Maize Genetics Cooperation Newsletter vol 81 2007

Grassy tiller and sweet corn

--Brewbaker, JL

Tillering is a characteristic of early American sweet corns (sugary1), but is rarely found in other races or types of maize. We report here that all of the early sweet corns we've tested carry the gene, grassy tiller (gt). Grassy-tillered plants also produce leaves that extend the husks ("husk leaves" or "flag leaves"), a feature not noted in the genetic literature but of utility to processors of temperate sweet corn for removal of husks.

Our breeding of tropical sweet corns in Hawaii has been based entirely on hybrids of temperate (tillered) and tropical (non-tillered) types. All of the >20 open-pollinated populations we�ve released of this type segregated tillered plants as a recessive trait (Brewbaker, HortSci. 33:1262-4). Tillered plants were also marked by presence of husk leaves that segregated as a recessive monogene (MNL 79:14), now known also to be gt1.

In the present study, temperate sweet corn inbreds provided by Bill Tracy (U. Wis.) were crossed with two sources of gt, one based on population WGRComp2 from Jim Coors (U. Wis.) and one, gt^Hi27 from our near-isogenic line series (MNL 69:58-9). The temperate inbreds were:

sugary1: 101t, C5, C40, Hotevilla AZ, P39, P51

shrunken2: Ia453sh2

Hybrids of these sweet inbreds with gt stocks were all highly tillered, with long flag leaves (Figure 1). All F2 populations grown from these hybrids were also 100% tillered. One recombinant inbred population (SET M) based on the cross of Ia453sh2 (tillered) with Hi38bt (no tillers) segregated 19 tillered and 27 non-

Figure 1. Grassy tillered hybrid of NIL gt^Hi27 with sweet corn inbred P51. (For color see online.)

tillered RILs, while the F2 of this cross segregated 3:1 for normal to grassy tillered. One of the tillering NILs, M23, was crossed to a gt stock and produced only grassy-tillered hybrids.

The number and size of tillers and husk leaves is highly correlated with plant vigor. Experimental trials at Waimanalo, Hawaii, are planted year-round, and corn biomass yields in summer are roughly double those in winter. Yields are reduced largely by low light in our wet winters (Jan. avg. 275 cal/cm^-2day^-1) vs. the dry summers (July avg. 450 cal/cm^-2day^-1). Tiller numbers are reduced in winter; the tiller heights of inbred gt^Hi27 were reduced to <6� in winter vs. >18� in summer. Vigorous +/gt hybrids often produce small flag leaves in the summer also. High plant density and low nitrogen fertility reduce the expression of tillers and flag leaves. Husk-leaf extension increased greatly in Hawaii�s summer trials for many hugely tillering Korean genotypes (MNL 59:14).

Other highly tillering genes include Tlr (tillering) and tb (teosinte-branched), and both of these mutants also have long flag leaves. The genes are on long arm of chromosome 1 and possibly allelic. Both genes have a major effect on ear morphology, unlike gt. The Tlr/Tlr homozygote is extremely grassy in Hawaii and has abortive ears. It resembles Cg (corngrass), a mutant that also leads to tillering and flag leaves.

Teosinte species tiller abundantly like most grasses, presumably based on genes like Tlr. Our hybrids of maize with Jutiapa teosinte and with Zea diploperennis were all highly tillered, showing tillering to be dominant (cf. Fig. 1, Srinivasan and Brewbaker, Maydica 44:353-370). In the referenced study Srinivasan produced 11 hybrids between tropical maize inbreds (with only the single main culm) and Z. diploperennis (avg. 18.3 tillers in winter, 29.3 in summer). In summer plantings the F1 plants averaged 5.4 tillers, F2�s averaged 3.4, backcrosses to maize averaged 1.6 and backcrosses to Z.d. averaged 2.9. Generation mean analysis showed that narrow-sense heritability was high (81%) and based largely on dominance and epistatic (dd) interactions. At least two loci were inferred. Winter data for the Z.d. x maize populations showed that tillers were reduced an average of 13.8% for the four generations, with similar reduction in heritability.

We have bred a broad-based population, HIC9d, from backcrosses of these Z.d. hybrids to maize. It segregates about 10% tillered plants. The population is highly heterogeneous for tiller and husk leaf extension, and for vigor, prolificacy and many ear traits. It is being tested for allelism of tillering genes to gt and Tlr.

The Maize Genetics Coop gene gt is located near the centromere on chromosome 1 and is attributed to Don Shaver (MNL 39:18-22), who writes (pers. commun.) �Earl Patterson had told me that E. G. Anderson found it or discovered it (at Cal Tech)�. The gt in our NIL set (reported in MNL 69:58-9) derives from the MGC stock gt/gt id/id (66Cal, 3327x28) that seems to have the same origin, out of mutants from Bikini in Anderson�s collection in 1948, a nursery in which I was privileged to work with Earl, Ed Coe and Andy. However, Walt Galinat (pers. commun.) notes his early interest in tillering and the possibility that the N.E. sugary lines in his program provided the gt locus of Shaver, who made hybrids of Galinat�s sweet corns with id (also found on chromosome 1L) and pe stocks in studies of perennialism in maize (Shaver, J. Hered. 58:270-273; MNL 79:39-41). In any event, the two sources appear to be identical alleles.

In view of the rarity of tillering in maize, the independent origin of gt in early American sweet corns or their progenitors appears highly probable. Mysteriously eluding early authors on this subject was the fact that gt also controls husk-leaf extension, a feature that became of value to the temperate sweet corn industry by facilitating husk removal during processing. In Thailand, the tropical supersweets with Hawaiian ancestry (many husks but no husk leaves) from 150,000 A. annually are husked following sprays with hot water (Taweesak Pulam, pers. commun.). It is unclear whether genotypes exist with flag leaves but no capacity or totipotency for tillering. We suspect that source of cytoplasm must be considered in unravelling the perennialism of Z. diploperennis that has been elusive in maize hybrids with genes like gt, Tlr, id and pe.

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