IN THIS ISSUE

Of Abscission and Other Breakthroughs

One full moon during the latter half of the seventeenth century, so the story goes, a falling apple inspired Isaac Newton to describe gravity as a force proportional to the inverse square of the distance between two objects. Now it would be a pity to overanalyze the veracity of the inspiring event, and the historical and scientific significance of Newton's theses can hardly be overestimated. Nevertheless, in the interests of plant science, I would like to point out the active role that the apple tree played at that inspirational moment when Newton gazed at the moon and heard the acclaimed fruit fall. Gravity may indeed have brought the fruit to the ground, but it was a complex developmental process that actually caused the apple to abscise from the tree.

Abscission refers not only to the release of ripening fruits, but also to the shedding of entire organ systems during the course of normal development and in response to tissue damage (Bleecker and Patterson 1997 ). Many of the physiological bases of abscission have been described in recent years, and it is clear that the process involves a series of programmed events that culminate in altered cell morphologies and tissue rearrangement. The process is hormonally driven by ethylene, and auxin appears to contribute fine tuning (Sexton and Roberts 1982 ; Abeles et al. 1992 ). Ironically, one of the first plant hormones to be discovered, abscisic acid, proves not to be required for the process after which it was named.

The signal transduction mechanisms that are invoked during abscission have yet to be elucidated, but there is ample evidence that a number of cellulase genes are induced (del Campillo and Bennett 1996 ). The specific expression of catabolic enzymes had long been suspected to support abscission, inasmuch as the degradation of the middle lamellae that separate cells at the site of breakage had been implicated in earlier studies (see, e.g., Morre 1968 ). The induction of a number of other proteins has also been reported during abscission, which may reflect the broader developmental program to which abscission belongs. In particular, processes of senescence and pathogen resistance are coordinated with abscission (del Campillo and Lewis 1992 ; Eyal et al. 1993 ).

At the microscopic level, the site of abscission develops as a specialized zone of cells of varying morphologies and tissue derivation. Among the histological characteristics that accompany the onset of abscission is the accumulation of small, cytoplasmically dense cells that often cause the macroscopic appearance of a groove at the surface of the abscinding appendage. Concomitantly, cells at the proximal face of the fracture plane appear to expand, and the components of the cell walls break down at the abscission surface. The expanded, rounded cells that are subsequently left behind at the proximal face differentiate into peridermal scar tissue. The multiple cell types that contribute to the abscission fracture undoubtedly reflect the cell–cell interactions that generally support processes of differentiation, and so the study of abscission may provide insights into other complex developmental events.

The jointless mutant of tomato offers additional evidence that abscission is but one aspect of a larger developmental program. Not only do jointless plants fail to form abscission zones at pedicel midpoints as wild-type plants do, but they are also affected with respect to flowering. Specifically, whereas wild-type shoot axes terminate growth upon the formation of inflorescences (so that axillary growth alone can extend a given shoot), jointless primary shoot axes continue to grow vegetatively subsequent to inflorescence and fruit production.

The molecular biology of the jointless locus has not been worked out, but the wild-type function is clearly involved in the determinacy of the inflorescence meristem. A detailed understanding of Jointless is thus germane not only to abscission, but also to questions of developmental biology as they are determined within the shoot meristem. On a more practical note, an understanding of abscission and the manifestation of jointless characteristics is of agronomic interest, because the pleiotropic nature of the mutation results in the mechanical harvestability of stem-free, calyx-free tomatoes.

On pages 159–175 of this issue, Szymkowiak and Irish describe the cell biology that underlies the jointless-conferred phenotype. They also utilize the mutation to investigate how distinct tissue types within the shoot meristem communicate so as to elaborate the tomato pedicel abscission zone and produce determinate inflorescences. With neither sequence information pertaining to the jointless locus nor any molecular probe to follow its activity, the authors have combined wild-type and jointless tissues within a series of chimeric tomato plants in order to evaluate the developmental consequences. Their results very powerfully corroborate the evolving view of plant development according to which positional cues defined at the organismic level, and not genetic constitution alone, incisively direct specific cells along pathways of differentiation. At the same time, the authors demonstrate that certain routes of informational flow are not open between certain cells, so that the apposition between neighboring cells and tissues can act to preclude, as well as to promote, developmental instruction.

