Yuxiang Huang

Shoot branching is an important crop agronomic trait that directly affects plant architecture and crop productivity. Although phytochrome B (phyB), BRANCHED1 (BRC1), and abscisic acid (ABA) mediate axillary bud outgrowth, it is unknown if there is any integrating factor among them in the Plantae. We report that mutation of CsphyB or inactivation of CsphyB by shade inhibits lateral bud outgrowth in cucumber. Cucumber PHYTOCHROME INTERACTING FACTOR 4 (CsPIF4) interacts with CsphyB and directly binds to the promoter of CsBRC1 to activate CsBRC1 expression. CsBRC1 also directly promotes the expression of ABA biosynthesis gene 9‐CIS‐EPOXICAROTENOID DIOXIGENASE 3 (CsNCED3). Functional disruption of CsPIF4 decreased expression of CsBRC1 and CsNCED3, reduced ABA accumulation, and increased bud outgrowth in cucumber. Csnced3 mutants had reduced ABA levels and increased lateral bud outgrowth. These results suggest that a regulatory network involving CsphyB‐CsPIF4‐CsBRC1 exists that integrates light signaling and ABA biosynthesis to modulate bud outgrowth. This provides a strategy to manipulate branch numbers in crop breeding to realize ideal branching characteristics to maximize yield.

Complete disruption of critical genes is generally accompanied by severe growth and developmental defects, which dramatically hinder its utilization in crop breeding. Identifying subtle changes, such as single nucleotide polymorphisms (SNPs), in critical genes that specifically modulate a favorable trait is a prerequisite to fulfill breeding potential. Here, we found two SNPs in the E-class floral organ identity gene cucumber (Cucumis sativus) SEPALLATA2 (CsSEP2) that specifically regulate fruit length. Haplotype (HAP) 1 (8^G2667^A) and HAP2 (8^G2667^T) exist in natural populations, whereas HAP3 (8^A2667^T) is induced by ethyl methanesulfonate mutagenesis. Phenotypic characterization of four near-isogenic lines and a mutant line showed that HAP2 fruits are significantly longer than those of HAP1, and those of HAP3 are 37.8% longer than HAP2 fruit. The increasing fruit length in HAP1-3 was caused by a decreasing inhibitory effect on CRABS CLAW (CsCRC) transcription (a reported positive regulator of fruit length), resultinged in enhanced cell expansion. Moreover, a 7638^G/A-SNP in melon (Cucumis melo) CmSEP2 modulates fruit length in a natural melon population via the conserved SEP2-CRC module. Our findings provide a strategy for utilizing essential regulators with pleiotropic effects during crop breeding.

Lateral branches are important components of shoot architecture and directly affect crop yield and production cost. Although sporadic studies have implicated abscisic acid (ABA) biosynthesis in axillary bud outgrowth, the function of ABA catabolism and its upstream regulators in shoot branching remain elusive. Here, we showed that the MADS-box transcription factor AGAMOUS-LIKE 16 (CsAGL16) is a positive regulator of axillary bud outgrowth in cucumber (Cucumis sativus). Functional disruption of CsAGL16 led to reduced bud outgrowth, whereas overexpression of CsAGL16 resulted in enhanced branching. CsAGL16 directly binds to the promoter of the ABA 8'-hydroxylase gene CsCYP707A4 and promotes its expression. Loss of CsCYP707A4 function inhibited axillary bud outgrowth and increased ABA levels. Elevated expression of CsCYP707A4 or treatment with an ABA biosynthesis inhibitor largely rescued the Csagl16 mutant phenotype. Moreover, cucumber General Regulatory Factor 1 (CsGRF1) interacts with CsAGL16 and antagonizes CsAGL16-mediated CsCYP707A4 activation. Disruption of CsGRF1 resulted in elongated branches and decreased ABA levels in the axillary buds. The Csagl16 Csgrf1 double mutant exhibited a branching phenotype resembling that of the Csagl16 single mutant. Therefore, our data suggest that the CsAGL16–CsGRF1 module regulates axillary bud outgrowth via CsCYP707A4-mediated ABA catabolism in cucumber. Our findings provide a strategy to manipulate ABA levels in axillary buds during crop breeding to produce desirable branching phenotypes.