More about associations of the growth-regulating genes
--Emil Khavkin and Ed Coe

Previously (MNL 68:61, 1994) we have hypothesized that genes for plant growth and development are associated into functionally significant clusters. Sixteen initially delineated clusters, 10-30 cM long (2 to 3 bins of the molecular map), comprise almost 90% of already mapped growth genes. The majority of recently reported QTLs for plant architecture, growth pattern and yield (Ajmone-Marsan et al., Maydica 39:133, 1994; Beavis et al., TAG 83:141,1991; Doebley and Stec, Genetics 134:559,1993; Edwards et al., TAG 83:765, 1992; Higginbotham, MNL 65:65, 1991; Stuber et al., Genetics 132:823, 1992; Zehr et al., TAG 83:903, 1992), growth and differentiation in vitro (Armstrong et al., TAG 84:755, 1992; Cowen et al., TAG 84:720, 1992; Wan et al., TAG 85:360,1992), pollen growth (Sari-Gorla et al., Heredity 69:423, 1992) and ABA accumulation (Quarrie et al., Russ. J. Plant Physiol. 41:565,1994) map within these clusters. We consider this evidence a corroboration to the cluster model.

It is no wonder that QTLs for plant height precisely match naked eye polymorphisms (NEPs) for reduced plant stature. More impressive are those map segments where the closely mapped NEPs expressing various growth and developmental manifestations coincide with QTLs for growth traits that are widely different. QTLs co-mapping with NEPs do not always correspond by their apparent functions.

These cases are exemplified by bin 1.11 (d8, kn, phy1 and tb1) and QTLs for plant height, architecture and yield; bin 1.12/13 (ij2, py2, rd1, tls1, tlr1, ts6, and vp8) and QTLs for embryogenesis in callus culture, plant height, ear and tassel development and days to pollen; bin 2.06 (ba2 and ts1) and QTLs for plant height, leaf length, ear architecture and embryogenesis in vitro; bin 3.05 (abp1, d2, rd3, rg1, te1, tp3, ts4 and vp1) and QTLs for pollen growth, plant growth and architecture; bin 3.06/07 (ba1, bif1, lg2, na1, les14, les17 and yd2) and QTLs for embryogenesis in vitro, days to pollen and the numerous parameters for plant architecture and yield substantiated by reports from three laboratories; bin 4.03-4.04 (la1, orp1, st1, tga1 and ts5) and QTLs for pollen germinability and many traits of plant shoot and inflorescence architecture; bin 5.06/07 (br3, bv1, gl17, nec3, nec6, td1, vp2 and vp7) and QTLs for pollen growth, plant architecture, yield and days to tassel; bin 9.03/04 (baf1, d3, gl15 and les8) and QTLs for embryogenesis in vitro, pollen growth, plant height and yield, and ABA synthesis. In these cases, the choice between polygenic interactions or monogenic pleiotropic expression of physiologically linked traits will provide a prospective means to dissect the complex traits and to search for key genes working as developmental switches.

Several additional clusters can be presumed that combine the mapping positions for single NEPs and for several molecular probes, co-segregating with growth-related QTLs. The most promising locations are bin 2.01 (lg1 and QTLs for plant height, ear length and days to pollen); bin 4.07/08 (tu1, nec5 and numerous QTLs for plant architecture, growth and yield reported from four laboratories); bin 6.01/02 (rgd1, si1, wi1 and QTLs for embryo differentiation in callus culture, pollen tube growth, days to tassel and days to pollen, plant height and architecture).

Two map segments devoid of NEPs for growth and development contain the major QTLs for embryogenic callus initiation, plant regeneration, plant yield and apparently ABA synthesis (bin 9.05/06); and for plant growth, days to pollen and plant yield (bin 10.07/08). Using the hybridization probes for the already known growth genes to map these segments of the genome could help reveal the genes manifesting these quantitative traits. 

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