The construction of plant chimeras such as those studied by Szymkowiak and Irish, in which the genetic mosaicism persists into propagated shoots by virtue of chimeric meristematic tissue, has been known since the beginning of the present century (reviewed in Szymkowiak and Sussex 1996 ). The development of "periclinal" chimeras (meaning that the genetically distinct tissues are oriented in fully distinct layers that run longitudinally along the shoot axis) was in fact one of the first experimental verifications that the shoot apical meristem of angiosperms is functionally arranged into three (periclinal) layers. The L1, or outermost layer, gives rise to epidermal tissue; L2 is the source of a discrete layer of subepidermal tissue in the shoot; and L3, the innermost layer, gives rise to internal shoot tissue and supplies and replenishes the cells of the meristem corpus. Through judicious grafting, it is thus pos-sible to generate chimeric meristems in which L1, L2, and L3 will persistently develop into genotypically distinguishable tissues and organs.

The realization of the "judicious" planning that went into Szymkowiak and Irish's work represents something of a technical coup, and a consideration of their basic strategy is probably necessary in order to appreciate their ultimate conclusions. How, after all, would one determine which tissues within chimeric plants are genetically jointless in the absence of molecular probes? Clearly, one would need at least three phenotypic markers, one for each tissue layer, so that the genetic origins of L1, L2, and L3 could all be defined by visual inspection of experimental chimeras.

In this respect, the authors were able to rely on previous efforts in which they had established a triple mutant (h/h, ag/ag, Xa-2/+) marker stock (Szymkowiak and Sussex 1992 ). In this way, the hairless (h) mutation, resulting in plants devoid of epidermal trichomes, serves as a signifier of L1-derived tissue. The anthocyanin gainer (ag) mutation abolishes the production of anthocyanin, so that cytological inspection of subepidermal tissue will identify L2 derivation, and the innermost shoot tissue can be defined by the yellow phenotype associated with the Xanthophyllic-2 (Xa-2/+) genotype, which also marks both L2- and L3-derived leaf tissue. (Homozygosity for Xa-2 is lethal.) Finally, the introduction of jointless into the triple marker background produces plants that can be designated j·j·j, in recognition of the jointless mutation within all three (L1·L2·L3) periclinal layers.

Developmental analyses are then initiated by grafting donor shoots (known as scions) from the j·j·j mutant (h, ag, Xa-2 background) onto wild-type (+·+·+) stock (or vice versa). The subsequent wounding of an established graft, precisely at the graft junction, results in a callus from which periclinal chimeric shoots are collected and propagated. In this manner, the authors were able to isolate the six possible combinations of periclinal chimeras with respect to the jointless locus: 1) j·+·+, 2) +·j·+, 3) +·+·j, 4) j·j·+, 5) j·+·j, and 6) +·j·j. Layers bearing the jointless mutation can be discerned upon the expression of either the h, ag, or Xa-2 marker, and the developmental effects of jointless can thus be assigned to particular meristem layers.

A priori, it was of course possible that the expression of the jointless-conferred phenotype, affecting multiple cell types, would require the mutation to be localized within all three layers of the shoot meristem. Previous results from multiple plant systems, however, have demonstrated that a collection of meristematic "signaler" cells often acts to instruct the remaining "responder" cells of the meristem to follow particular paths of development (Szymkowiak and Sussex 1992 , Szymkowiak and Sussex 1996 ). To the extent that jointless could be regarded as a perturbation in information flow among meristem layers, therefore, the six periclinal chimeras studied by Szymkowiak and Irish could be relied upon to determine any blockage in anticlinal (i.e., between layers) communication that jointless might pose.

In fact, the authors show most decisively that the mutant phenotype is expressed only when jointless is borne by L3 cells of the meristematic tissue. In other words, normal pedicel abscission requires that L3 cells be wild type. Moreover, the two developmental characteristics that are affected by jointless, that is, pedicel abscission and inflorescence determinacy, remain linked within all chimeric combinations of L1, L2, and L3 tissues.

