of protein translocation is on likely of data from mass exudate a and apparently functional defense has been et al., and a more on of by has proteins with the to and defense It likely that these proteins provide the vasculature with to the of and but it is also that to a range of are as a consequence of the that is in in collected from insect stylets it be that the proteins secreted to ST in the of include defense that may also be translocated long (Will and van Bel, 2006). A number of proteins have been in phloem that and there is evidence for translocation of et al., 2005). the likely roles for translocated in gene expression this of proteins may provide the means to and signals long distance such as have been that may as to other proteins or the ST to proteins as they through the ST et al., et that the protein composition found in xylem sap two species of Brassica and two The 50 to 100 and following data that and a number of While phloem found to be the likely of of the proteins or to xylem is could be found for all the that with of these proteins to the root A suite of but also found in xylem sap from and in a proteins et al., of xylem exudate from has many of which to found in and with While all these that xylem proteins are in or their long distance movement in the transpiration stream and their as a consequence of are to be provide an to two long distance of development and root development is there is an of signals between the roots and shoot to regulate the number of that This signaling also to be to of the not by which legumes regulate the in response to root where part of a root with and on the other that the A number of or in in in and in have been where this is et al., for and 2006). with some of these that the shoot A in the roots is translocated and in the shoot a that development of the have been and to a to from et al., et al., et al., et al., the of the have not been Kinkema et al., 2006; et al., or roots are roots with growth that are a of many species in the as a response to levels roots the to by the root for and of and which in the that is to or lupin this root response to and the species has been as a for plant to and the et al., et al., 2005). of roots in lupin is by a from the shoot et al., 2005). et have recently and phloem transport to be components in gene expression in lupin roots, that the to sample phloem streams could be exploited in the translocated The vasculature of the plant provides an ideal pathway for rapidly information from to together with assimilates and some in solute composition of to translocation there that the of xylem and phloem It is likely that some assimilates in regulatory but roles for the of other the plant growth and in phloem are The transpiration stream also provides a means for the rapid long distance movement of signals that in roots and in legumes also in and which are postulated to regulate in the shoot. to translocation of solutes in phloem as the of a a that of to plant at and has been the single most important exploited in of a species and by plant the significance of this the means by which it is regulated remain but some of signaling mechanism has been that in plants and are to provide information some between root and shoot development to be but in the of the for a to of information between is in the of the as role in Suc as an important signaling in plants et al., 2006). from that phloem may be regulated by Suc that of Suc in and et al., the plant to regulate at this The developed the of a Suc but this has to be et al., from the phloem in may as of development with the high of to Suc by to in in development in et al., 2005). the of Suc in the in development of cells on the et al., and the in the to of after of of Suc in the cells et al., et al., is a in and a signaling role has been in plants et al., as a and the et al., 2004). A signaling function has been for translocated et al., acids in phloem and xylem of lupin et al., Pate et al., and other While there is evidence for in plants that could also et al., 2004) evidence translocation of these with functional regulatory is to be the many analyses of that the of all plant growth the and their or in xylem a functional in relation to or as a consequence of their translocation has in most to be The data are and, in that in development or in may be in of these hormones in translocation the of such to processes is from of proteins in phloem has also a likely role for the vasculature in some in et al., and is also evidence for of expression of of the in phloem with et al., 2004). the most is movement of through xylem to of action in the water to function and the water of However, of of in lupin et al., using the sap and that to and water in the species found that of the roots in xylem in from the shoot following in the root with the in the shoot. the plant by applying a solution to roots the of from the shoot to the root with more than A et al., using increased of and equivalent in has that of phloem to xylem in roots and between xylem and phloem be to a more understanding of translocation. The significance of long distance transport of in plant development is from and 2006; et al., xylem and phloem but information on the forms and their is at a that the translocated forms are the of from lupin that the composition of forms in translocation is from et al., from to in both xylem and phloem to be to in reproductive development and a of which species in are also in xylem sap from and in developing are the et al., The of these in species is to be but it from that the between and the of two separate and the other 2006). lupin has also evidence for the of in phloem and xylem and their likely translocation the of and there some evidence that phloem collected from different on the plant a different of (Thompson et al., et al., However, there has been data as to which species are and in is is et al., roles for each of the species are to be that functional significance for that are translocated either in xylem or phloem be The and