Question

(b) The genes SHP1/2, FUL, and RPL, are involved in regulating Arabidopsis fruit development. What are the roles of these genes in the growth of the valve, replum, and dehiscence zone? (4 pts)

(c) Use a diagram to illustrate the interactions between SHP1/2, FUL, and RPL gene products. Use ‘®’ for positive interactions (i.e., activation) and ‘ ¾|’ for negative interactions (i.e., repression). (3 pts)

6. Please answer the following question about the Quiescent Center (QC) of the root meristem. (10 pts in total)

(a) What is the QC of the root meristem? (4 pts)

(b) How is QC identity established? (Describe the plant hormone(s) and genes that are involved in QC development) (6 pts)

Answer 1

b)

FRUITFULL (FUL) is necessary for proper valve development and represses SHATTERPROOF 1/2 (SHP 1/2) SHP1/2 are necessary for valve margin development. REPLUMLESS (RPL) is necessary for replum development and represses SHP1/2 .The repression of SHP1/2 by FUL and RPL keeps valve margin identity to a small strip of cells. SHP1/2 activate INDEHISCENT (IND) and ALCATRAZ (ALC), which are both necessary for the differentiation of the dehiscence zone between the valves and replum.

FUL orthologs have a conserved role in regulating proper fruit development even in fruits with diverse morphologies.genes control proper pericarp cell division and elongation, endocarp identity, and promote proper distribution of bundles and lignified patches after fertilization. RPL orthologs have not been characterized in core eudicots, but an RPL homolog in rice is a domestication gene involved in the non-shattering phenotype, suggesting that the same genes are important to shape seed dispersal structures in widely divergent species .SHP orthologs play roles in capsule dehiscence.

The expression of ALC in the valve margin is regulated by SHP1/2 and FUL in Arabidopsis.In Arabidopsis, the most closely related bHLH protein to ALC is SPATULA (SPT). Both SPT and ALC share conserved atypical E-box elements in their cis-regulatory sequences . This sequence is required for SPT expression in the "valve margin" and "dehiscence zone", however, similar expression studies are lacking in ALC.SPT orthologs have been identified across the seed plants.Sequence analyses of Brassica rapa IND and Arabidopsis IND identified a shared 400 bp sequence in the cis-regulatory regions with high similarity This region was able to direct expression in the valve margin and its expression was regulated by FUL and SHP1/2.

(RPL) is necessary for "replum development "and represses SHP1/2 . The repression of SHP1/2 by FUL and RPL keeps "valve margin" identity to a small strip of cells. Here are 13 BLH proteins in Arabidopsis and the most closely related paralog to RPL in Arabidopsis is PNF .PNF and RPL orthologs throughout the angiosperms indicating that a duplication occurred at the base of the angiosperms before they diversified . RPL is integral for replum formation in the Arabidopsis fruit and represses SHP1/2 . However, RPL "[also called PENNYWISE (PNY), BELLRINGER (BLR), and VAAMANA] "has multiple roles in Arabidopsis development including meristem development, inflorescence, and fruit development.In Arabidopsis, RPL represses SHP1/2 to keep valve margin identity to a few cell layers.These cell layers later become lignified and are important for " fruit dehiscence". SHP orthologs also play roles in capsule dehiscence.

SHP genes is responsible for the conversion of replum cells into valve margin cells and that RPL function is to negatively regulate SHP genes in the replum, avoiding the differentiation toward valve margin of replum cells. In this way, RPL plays the same role in the replum as FUL in the valves. FUL and RPL restrict SHP expression to a narrow strip of cells that will develop into valve margin, thus ensuring fruit dehiscence.  

Genetic and molecular studies have contributed to our knowledge about which genes are needed to create the detachment line responsible for fruit opening, SHP1, SHP2, IND and ALC and how these genes have to be actively repressed in the valves by FUL and in the replum by RPL, to avoid the spread of valve margin identity.

c)

Genetic pathway controlling fruit development in Arabidopsis . The differentiation of tissues is controlled by two sets of genes that either promote valve margin development, i.e. SHP1 /2 (MADS-box genes), IND and ALC (basic helix-loop-helix-type transcription factor genes) or restrict it from surrounding regions ( FUL , RPL ). Proper establishment of the dehiscence zone is controlled by SHP1/ 2 , IND.

6 )a) In the root apical meristem (RAM)," the quiescent center (QC) maintains a stem cell population to form a stem cell niche".The  part of root-promeristem, which contains the initials of epidermis, cortex and stele and divides less frequently, is referred to as quiescent centre.

b) Transcription factor SCARECROW (SCR) is required for maintenance of the root meristem .SCR is expressed in the quiescent center (QC) of the root meristem in cortex initials and in differentiated endodermis .Activity of SCR is required for " QC cell identity" and root growth.

Typically, an auxin maximum forms within the distal stem cell region, via local biosynthesis and polar auxin transport directed by the PIN efflux facilitators, and such a maximum is required for QC function.The auxin-inducible genes encoding PLT1-PLT4 transcription factors are master regulators of root stem cell activity, and their expression gradient is dependent on auxin distribution and is required for the maintenance or differentiation of stem cells in the RAM . Brassinosteroids (BRs) are a group of naturally occurring polyhydroxy steroidal hormone isolated from plants.BR is involved in the promotion of QC renewal and distal stem cell differentiation. Interestingly, mutation in BR biosynthesis genes or exogenous application of BR all lead to the reduction of root meristem size, suggesting that balanced BR signaling is critical for root meristem maintenance.During the postembryonic development of the root, ethylene could restrain cell division of the QC, and the cells formed through ethylene-induced divisions retain characteristics of the QC.