Understanding the basis of a novel fruit type in Brassicaceae: conservation and deviation in expression patterns of six genes

Abstract

Background: Variation in fruit morphology is important for plant fitness because it influences dispersal capabilities. Approximately half the members of tribe Brassiceae (Brassicaceae) exhibit fruits with segmentation and variable dehiscence, called heteroarthrocarpy. The knowledge of the genetics of fruit patterning in Arabidopsis offers the opportunity to ask: (1) whether this genetic pathway is conserved in taxa with different fruit morphologies; (2) how the pathway may be modified to produce indehiscence; and (3) whether the pathway has been recruited for a novel abscission zone.

Methods: We identified homologs of ALCATRAZ, FRUITFULL, INDEHISCENT, SHATTERPROOF, and REPLUMLESS from two taxa, representing different types of heteroarthrocarpy. Comparative gene expression of twelve loci was assessed to address how their expression may have been modified to produce heteroarthrocarpy.

Results: Studies demonstrated overall conservation in gene expression patterns between dehiscent segments of Erucaria erucarioides and Arabidopsis, with some difference in expression of genes that position the valve margin. In contrast, indehiscence in heteroarthrocarpic fruit segments was correlated with the elimination of the entire valve margin pathway in Erucaria and Cakile lanceolata as well as its absence from a novel lateral abscission zone.

Conclusions: These findings suggest that modifications in the valve margin positioning genes are responsible for differences between heteroarthrocarpic and Arabidopsis-like fruits and support the hypothesis that heteroarthrocarpy evolved via repositioning the valve margin. They also highlight conservation in the dehiscence pathway across Brassicaceae.

Figure 1

Diagrammatic representation of fruits of focal species. Left panel: Erucaria erucarioides fruit in medial view. Right panel: Cakile lanceolata fruit in medial view. Central panel, top: transverse section through the distal segments of Erucaria and Cakile. Central panel, bottom: transverse sections through proximal segment of Erucaria (left) and Cakile (right). White arrowheads in left and right panels indicate the position of the joint, black arrowheads in the central panel indicate the position of the valve margin, dotted lines represent abscission zones at fruit maturation. o, ovules; ow, ovary walls; r, replum; s, septum; v, valves; *, endocarp b layer.

Figure 2

In situ hybridization expression of EeSHP2, EeALC, EeIND, and EeFUL1 during Erucaria erucarioides carpel development. (a-c) Transverse sections through buds showed EeSHP2 expression. (a) Expression in the developing septum of a young bud. (b) Expression at a later stage in the septum, ovules, and valve margin of the proximal segment. (c) Expression in the ovules of the distal segment. (d) EeSHP2 sense control in a transverse section. (e) Longitudinal section of a differentiated carpel showed EeSHP2 expression in septum and ovules. (f) EeALC expressed in the valve margin, septum, endb, and exocarp of a proximal segment, transverse section of an old bud. (g) Signal of EeALC in the distal segment was detected in the septum of differentiated carpel, transverse section. (h) Transverse section of an undifferentiated carpel showed EeIND expression in ovule primordia. (i-k) EeIND was expressed in valve margin and ovules of the proximal segment of differentiated carpel, transverse section (i), but only septum and ovules of proximal as shown in transverse (j) and longitudinal (k) sections of differentiated carpel. (l) Transverse section showed EeFUL1 expression throughout inner valves, but not replum or presumptive valve margin, of undifferentiated carpel. (m, n) EeFUL1 was expressed in inner valves of the proximal segment, as seen in transverse (m), but not in the ovary walls of the distal segment of differentiated carpel, longitudinal (n). Arrows indicate the position of the joint, arrowheads indicate the position of the valve margin. o, ovules; ow, ovary walls; r, replum; s, septum; st, stamen; v, valves. Scale bar: 50 μm (a), 100 μm (b, c, d, f, g, h, i, j, k, l, m), 500 μm (e, n).

Figure 3

Reverse transcriptase polymerase chain reaction. RT-PCR expression analysis of (a) EeSHP2, EeALC, EeIND, EeFUL1, EeFUL2, EeRPL, EeACTIN, and (b) ClSHP1, ClSHP2, ClALC, ClFUL1, ClFUL2, ClRPL, and ClACTIN in young buds (YB), old buds (OB), flowers (FL), young fruits (YF), mature fruits (MF), and leaf tissue (LF).

Figure 4

In situhybridization expression ofClSHP1,ClSHP2, andClALCduringCakile lanceolatacarpel development. (a, b) Transverse section through an old bud showed ClSHP1 expression in the ovules of (a) distal and (b) proximal segments. (c) ClSHP1 signal observed in the ovules, septum, and nectaries of an old bud, longitudinal section. (d) ClSHP2 signal was observed in the developing placental tissue. (e-g) ClSHP2 expression was observed in the ovules and septa of (e) distal and (f) proximal segments, transverse sections, and in longitudinal (g). (h, i) Transverse section through an older bud showed weak ClALC expression in septum, ovules, and funiculus in (h) proximal and (i) distal segments. (j) ClALC sense probe of an older bud. Arrows indicate the position of the joint. o, ovules; ow, ovary walls; r, replum; s, septum; v, valves. Scale bar: 100 μm (a, b, d, e, f, h, i, j), 500 μm (c, g).

Figure 5

Comparison of expression patterns of valve margin pathway genes between Arabidopsis,Erucaria, andCakile. (a) Valve margin pathway in Arabidopsis (modified from [11]). (b) Summary of valve margin pathway expression in distal (top) and proximal (bottom) of Erucaria based on in situ and RT-PCR data. (c) Summary of valve margin pathway expression in distal (top) and proximal (bottom) of Cakile based on in situ and RT-PCR data.