signaling role of is a significant of and gene in A role for is also a of plant development and a number have been in of important processes through (e.g. the et al., there has been evidence for their long distance translocation in phloem and action as a However, et has to the of in the and have in and 2006) with many more to be and 2006). The high is in vascular and in in phloem or phloem et al., the protein to the ST for long distance A of proteins in lupin phloem by and using extended by et to more than 100 proteins and These this to exudate collected a solution from and in the of the They data for in the mass range to of which specific to or increased in phloem from plants to data et al., has of the gene in in the translocated to the shoot However, there is that the protein may be and 2006) and this is by the of in phloem exudate of et al., 2006). the found by et are also to be is that they with the to the as a in thought to be in phloem of causing an response to in and other distant from the of Phloem by and more or specific expression of in phloem et al., has also in the of vascular cells and 2004). However, that is to the vasculature where it to a specific a number of release of and to be the long distance that wound response initially to the and 2006). The gene for the in a number of legumes has been as a to et al., et al., et al., et al., 2005). shoot and in as part of a protein that is by a et al., The of the to that the long distance signaling translocated from root to shoot may be a to that is in is that the of a number of species in phloem and the that some of these may function as specific of gene expression has up a and role for long distance translocation. The many that the have to in plants as a consequence of some translocated or may have the for the of that gene expression at a outside local long distance signals may have This of information will provide a means for the for development of a to plant growth and are a number of of that are translocated in phloem et al., et al., and some where translocation has been to result in in plant development et al., et al., et al., 2006). distance movement of the ST recently in of in in response to is part of a in plants of two and for of the in in that translocation of for other members of this may regulate other processes (Banerjee et al., 2006). Phloem of legumes has to be to which are an important role in gene in of and have been in phloem exudate et al., and These with a and the of their which initially through their role in of a part of the plants defense mechanism They to and of the is a is to a response in distant of in is also by et al., 2006). to these that there are two recently of that are after of a single by a and are by with the by the of et al., et al., et al., 2005). are as a result of of two and The to et al., and et al., 2006). Much of the evidence for translocation of to gene is on of gene from to and the of the in from the et al., et al., or expression in of the shoot to a and et al., of phloem exudate is not in the plants that provide in this However, et movement of between a and a supporting the that may the for gene However, and in the phloem of plants and their translocation through the not These and in which to that other or could also as signaling in long distance transport of gene 2005). of a protein in pumpkin phloem that to single the of translocation of in phloem. to movement of single not but movement of these in phloem not et al., 2004). from this is that they are these could not move between cells A protein of to also in phloem from along with a of et al., 2004). are to move to defense and it is that and have but this to be are important of plant development and to The of their are and they an important role in regulatory from to a in et al., are the of leaf where to of and to of the leaf et al., and 2004) and that development to expression of in and of the to development of and of as as the and are in response to and to the plant to is that is in of and the of a to regulate et al., et al., 2006; et al., 2006; et al., 2006). et in phloem and that a of in phloem of castor and have different in lupin phloem and of expression of in that the is the leaf and is to of action in cells on the of the leaf where it expression of and to et al., and 2004). the in phloem. of movement proteins that trafficking cause the of for a functional role in normal leaf development et al., 2004). have from phloem exudate of lupin and et also a that in expression of in response to and also this from lupin phloem exudate and However, there is evidence for translocation of these and a number of that their expression is or that they have where a from a in and there translocation of the the to a et al., and where action with containing a and with expression from the gene they in the cells in which they are et al., 2005). it is to that their in to could move to regulate gene expression in response to there is evidence for this the that the for lupin is not and is not likely to in the the genus a for that long distance translocation with plant growth and It could be particularly in long distance that to and root a number of lupin species are that of high protein and in this the information on translocation in this genus could as for other unique that to the transport fluids of xylem and phloem at a number of on both vegetative plants and plants reproductive development has potential to provide information about the that are regulated as consequence of translocation of solutes and both and Most phloem can be readily collected at both and (Pate et al., unique to that of of the genus can be et al., et al., and a number of species can be and et al., et al., et al., 2005). for which phloem most are but the root can be using et al., 2005). this species as a for translocation more information would be particularly which from information about the would and development of to in et al., The data that is through with other species, will also provide a to the information by of translocation in