What do these results have to say about the flow of information within the developing meristem? Histological evaluations of pedicel tissues from all periclinal chimeras establish that the Jointless wild-type allele is not only necessary for the elaboration of normal abscission zones, but is also sufficient. Indeed, wild-type L3 tissue can rescue mutant L2 and L1 tissues to the extent that j·j·+ chimeras form pedicels that appear fully wild type (i.e., +·+·+) and abscise normally. In this way, the signaling that involves Jointless is unidirectional and radial, initiating primarily within the meristem corpus, although the authors also offer some evidence for signaling from L2 to L1.

But what about lateral communication between the cells of a given meristem layer? Can chimeras help here, too? Indeed, Szymkowiak and Irish suggest that within layers, cell development is autonomous. This conclusion is based on chimeras in which mosaics of jointless and wild-type tissues are generated within each of the meristem layers. The characteristics of such mericlinal chimeras are intriguing, because even small sectors of wild-type cells in the L3 prove sufficient to direct L2 and L1 to maintain the wild-type phenotype. In multiple mericlinal chimeras, however, lateral communication of Jointless is precluded. These results collectively show that regulation of information processing among cells can be exerted even when the information per se is formally present within a given tissue. To be certain, the future molecular characterization of the jointless locus will help to clarify how the periclinal arrangement of meristematic tissues is related to developmental programs. Until then, we can happily rely upon elegant genetic and developmental studies of the type presented this month by Szymkowiak and Irish.

REFERENCES

Abeles, F.B., Morgan, P.W., and Saltveit, M.E., Jr. (1992). Fruit ripening, abscission, and postharvest disorders. In Ethylene in Plant Biology, 2nd ed. (San Diego, CA: Academic Press), pp. 182–221.

Bleecker, A.B., and Patterson, S.E. (1997) Last exit: Senescence, abscission, and meristem arrest in Arabidopsis. Plant Cell 9:1169-1179[CrossRef][ISI][Medline].

del Campillo, E., and Bennett, A.B. (1996) Pedicel breakstrength in cellulase gene expression during tomato flower abscission. Plant Physiol. 111:813-820[Abstract].

del Campillo, E., and Lewis, L.N. (1992) Identification and kinetics of accumulation of proteins induced by ethylene in bean abscission zones. Plant Physiol. 98:955-961[Abstract/Free Full Text].

Eyal, Y., Meller, Y., Lev-Yadun, S., and Fluhr, R. (1993) A basic-type PR-1 promoter directs ethylene responsiveness, vascular and abscission zone specific expression. Plant J. 4:225-234[CrossRef][ISI][Medline].

Morré, D.J. (1968) Cell wall dissolution and enzyme secretion during leaf abscission. Plant Physiol. 43:1545-1559[Medline].

Sexton, R., and Roberts, J.A. (1982) Cell biology of abscission. Annu. Rev. Plant Physiol. 33:133-162.

Szymkowiak, E.J., and Irish, E.E. (1999) Interactions between jointless and wild-type tomato tissues during development of the pedicel abscission zone and the inflorescence meristem. Plant Cell 11:159-175[Abstract/Free Full Text].

Szymkowiak, E.J., and Sussex, I.M. (1992) The internal meristem layer (L3) determines floral meristem size and carpel number in tomato periclinal chimeras. Plant Cell 4:1089-1100[Abstract/Free Full Text].

Szymkowiak, E.J., and Sussex, I.M. (1996) What chimeras can tell us about plant development. Annu. Rev. Plant Physiol. Plant Mol. Biol. 47:351-376[CrossRef][ISI].

Related articles in Plant Cell:

Interactions between jointless and Wild-Type Tomato Tissues during Development of the Pedicel Abscission Zone and the Inflorescence Meristem

Eugene J. Szymkowiak and Erin E. Irish
Plant Cell 1999 11: 159-176. [Abstract] [Full Text]  

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